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Audio Component Grounding and Interconnection

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About the Author

David Davenport

Dave Davenport’s interest in
audio was sparked as a teenager, building hi-fi kits and learning electronics.
Upon retiring from a 30-year career as an engineer with IBM, Dave is now active
writing articles and providing audio kits through his business, Raleigh Audio -
www.raleighaudio.com

Audio Component Grounding and Interconnection

David Davenport
,

21st April 2010

This article focuses on audio system design for the DIY enthusiast
implementing a stereo system for home use. There is a lot of information
available for designing audio circuits, but precious little on interconnecting
those circuits into a total audio system to achieve maximum performance.
Interconnecting circuits to create an audio system involves connecting signals
and grounds , both within a single component as well as between components. This
article has the most benefit for the person constructing his or her own
equipment, rather than merely interconnecting finished components. Although much
of the information presented is applicable for professional installations, those
installations face a myriad of problems due to physical size, number of
components and different types of components, which are not addressed here.

Although I use electron tube circuits for many examples in this article,
the concepts presented are valid for solid state circuits as well. Most, if not
all digital circuits will use integrated circuits and I cover interconnecting
digital electronics with analog electronics.

Chapter 1 - Ground

The term “ground” is problematic because it can
mean so many different things. Often a designer will lump all of the meanings
together and in an indiscriminating manner, connect everything that needs to be
“grounded” together. Lumping all of the meanings together and treating them all
the same causes problems that wouldn’t happen if the different uses of ground
were kept separate and each treated in a manner appropriate for its use.
Therefore a good place to start would be to tease all of the meanings apart so
we can address them separately.

The first ground we encountered was
playing in the dirt as a child: we call the place where the grass and trees grow
ground. However, considering the electronics in an airplane or a cell phone,
this ground is not necessary for electronic circuits to function – its only
interest in electronics is as a sink for lightning strikes. I’ll refer to this
ground as “earth.”

Recognizing that high voltage is lethal, the various
standards organizations around the globe have instituted safety standards for
mains-attached electrical equipment. The first assumption in these standards is
that a human body in the vicinity is at earth potential. Therefore the standards
require that a metal chassis and any exposed metal parts be at earth potential
by specifying a wire be connected between the chassis and a connection to earth
at the circuit breaker panel. This ensures that any electrical fault within the
equipment will not be a hazard to anyone that comes in contact with the chassis.
A second benefit of this earth connection is that it provides a low-impedance
path to earth for a lightning strike. This minimizes potential circuit component
damage as well as providing protection from electrocution. A third facet of the
standards is the means by which a fuse will be blown or a circuit breaker
tripped in the event of a fault where the mains voltage is connected to the
chassis. As shown in figure 1-1, in the event of a fault, a large current will
flow from the circuit breaker panel, through the electrical wiring to the
equipment, through the fault to the chassis, and then through the safety wire
back to the circuit breaker panel. If there is an appropriately-sized fuse in
the equipment, it will blow; otherwise the circuit breaker will trip. It is a
good idea to provide an appropriately-sized fuse in the equipment because it
will blow a lot sooner than the circuit breaker, thereby quickly eliminating the
large current which may damage expensive components. Note that the connection to
earth at the circuit breaker panel does not come into play for a fuse to blow or
breaker to trip. Also, no current flows in the safety wire when there is no
fault present. I’ll refer to this ground as “safety ground.”

Figure 1-1, Safety
ground: showing the current from a fault between the power line and the chassis.

Adhering to the safety standards by providing a safety ground
provides some protection for equipment manufacturers from expensive liability
law suits. While you may not be worried about law suits – you are liable for any
damage, injury or death caused by the equipment that you build – you should be
very concerned about your personal health and well being as well as that of your
loved ones. Equipment lacking a safety ground has caused fires, injury and
death. Do not under any circumstance, for any reason, disconnect or disable
an existing safety ground, or fail to include a safety ground in any equipment
that you build. But, you say, “what about my CD player, it has a two-wire
power cord without a safety ground?” Consumer component manufactures have
engineers with the skills to design a double-insulated product which is
inherently safe without a safety ground. This is not a trivial task and should
not be attempted by amateurs that do not have the necessary skills. Double
insulated products are designed such that any single fault cannot cause power
line voltage to be present on any exposed part, including the chassis. Special
provisions must be made to preclude a winding-to-winding fault within the power
transformer.

The good news is that, in a properly designed audio system,
a safety ground will not hurt and may actually help produce optimal sound
quality. We’ll see how this works a little later.

Most audio components
will have one or more power supplies. I think of the purpose of a power supply
is to provide the operating environment for the audio circuitry. That is to say,
it provides the necessary voltages and currents to establish an idle state
separate from any function that the circuit is intended to produce. For example,
in a triode vacuum tube amplifier, the power supply supplies plate current,
plate voltage and a bias voltage. It will also supply screen grid voltage for a
pentode amplifier. Current will flow from the positive terminal of the supply,
through the load and back into the negative terminal of the supply. For
convenience in making voltage measurements, understanding and using the power
supply, a designer will usually designate some point in the power supply as
“ground.” This is usually the negative terminal of a positive supply, or the
positive terminal of a negative supply. I will refer to this ground as “power
common.”

Figure 1-2, Power Common:
In this example the negative terminal of the power supply is chosen to be power
common.

What would happen if some node with voltage from the power
supply were to become shorted to the chassis? The chassis is at earth potential
(through the safety ground) so the shorted node would be at earth potential – so
far, so good. We would like to detect the fault and cause the fuse to blow,
which requires a complete path for the fault current to flow from the power
supply, through the fault to the chassis and back to the power supply. Although
it doesn’t have to be, to avoid confusion most designers choose power common as
the point to connect to the chassis for the return path. Note that under normal,
non-fault, conditions, no current will flow in the connection between power
common and the chassis. The only purpose for this connection is to direct fault
current.

Figure 1-3, Power Common
and a fault

Figure1- 3 shows that a fault causes a large current to
flow in the secondary of the power transformer, which in turn induces a large
current in the primary of the transformer. Note that no current flows in the
safety ground for this fault.

Power common should also be used as the
“ground” reference for any vacuum tube filaments. This may be by a direct
connection or by a bias voltage.

It goes without saying, that in order
for an audio component to be worthwhile, it must process an audio signal in some
manner. For a signal voltage to be meaningful, it must be referenced to
something. In some cases, as in a differential amplifier, the signal is
presented to the positive input and the reference is the negative input. In
other cases, as in a single-ended amplifier, the signal is presented to the
input and the reference is “ground.” I will call this ground “signal reference.”
A triode is a three-terminal device: the input signal is presented to the grid
with the cathode as the reference, and the output is taken from the plate with
the cathode as the reference. The cathode is usually connected to power common,
either directly or through a bias resistor. Thus, signal reference is connected
to power common; however no current flows in the connection between power common
and the signal reference. Each circuit has a signal reference and some circuits
may be analog while other circuits are digital. It is a good idea to keep the
digital references separated from the analog references.

The last use of
the term “ground” that I will address is shields. There are basically two
different shields in an audio system – chassis and cable shields. There are two
important characteristics of a shield: the continuity of the enclosure and the
material it is made of. By continuity, I mean that for a shield to be maximally
effective there must be no holes or gaps in the chassis or in the connection
between the cable shield and chassis. This may not be practical in an audio
system, but it is good to keep in mind as a goal. Of course, plastic or wood
will offer no shielding, while copper or aluminum will provide electrostatic
shielding and steel or mu metal will provide both electrostatic and magnetic
shielding.

Rule 1. In Morrison [1] (page 39) states, “An electrostatic
shield enclosure, to be effective, should be connected to the zero-signal
reference potential of any circuitry contained within the shield.”

It is
Important to not have a chassis isolated – it must have some ground. Everything
has a potential to everything else. Either control it or you have noise
potential. Thus a chassis or cable shield must be connected to signal reference;
however, no current flows in the connection between the shield and the signal
reference.

I use the terms “ground buss” and “star ground” in this
article. A ground buss is a piece of wire at ground potential. A star ground is
two or more grounds connected to one physical point. A star of stars is just
that – several star grounds in turn all connected to a single point.

So
in an audio system we have earth and possibly multiple safety grounds, power
commons, signal references and shields; all interconnected. I have said that
under normal, non-fault, conditions that no current will flow in these
interconnections. This is not entirely true for there is noise in the system
which may flow between the different “grounds” and cause degradation of the
sonic performance of the audio system. Therefore the goal is to minimize the
noise traffic in a system by first reducing the noise and second, by reducing
the propensity for the remaining noise to move around; or alternatively to
control the paths so as to minimize the effect. Let’s start by taking a look at
noise.

Chapter 2 - Noise

All electrical noise, with the exception of
lightning induced noise, is produced by man-made electrical or electronic
devices, including all of those in your home. Of course we can’t exclude the
audio system itself from producing noise; each component affecting itself as
well as all of the others in the system. The noise is either radiated through
space or conducted through wiring into the audio system as well as between the
components in the system.

We think of the mains power as a solid 120V,
60 HZ (or 230V, 50 HZ) but in reality it isn’t very clean. It harbors noise from
DC, to harmonics of the power line frequency, to spurious junk into the
megahertz range. All of this noise can get into the audio circuitry and produce
readily identifiable noise in the form of hums and buzzes, or aberrations in the
sound, which while not something you can put your finger on, messes up the
sound.

The noise is running on a two-way street with power line noise
projected into the power supply and power supply noise projected into the power
line. Additionally, noise from both sources is projected into the chassis, and
thus the system of grounds. This is discussed in detail by Eric Juaneda here and by Bill Whitlock in Jensen AN-004.

Power line noise comes in two flavors,
differential noise and common mode noise. With differential noise, the noise
voltage is impressed between the two power lines – hot and neutral. With common
mode noise, there is no noise voltage difference between the two power lines;
rather, the noise voltage is impressed between safety ground and the two power
lines.

So, what can we do to ameliorate the noise from the power
line?

Clean up the power line. Any electrical device in your home that
contains a switching power supply is dumping tons of electrical garbage into
the power and safety ground wires. Of particular interest are TVs and DVD
players that are interconnected with your audio system or plugged into the
same AC power branch circuit. In an AudioAsylum post here, Charles Hansen identifies the problem and recommends
disconnecting and un-plugging all such components. Merely turning them off
doesn’t cut it – even though you think that they are off, their power supplies
are still on and are injecting noise into the power system. The problem is not
limited to TVs and DVD players; rather, devices such as computers and
microwaves anywhere in the home, and particularly in the same branch circuit
as the audio system, are also culprits.

Clean up the safety ground. Safety ground is really the flip side
of the power line because the power is referenced to the ground. By cleaning
up one, you can’t help but clean up the other. Even though safety ground is
connected to earth, it is a long wire that is routed around the house, picking
up all sorts of electrical noise because it is an RFI antenna. Some folks have suggested adding a second earth ground directly
from the audio system, while others advise that an additional ground may actually cause
more problems than it solves. A single, dedicated safety ground along with a
dedicated power branch may be a good idea; however this is expensive because
it must be installed by a licensed electrician. Much the same result can be
achieved by dedicating one of your existing branches to your audio system.
This branch should ideally have no other equipment connected to it, or at
most, only what I call “benevolent loads” – for example, ordinary incandescent
light bulbs without a dimmer control.A thorough explanation of a “Technical
Ground” is contained in Jim Brown’s grounding article here.

Class 2 devices side-step the issue of a noisy
safety ground because they, of course, do not have a safety ground. However,
it is important to note that this solves the problem only as long as all
powered devices in the audio system are Class 2 devices. That is, there can be
no connection to earth anywhere in the audio system, or in any attached
component, for example, a computer that is not galvanically isolated. The
problem of interconnecting Class 2 devices (without a safety ground) with
Class 1 devices (with a safety ground) is shown in figure 5 of the Juaneda article and discussed in detail in the Jensen AN-004.

Eliminate DC on the power line. We normally think of noise as some
sort of alternating current phenomena; however direct current on the primary
of a power transformer is also noise. A thorough discussion of the problem and
a solution is presented in Rod Elliott’s article.

Choose the proper power line polarity on each component. Noise from
the power line in the form of AC leakage current will be transferred to the
core and frame of the power transformer through the parasitic capacitors as
shown in the Juaneda article. Because of the way a power transformer is
constructed, the parasitic capacitors are different values; and because the
neutral power line is connected to the safety ground, the amount of noise on
the two sides of the power line is different. These two factors work together
to cause the amount of leakage current to be different depending on the
polarity of the connection of the power to the transformer. The result is a
voltage, with respect to earth, impressed on the chassis. It is most unlikely
that this impressed voltage will be the same on two different chassis in the
audio system and when those chassis are connected together, noise voltage will
flow in the interconnection. This is discussed in Charles Hansen’s AudioAsylum
post here and a way to test the polarity is shown here. Remember, the final polarity change should be done
where the power line connects to the power transformer so as to keep the fuse
and power switch on the hot side of the line. Note that this problem exists in
class 2 devices (without a safety ground) as well as class 1 devices (with a
safety ground.) This is discussed in detail in Jensen AN-004.

Include a line filter on equipment that you build. The best kind to
use is one that has the filter integrated into the power entry module, like
that shown here. Unfortunately, a power line filter is a two-edged
sword; although it does filter the noise, it dumps common mode noise into the
safety ground (which we want to clean up.)

Use a power conditioner. By its very nature a power conditioner
isolates its attached components from the power line. The conditioner can be
used two ways: First, for isolating sensitive audio components from the noisy
power line, and second, for isolating noisy devices that contain switching
power supplies from the power line. Of course, the audio components and noisy
devices should not both be connected to the same power conditioner. Although
the power conditioner does a good job of cleaning up the power line, the
safety ground is passed directly through the power conditioner. Therefore, the
suggestions for cleaning up the safety ground discussed above may still prove
beneficial.

Use a shielded power transformer. A shielded transformer has a
shield that eliminates the parasitic capacitors between the primary and
secondary windings, thus eliminating the AC leakage currents between those
windings. It isn’t often that a shielded power transformer is needed but it is
good to know that it is available for that rare case.

As well as
minimizing the noise coming from the power line, it is important to minimize the
noise that a component injects back into the power line. We will see why this is
so later when we talk about interconnecting components.

Power Supply
noise can be minimized by careful design of the power supply, paying attention
to the choice of rectifiers, snubbers and filter networks. What can we do to
minimize the noise injected back into the power line?

Don’t include switching power supplies in any equipment that you
build.

Provide a more constant load. Because audio components are not
resistive loads, the manner in which they draw current from the power line is
not constant. This irregular load presented to the power line contributes to
the noise on the power line. Some things you can do here are regulated
supplies, class-A circuits, choke-input power supplies and Minimal
Reactance Power Supplies.

Use low-noise rectifiers. Different kinds of rectifiers used in a
power supply convey different characteristics to the sound of an audio
circuit. Some sound better for one circuit and worse in another circuit.
Whichever one you choose to use for a particular circuit, low noise is an
important criterion. Tube rectifiers, Schottky diodes and Fast Recovery
Epitaxial Diodes (FRED) are good choices.

Use a snubber on the rectifiers if needed.

Include a line filter on equipment that you build. Since the noise
is on a two-way street, the filter will help both ways.

Shielded power transformer. Again, rarely needed but good to have
available.

We have talked about electrical noise and what can be
done to minimize the noise. All the electrical noise in the world doesn’t matter
a bit unless it affects the signal. On the other hand, just a tiny bit of
electrical noise can wreak havoc if it shows up in the wrong place. In order for
the noise to affect the signal it must be routed in such a way as to interact
with the signal. Careless mixing and interaction of grounds is the biggest cause
of audible degradation of the music in an audio system, so let’s take a look at
how this works.

To understand what can go wrong we need to understand
something called Common Impedance Coupling. Simply stated, Common Impedance
Coupling is the way that noise gets mixed in with a signal. Noise can get mixed
with the signal in a couple of ways: first, “conductive coupling,” when two
circuits share the same path, and second, “radiated coupling,” when noise from
one circuit is radiated into another circuit carrying the signal. [12]

Figure
2-1, Two forms of Common Impedance Coupling.

The circuit on the left
of figure 2-1 shows two current loops, one loop with the signal and a second
noisy loop. Impedance Z1 is a sensitive spot in the circuit, perhaps a signal
reference buss, which is common to both loops. The signal current, I1, and the
noise current, I2, both pass through Z1 creating a voltage that is the product
of the impedance and the sum of the two currents. The circuit on the right of
figure 2-1 also shows two current loops, one with the signal and the other with
noise. In this case the sensitive impedance, Z2, is not common to both circuits.
However the two loops are situated such that there is capacitive or inductive
coupling between them and the noise voltage is coupled into the signal loop. The
resultant current is a composite of the signal current and the noise current
which creates a voltage across Z2 that contains a mix of the signal and noise.
There are two loops in each of these examples, but in reality the Common
Impedance Coupling could be the result of several loops or a combination of
conductive coupling and radiated coupling.

An example of a problem
caused by radiated coupling is with the orientation of transformers in a vacuum
tube amplifier. Depending on the orientation of the transformers with respect to
each other, the output transformer may pick up the radiated field from the power
transformer, causing an audible hum. Simply rotating one of the transformers may
be sufficient to remedy the problem. Otherwise, increasing the distance between
them will eliminate the problem.

Careless or poorly chosen routing of
wiring can also cause problems with radiated coupling. It is important to keep
filament wiring tightly twisted and laid close to the chassis. Low-level signal
wiring should be kept as far as possible from power and filament wiring. It is a
good idea to use a twisted pair for signal wiring, the signal wire twisted with
its signal reference. When signal wiring must cross power or filament wiring,
they should do so at right angles.

Chapter 3 - Interconnecting Components

There are two aspects to an
audio connection: the signal and the ground. While both are equally important,
designers have focused on the signal and left the grounding to chance. Thus
poorly chosen grounding is the largest cause of audible hum and buzzes in an
audio system. We will cover both aspects of an audio connection.

Audio
components have either balanced or unbalanced interfaces. Those interfaces are
similar in that they both have two wires – a signal and a reference. The
difference is: on a balanced interface the impedance to ground is the same for
the signal and reference; while on an unbalanced interface the impedance to
ground for the signal is different than the impedance to ground for the
reference. Usually the reference of an unbalanced interface is connected to
ground and the interface is called “single-ended.” This term is unfortunate
because it can obscure the fact that it is still a two-wire interface and lead
designers to indiscriminately choose any convenient ground as a reference. Note
that the kind of connecter used for the interface has nothing to do with if it
is balanced or unbalanced; however most balanced interfaces use an XLR connector
and most single-ended interfaces use an RCA connector. Why would you want to use
one over the other? The ubiquitous single-ended interface is cheaper to build;
while the balanced interface gives better noise performance. That being said, if
you are using commercial components, it really comes down to what those
components hand you.

Let’s start by looking at the ground aspect of
interconnections.

3.1 - Balanced Interconnections: Grounding

The good news is,
compared to a single-ended connection, it is relatively easy to get a balanced
interconnection right; the bad news is, it is also easy to get it wrong. In fact
many, if not most, of the balanced interfaces available on both commercial and
professional audio equipment got it wrong. [12]

To see what could go
wrong let’s take a side trip into power supplies. It goes without saying that
the largest contributor of noise within an audio component is its power supply.
I won’t go into power supply design here but I will cover one important area –
ripple noise on an internal power supply ground buss. Figure 3.1-1 shows a
schematic diagram of a simple power supply with an internal ground buss. Ripple
current flows through this buss from the filter capacitors to the center tap of
the power transformer. There is more ripple current in the capacitor closest to
the transformer, with progressively less as the filter progresses. Even though
the buss may be a short wire or printed circuit trace, it has finite resistance
so a noise voltage is developed by the ripple current flowing through the buss.
If signal reference current is routed through the power supply buss, this noise
voltage will be impressed onto the signal by Common Impedance Coupling.

Figure
3.1-1, A typical power supply.

Now let’s see what happens when we
combine this power supply with a poorly chosen grounding system, which is
employed in many components. Figure 3.1-2 shows an example of a piece of
equipment that has a severe ground loop problem. Knowing that the center tap of
the power transformer is the noisiest point in the device, the circuit designer
connects this point to the chassis at the same point that the safety ground
connects to the chassis. He thinks that somehow this will drain all of the noise
to earth. Unfortunately, the earth is not a huge electron vacuum cleaner – in
order for current to flow anywhere, there must be a return path and a voltage
difference between the input and return of the loop. Of course the power supply
must be connected to the audio circuit so the designer dutifully connects the
quietest point on the power supply ground buss to the audio circuit ground. Come
time to connect the input XLR connector to the circuit, the signal on pin 2 and
the reference on pin 3 is connected to the + and – inputs of the differential
amplifier. What to do with pin 1 on the connector? Well, pin 1 is the shield and
the shield should be grounded, and the shield is on the same cable with the
signal, so why not connect pin 1 to ground of the circuit where pins 2 and 3 are
connected? Okay, done. The designer should have been thinking outside of the box
(his own box) when he designed this grounding arraignment. If he had, he would
have seen that he had created a ground loop with the attached component. The
attached component connects the safety ground to the cable shield, either
directly by the chassis (as it should be), or worse, indirectly through a lame
grounding arrangement like the one in the first device. The loop contains the
power supply ground buss and the signal reference. The noise current on the
ground buss together with the finite resistance of the buss, provides a voltage
across the buss which will drive noise current through the loop. The noise
current through the loop together with the finite resistance of the signal
reference will develop a noise voltage in the signal reference. Now that the
signal reference is dirty there is no hope in achieving a clean signal.

Figure
3.1-2, The Pin 1 Problem.

Neil Muncy identified this problem and
dubbed it the “Pin 1 Problem” in his AESJ article [12] because pin 1 of the XLR
connector was connected to the wrong place. The shield and pin 1 should have
been connected to the chassis as shown in figure 3.1-3 rather than the signal
reference.

Figure
3.1-3, The Pin 1 Problem Fix.

By connecting the shield to the
chassis rather than the signal reference, there is no longer a loop for noise
current to flow in - thus the signal reference is clean. The internal ground
structure is still not right and I’ll address that when I cover unbalanced
interconnections.

It is important for the shield to be connected to the
chassis at both ends for several reasons: improved shielding, improved headroom
and maximum CMRR as is discussed here. Galvanic isolation provides the possibility of
eliminating a Pin 1 Problem, it does not guarantee it. Notice that the cable
shield provides a second path (with the safety ground) between the grounds in
the two chassis. If either component has poorly chosen grounding, a Pin 1
Problem will surface. There is controversy in the pro-audio community whether
the shield of a balanced connection should be connected at both ends or only at
the driver end. There are pros and cons to each with the biggest factor being
the complexity of interconnecting a large professional system. This, together
with the large number of devices suffering from a Pin 1 Problem has led many
installers to favor the driver-end-only solution. The technical aspects of the
question are discussed in a side bar in Bill Whitlock’s article on audio interfaces. A home stereo system is a lot
simpler than a pro instillation and the cables are a lot shorter thereby greatly
reducing the common mode noise problem. Assuming that you can eliminate any Pin
1 Problems, the advantages of connecting the shield at both ends win out in a
home stereo system. However, if you cannot eliminate a Pin 1 Problem in an
attached component, the driver-end-only is an easy solution. Note that
off-the-shelf cables have the shield connected to pin 1 at both ends, so you
will need to modify the cable to implement the driver-end-only connection. I
prefer modifying the cable rather than the wiring of a component because at some
later date you may want to attach a different component and don’t want to have
to remember to re-wire the interface. It is a lot easier to keep a pair of
driver-end-only cables in your stash of cables – but remember to label them as
such.

Before I go into single-ended interconnections I want to clarify
one thing about safety ground in the figures. The figures show safety ground
directly connecting the two chassis. Of course this is not accurate; rather both
chassis are connected to safety ground at the power outlet. I am assuming that
both devices are plugged into the same power outlet, thus their safety ground
wires are directly connected in the outlet. I have dropped the power outlet from
the pictures to simplify them. The absolute value of the voltage with respect to
earth of the safety ground at the power outlet is not germane to the discussion
because it is common mode to both devices; remember – earth is not a vacuum
cleaner.

3.2 - Single-ended Interconnections: Grounding

As shown in figure
3.2-1, a single-ended connection can suffer from a noisy ground loop. Even
though there is no XLR connector to have a pin 1 in a single-ended connection,
the problem is still called the Pin 1 Problem because it is the same structure
as that in a balanced connection.

Figure
3.2-1, The Pin 1 Problem in a single-ended connection.

The problem
is worse in the single-ended case than the balanced case because the shield is
also the signal reference. The noise current through the shield develops a
voltage across the finite resistance of the shield and the resultant noise
voltage on the reference is impressed on the signal in the amplifier.
Consequently, the problem is harder to fix in the single-ended case than for the
balanced case. However since connecting the shield to the chassis solved the
problem for the balanced connection; let’s take a look at that.

Figure
3.2-2, Connecting the shield to the chassis.

Connecting the shield
to the chassis may help a little because it shorts out the loop that includes
the cable shield. However it does not solve the problem because it causes
another noisy loop within the device, that is: chassis -> signal reference,
-> noisy power supply buss, -> chassis. Well how about if we were to
connect the shield just to the chassis and not to the signal reference? Nope,
there is still a problem; now the signal reference of the amplifier is connected
to the signal reference of the cable through the noisy power supply. Hmmm… okay,
is there something that we can do to reduce the noise current in the loop? Sure,
we can add some resistance to the loop like shown in figure 3.2-3.

Figure
3.2-3, Safety Loop Breaker Circuit.

The Safety Loop Breaker
Circuit, as explained in Rod Elliott’s article, is a
clever circuit that inserts a high impedance in the loop for low voltages, and a
low impedance in the loop for high (fault) voltages. It is effective; however it
attempts to ameliorate the symptom rather than eliminate the problem. The
problem is twofold – the loop and the noise generator; so let’s address them.
Figure 3.2-4 shows a better power supply solution.

Figure
3.2-4, A clean power supply.

In figure 3.2-4, the internal ground
buss is collapsed into a point, forming a local star ground. Bringing everything
to a point forces us to make a connection to that point – no more multiple-point
connection over which a noise voltage could form. Before, we had two
connections: a high noise one connected to safety ground and a low noise one
connected to signal reference. Note that now the high noise point is directly
connected to the power common where it can be connected to safety ground to
drain the AC leakage current, and the low noise point is directly connected to
the power common where it can be connected to signal reference.

A
power supply is a two-terminal output device – a voltage and power
common. Do not make any external connections to internal points in the
power supply. Of course, if the power supply produces both a positive and
negative voltage output with a shared power common then it is a three-terminal
output device.

Figure
3.2-5, A clean power supply attached to the loop.

We have solved
one source of Common Impedance Coupling but there is still another lurking. The
next noise generator is the AC leakage current from the power transformer
thorough the power supply to power common. In this case the Safety Loop Breaker
Circuit may be detrimental. The Safety Loop Breaker Circuit will inhibit this
current from reaching safety ground, and the current will take the path of least
resistance through the signal reference and shield to find safety ground through
the attaching device. Another problem is that the signal reference is not
directly attached to the chassis so the chassis is not as an effective shield as
it could be. I’ll come back to the appropriate use of a Safety Loop Breaker
Circuit later but for now let’s get rid of it and attach the signal reference
directly to the chassis.

Up until now we have looked at the Pin 1
Problem only on an input connection. Let’s now look at the problem from an
output connection perspective.

Figure
3.2-6, Pin 1 Problem on an output.

A Pin 1 Problem on an output
really isn’t any different than a Pin 1 Problem on an input. As we are getting
close to a final solution on the input side, let’s apply what we have found to
the output side as well.

Figure
3.2-7, A clean power supply in both components.

Okay, the loop is
shrinking and there is no longer any reason for the AC leakage current to choose
the shield rather than the Safety ground. We have ameliorated the conductive
coupling problem but we still have a radiated coupling problem so let’s take a
look at that next. We still have the shield current going to the chassis through
the signal reference. Figure 3.2-8 shows all of the different grounds connected
together in a star of stars configuration.

Figure
3.2-8, Star of stars.

It is now clear that the shield and safety
ground no longer form a problematic ground loop and are now merely parallel
paths.

Loops aren’t bad – it depends on what is on the
loop. Unless there is a voltage generator to drive a current around the
loop, or radiated current into the loop, it is merely a parallel path. Consider
the parallel shields of a left and right channel stereo cable. However, parallel
paths do form a loop antenna and can pick up RFI by radiated coupling. Therefore
minimize the use of parallel paths to only where necessary and then minimize the
area of the loop.

Speaking of RFI radiated coupling, it is possible for
audio cabling and input circuitry to pick up RF noise. In extreme cases,
shielding won’t resolve the problem and more aggressive techniques like RF
filters on the audio inputs must be employed. A Ham’s Guide to
RFI, Ferrites, Baluns, and Audio Interfacing by Jim Brown is a particularly
good coverage of filtering RFI.

There is still one thing left to address:
Since it is certain that AC leakage current will flow through safety ground and
the shield from both components, the two chassis and thus the two signal
references will be at different AC potentials. This result s in signal noise:
much less than we had with the Pin 1 Problem, but still some noise. We can
reduce the voltage difference between the two chassis by reducing the impedance
between them. First we use a larger safety ground wire in the power cord – the
larger, the better. And of course you could even go to a hefty silver-wire power
cord.
Second, use a shielded twisted pair for the interconnect cable, with one of the
wires in the pair (as well as the shield) being the signal reference. If all of
this is not enough, you could consider a Parallel Earth Conductor (PEC). A PEC
is simply a heavy wire connecting the two chassis. Jim Brown calls this "local
bonding" in this article.

Figure 3.2-8 shows the star grounds on
the chassis where the safety ground comes in. The star does not have to be there
and it may be more convenient to move the star onto a PC board. A couple of
examples of this are shown in figure 3.2-9.

Figure
3.2-9, Some options for star grounding.

Let’s move on to the
signal aspect of Interconnections.

3.3 – Balanced Interconnections: Signal

A signal interface is
comprises of a signal and an associated reference. By definition, on a balanced
interface the impedance to ground is the same for the signal and reference. Most
balanced interfaces these days are implemented with electronic circuits, usually
op-amps or similar integrated circuits. Classical balanced interfaces were
implemented with audio transformers, and a few audio components still use
transformers on their interfaces. Notice that the two lines on the interface are
called “signal” and “reference.” These are different from the “signal reference”
ground within the component.

Figure
3.3-1, Balance interconnection with active circuits.

A cursory
search will provide many available active chips for a balanced interface. Bill
Whitlock goes into some of the circuits in his article here.

There is one other issue to be aware of with a
balanced interface when both ends have electronic circuits rather than a
transformer on one or both ends. Even though a balanced input takes the signal
between the two inputs without reference to a ground, it is important that the
two inputs have a reasonably close potential to ground, otherwise the CMRR will
suffer. Consider for a moment a tube differential amplifier with both its inputs
to the grids at plus or minus 50 volts. In this extreme case the tubes would be
either saturated or cut off. Thus, it is important to have an established ground
reference between the sending and receiving components.

Audio Transformer

A transformer? Yes, a transformer. Transformers
have received a bad rap for use in high-quality consumer audio equipment. They
are said to be large and heavy, exhibit poor frequency response and distortion,
and are expensive. As to the size and weight, we are talking line input and
output transformers here, not tube amplifier output transformers. The Lundahl LL1690 line
input transformer is PC board mounted and is lighter and smaller than many
high-quality film and foil capacitors that I use for coupling. As for frequency
response and distortion, take a look at the Jensen JT-10KB-D which is down -3dB at 0.5Hz (less than 1Hz folks!)
and 180kHz, with less than 0.001% THD at 1kHz. These are just a couple of
examples; both Lundahl and Jensen have several line transformers available to
meet your specific needs. High-quality transformers are expensive; there is no
getting around that. You get what you pay for and I suspect that most of the bad
reputation has been generated by the use of cheap transformers. Bill Whitlock
wrote the audio transformer chapter of the Handbook for Sound Engineers
that will give you an opportunity to understand audio transformers.

Figure
3.3-2, Balanced interconnection with transformers.

Okay, so what do
you get with a transformer? Galvanic isolation and an excellent common-mode
rejection ratio (CMRR) are the main things, but you can also get free gain or
attenuation and along with that an opportunity for a lower output impedance. The
primary advantage of a balanced transformer input versus a balanced active
circuit input is a vastly improved CMRR. This is explained in section 3.2 of
Bill Whitlock’s Understanding, Finding, & Eliminating Ground Loops in Audio
& Video Systems.

A good introduction to the advantages of
balanced circuits and transformer interfaces is presented in So
You Thought Your Amplifier Was Balanced? by Andy Grove and Peter
Qvortrup.

Figure
3.3-3

Figure 3.3-3, It doesn’t make any difference if one end of
the connection has a transformer while the other end has an electronic circuit.
The interconnection is the same for all varieties of balanced
interfaces.

3.4 Single-ended Interconnections: Signal

The ubiquitous
single-ended circuit is available on most audio components.

Figure
3.4-1 A single-ended interconnection using electronic circuits.

Most, if not all, single-ended output circuits will exhibit a DC
voltage offset, meaning that the quiescent interface will be at a voltage level
different from the reference, or shield. This voltage offset will produce a loud
audible “thump” when the component is powered on. At best, the thump is
startling and at worst it may damage a speaker. Also, some (not all) volume
controls can be damaged by DC current over a long period. Therefore the designer
of the output circuit will include a capacitor to block DC offset on the output.
Because there are no standards addressing the interface, not trusting that the
designer of the output circuit included a capacitor, the designer of the input
circuit will also include a capacitor there. Being in series, the effective
value of the combination of the two capacitors will be less than either alone.
Therefore, unless the designers greatly oversized the capacitors, bass response
may suffer. Also, the capacitors will interact with other impedances in the
circuit, creating a low-pass filter; thereby affecting the high-frequency
response. And of course, unless the capacitors are of the best quality, they
will degrade the quality of the audio signal. There is a way to eliminate the
capacitors.

Figure
3.4-2, A single-ended interface using transformers.

A transformer
is inherently a balanced device; however it can be used in a single-ended
circuit by just grounding one side of a winding.

Figure
3.4-3, Single-ended interconnection with transformer input.

Figure
3.4-3 shows two components interconnected with a single-ended interface. The
component on the right has a transformer input and notice that the shield, or
reference, is connected to only the transformer and specifically is not
connected to ground in that component. The only connection between the grounds
in the two components is through the safety ground. The signal and shield that
are connected to the primary winding of the transformer are connected to the
driver and ground in the left hand component. Thus, even though the transformer
is physically located in the right hand component, its primary winding and the
cable are part of the output circuit of the output circuit in the left hand
component. The signal connection between the components resides in the flux of
the transformer and not in any wires. This is what is meant by “galvanic
isolation.” Since there is only a single connection between the grounds in the
two components (by the safety ground,) galvanic isolation precludes the
possibility of any ground loops between the components. This completely
eliminates the Pin 1 Problem. The capacitor on the output of the component on
the left in the figure is still needed because, even though the transformer will
block any DC offset, a DC offset may saturate a small input transformer.

You may provide an RF connection between the cable shield and the
chassis on the right hand component by connecting a 10nF ceramic capacitor
between the chassis and the shell of the RCA jack. Keep the leads as short as
possible.

This same interconnection scheme could be used if both
components had transformers on their interfaces; however, in that case it would
make more sense to implement a balanced interface.

Figure
3.4-4, Single-ended interconnection with transformer output.

The
transformer output shown in figure 3.4-4 is the mirror image of the single-ended
transformer input shown in figure 3.4-3. However there is one important
difference between the two configurations: the output amplifier in figure 3.4-3
has a low output impedance, while the input amplifier in figure 3.4-4 has a high
input impedance. With a high input impedance, the attached cable and transformer
primary winding is an antenna which picks up RFI noise. Grounding the shield at
the source provides a low impedance and eliminates the problem. Therefore in
this case, both ends of the interface must be grounded. Depending on the bias
requirement of the input circuit, the input capacitor may not be needed.

3.5 - Mixed Interconnections

Sometimes you will have two components,
one with a balanced interface and the other with a single-ended interface; and
you wonder if there is a way you could interconnect them. Yes there is and we’ll
go into that now.

Single-ended to Balanced Interconnection

The single-ended to
balanced interconnection is pretty straight-forward, requiring that you only
make a special cable. I’ll skip the cases where both components have
transformers on their interfaces because in these cases it would be best to use
a balanced interconnection.

Figure
3.5-1, Single-ended to balanced interconnections.

In both of these
cases the component on the left has a single-ended interface and the component
on the right has a balanced interface. The only difference is that with a
transformer in the right-hand component, a blocking capacitor is needed in the
left-hand component. The capacitor is optional in the case of both components
having electronic circuits on their interfaces. Even though the component on the
right has a balanced input, the interconnection is unbalanced because it is
referenced to ground.

Figure
3.5-2 Single-ended to balanced interconnection cable.

You will need
to make a special cable from a shielded twisted pair with an XLR connector on
one end and an RCA connector on the other end. Pin 2 of the XLR connector is
connected to the center pin of the RCA connector, and both pins 1 and 3 of the
XLR connector is connected to the shell of the RCA connector. This is important;
if pins 1 and 3 are connected together at the XLR connector end instead of at
the RCA end, the noise rejection will be poorer.

Balanced to Single-ended Interconnection

The balanced to
single-ended interconnection is more complex than the single-ended to balanced
interconnection.

Figure
3.5-3, Balanced to single-ended interconnection with transformer
output.

With the interconnect scheme shown in figure 3.5-3 we have
a balanced output on the left, a single-ended input on the right with the two
interconnected. Even though the component on the left has a balanced interface,
the interconnection is unbalanced because it is referenced to ground.

In
this case, with a transformer output on the balanced interface, you can
use an interconnect cable similar to that shown in figure 3.5-2. This cable is
wired the same but the gender of the XLR connector is opposite of the cable
shown in figure 3.5-2.

Most equipment that provide balanced outputs do
so with differential amplifiers rather than transformers. The differential
amplifier is most often two single-ended amplifiers, one with its signal
inverted. When a single-ended output is desired, the negative output is not
used, thereby loosing one-half of the signal, resulting in a 6dB difference in
the output level between single-ended versus balanced operation. A transformer
output does not exhibit this 6dB difference in signal level because the whole
output signal is used for both balanced and single-ended operation.

Now
let’s consider the case where both components have active circuits on their
interfaces. This is where the complexity lies. A good explanation of the
interconnection for balanced and single-ended components is given in Jensen’s AN-003, and
I will present a summary here.

I have purposely shied away from showing
specific circuits in order to simplify the illustrations and also to suggest
that the examples are generic. However it is important to know the type of
circuit used for a balanced output driver if you want to connect it to a
single-ended input. Some circuits want to have the unused output grounded while
other circuits want the unused output left floating. Some circuits want the
unused output grounded at the driver end while others want the unused output
grounded at the far end. The wrong choice can degrade the sound or even damage
the circuit. Rather than sort out all of the possibilities, I am going to
side-step the issue and give you the best-quality solution. We know that a
transformer input will accept any balanced output circuit and we know that a
transformer can be used for a single-ended output, so let’s put those two
together and use a transformer for balanced to single-ended conversion.

Figure 3.5-4, Balanced
to single-ended interconnection.

An off-the-shelf version of this
solution is the Jensen ISO-MAX
PC-2XR. This unit is specifically designed to interface between professional
and consumer equipment. Professional equipment usually operates at a 12dB higher
level than does consumer equipment, so the PC-2XR provides 12dB of attenuation
to better match the devices.

We’ll come back to this, and similar
devices for some solutions to the Pin 1 Problem.

Composite Interfaces

We can see that there is not a whole lot of
difference between the configurations of a single-ended transformer input versus
a balanced transformer input; likewise for a single-ended transformer output
versus a balanced transformer output. Therefore, for the cost of an additional
connector and a switch you can have both.

Figure
3.5-5, Balanced connection with single-ended connection.

The input
transformer is connected to both an RCA jack and an XLR connector in parallel
and the signal is provided through one or the other connector. If the input is
single-ended, the ground for the shield is provided by the attaching component.
The output transformer is also connected in parallel to both an XLR connector
and RCA jack, providing the signal through one or the other connector. If a
single-ended output is desired, a cable is connected to the RCA jack and a
ground provided for its shield by closing the RCA/XLR switch. The switch is left
open if the XLR connector is used to provide a balanced connection.

3.6 - Pin 1 Problem Remedy

Even though you employ all of the proper
grounding practices in audio components that you build, you can still have a hum
if you attach to a piece of commercial equipment that has a Pin 1 Problem. You
have some choices in handling the problem.

First, you could modify the
offending device so that it has a proper grounding scheme. Sometimes this is not
practical because of the way the device is constructed so the next best thing
would be to add a Safety Loop Breaker Circuit to the problem device.

But
what if you have a piece of vintage classic equipment that you don’t want to
modify in any way so as not to impact its resale value? The solution is simple
if the equipment has a balanced interface – merely open the shield at the
receive end of the interconnect cable. As shown on page 27 of this article, it is important to open the shield at the receiving
end rather than the sending end to maximize CMRR. You might want to experiment
with placing a 10nF capacitor from the shield to the chassis. Keep the leads as
short as possible.

If the offending equipment has a single-ended
interface you can turn to a transformer to solve the problem. As discussed here,
Jensen has a complete line of isolators similar to the ISO-MAX PC-2XR that can
interface any type of connection to any type of connection while breaking the
problematic loop.

It is always better to solve the problem in the
offending device (by modifying it) or on its interface (with transformer
isolation) rather than to compensate for the problem in the equipment that the
offending device is attached to. We’ll see why this is when we cover
interconnecting equipment that use a Safety Loop Breaker circuit for isolation
and also in the section on ground isolation.

3.7 - Effective Interconnection Schemes

In this section I will
present four interconnection schemes that should cover just about every
situation. I’ll be using some terms rather loosely so I’ll define my usage
here:

Class 1 device – This is an analog component that has its chassis
connected to safety ground and does not have a Safety Loop Breaker Circuit or
a Pin 1 Problem. It does have single-ended non-galvanically isolated
interfaces.

Class 2 device – This is a commercial analog component that is
designed to be safe with no connection to safety ground. It has single-ended
non-galvanically isolated interfaces. An example of a Class 2 device is any
component that has a two-prong power plug, or is battery powered, like a
laptop computer.

SLB device – This is an analog component that has its chassis
connected to safety ground and has its system star ground isolated from the
safety ground by a Safety Loop Breaker Circuit.

Pin 1 Problem device – This is an analog component that has its
chassis connected to safety ground and does have a Pin 1 Problem. That is, it
has a poorly chosen grounding system that injects noise into the ground
system. Most, if not all, tower computer (not laptop) sound cards have a Pin 1
Problem.

Computer – This is a catch-all for any kind of digital audio device
that has digital power supplies dumping a lot of noise into its ground system.
An example is an SPDIF interface from a computer or a satellite TV system.

The first effective grounding scheme is a system with all Class 1
components shown in figure 3.7-1. This is the grounding scheme that we developed
in section 3.2. This interconnection scheme has a single-level ground system
with all of the signal shields and safety grounds connected to the system star
grounds.

This scheme is the only one of the four that allows noise from
the power line in the form of AC leakage current to flow in the signal ground
system. That noise will be transferred from the primary of the power transformer
through the parasitic capacitors to both the core and frame of the power
transformer as well as power common as shown in the Juaneda article and also Jensen AN004.
However, since the whole ground system has a low impedance, the level of the
noise will be low and should not be a problem. If it is a problem, the impedance
can be reduced using Parallel Earth Conductors (PEC) or “local bonding” as
described by Jim Brown here.

Figure
3.7-1, Simple system of Class 1 components.

The second effective
grounding scheme is a system with all Class 2 components as shown in figure
3.7-2. This interconnection scheme has a single-level ground system with all of
the signal shields connected to the system star grounds.

Figure
3.7-2, Symple system of Class 2 components.

This scheme does not
have a problem of noise from the power line in the form of AC leakage current
because the ground system is not connected to safety ground and thus there is no
return path for the leakage current. Without a path, there can be no current.
And of course, because there is only one connection between the components – the
shields, there can be no ground loops.

The third effective grounding
scheme is a system with all the components having Safety Loop Breaker
Circuits as shown in figure 3.7-3. This interconnection scheme has a
two-level ground system with all of the signal shields connected to the system
star grounds forming the first level. All of the safety grounds are connected to
the chassis forming the second level. The two ground levels are isolated from
each other by Safety Loop Breaker Circuits.

Figure
3.7-3, Simple system of SLB components.

This scheme does not have a
problem of noise from the power line in the form of AC leakage current because
the first-level ground system is isolated from safety ground and thus there is
no return path for the leakage current. Without a path, there can be no current.
You can think of the first-level ground system as the same as the ground system
in the Class 2 ground scheme. However, AC leakage current that has been injected
onto the chassis from the transformer does flow in the safety ground circuit.

Of course, no grounding scheme is very useful if it is restricted to
having only one type of component in the system. So let’s see how we can mix
different types of components in a ground system. We’ll start simply with just
two interconnected components and then expand the grounding to cover several
components.

Figure
3.7-4, Interconnection of a Class 2 device with an SLB device.

As
mentioned above, the first-level ground of an SLB device is the same as the
ground in a Class 2 device so they interconnect without any problem.

Figure
3.7-5, interconnection of a Class 1 device with either a Class 2 device or an
SLB device.

There is no ground loop in either of these cases;
however AC leakage current from the power transformers in the Class 2 and SLB
devices is directed to earth through the shield and safety ground in the Class 1
device. If this is a problem, the only recourse is to isolate the safety ground
in the Class 1 device from the class 2 or SLB device. This may be achieved by
either adding an SLB circuit to the Class 1 device (thereby making it an SLB
device) or galvanically isolating the interconnection with a transformer.

Figure
3.7-6, Interconnection of a Pin 1 Problem device with either a Class 2 device or
an SLB device.

There is no ground loop in either of these cases so
the Pin 1 Problem is remedied; however, just as with the Class 1 case above, AC
leakage current will flow through the shield. This may present more of a problem
with a Pin 1 Problem device than with a Class 1 device because the AC leakage
current will flow through the signal reference before being directed to the
safety ground. The solution is the same as that of the Class 1 device above.

Figure
3.7-7, Interconnection of a Pin 1 Problem device with either a Class 1 device or
another Pin 1 Problem device.

This is the classic case of a Pin 1
Problem that was discussed in detail in chapter 3.2. The solution is to either:
modify the Pin 1 Problem devices so that they no longer exhibit a Pin 1 Problem,
or add an SLB circuit to the Pin 1 Problem device, or galvanically isolate the
interface with a transformer. This is illustrated in figure 3.7-8.

Figure
3.7-8, Solution for the Pin 1 Problem device interconnection.

Since
the Pin 1 Problem injects noise into the ground system, the most effective way
to include such a component is to galvanically isolate it from the rest of the
ground system. The same solution effectively allows a computer sound card to be
attached to an audio system. The galvanic isolation for these devices can be
incorporated into the components or provided as separate, external devices, such
as the Jensen ISO-MAX.

In the case of the computer, since a computer
sound card is not in the realm of the highest quality audio devices, there is no
need to spend the money on the highest quality line transformer to provide
galvanic isolation. EDCOR has a line of very good line input and output
transformers that will serve well in this application.

Attaching a
computer sound card to the SLB grounding scheme presents the same problems as in
attaching a computer sound card to the Class 1 grounding scheme. The best and
total solution is galvanic isolation. However, for a very simple system as shown
in figure 3.7-9, directly attaching the computer sound card to the SLB isolated
amplifier may provide an acceptable solution. The SLB will inhibit the ground
loop. You will still have the problem of AC leakage current from the amplifier
flowing through the computer but, since the computer sound card as a poorly
chosen ground structure, it may not matter.

Figure
3.7-9 A simple SLB system.

However, if the computer is a laptop
then there is no need for the galvanic isolation because the laptop does not
have a connection to the mains power and therefore no ground loop.

There
is one additional problem to take care of here – the dirty safety ground on the
computer. We do not want to mix this dirty safety ground with the relatively
clean safety ground of the audio system so provide a separate AC mains branch
circuit for the computer.

Okay, let’s summarize where we are with the
interconnections of two devices:

There is no problem interconnecting devices of the same type: Class 1,
Class 2, or SLB.

There is no problem interconnecting a Class 2 device with an SLB device.

There is a minor problem of AC leakage current when interconnecting a
Class 1 device with either a Class 2 device or SLB device.

There is a minor problem of AC leakage current when interconnecting a Pin
1 Problem device with either a Class 2 device or SLB device. The major “Pin 1
Problem” is remedied.

There is a major “Pin 1 Problem” when interconnecting a Pin 1 Problem
device with either a Class 1 device or another Pin 1 Problem device.

A Safety Loop Breaker circuit helps a lot and may provide an adequate
solution.

A transformer will provide isolation, solving the Pin 1 Problem and the AC
leakage problem.

Let’s move on to interconnecting three or more
devices. Again, as long as all of the devices are of the same type: Class 1,
Class 2 or SLB, three or more can be interconnected without a problem. A mix of
Class 2 and SLB devices may be interconnected without a problem. Problems arise
when Class 1 or Pin 1 Problem devices are introduced into a Class 2 or SLB
grounding system. When there is only a single Class 1 or Pin 1 Problem device in
the system, the situation is the same as for interconnecting two devices as
discussed above. It gets interesting when two or more Class 1 or Pin 1 Problem
devices are introduced into the Class 2 or SLB grounding system.

When we
directly attach a Class 1 device to a Class 2 ground system we cause the whole
ground system to be connected to safety ground, thereby loosing the inherit
ground isolation in a Class 2 system. When we directly attach a Class 1 device
to the SLB first-level ground system, we cause the first-level ground system to
be directly connected to the second-level ground system. This nullifies the
benefit of all of the Safety Loop Breaker Circuits in the system. We now have
what amounts to a Class 1 grounding system. We don’t have an immediate problem
because we don’t have a noise generator in any loop. We do have a problem
waiting to happen when a Pin 1 Problem device is added to the system. This is
shown in figure 3.7-10.

Figure
3.7-10, A ground loop bypassing the SLB circuit.

The ground loop
flows between the Pin 1 Problem device and the Class 1 device, bypassing the SLB
circuit. The noise voltage is impressed on both the shield from the Pin 1
Problem device as well as the shield to the Class 1 device. Things can get
pretty complex and this figure doesn’t even hint at the complexity. The SLB
device can actually be several interconnected SLB devices and the Class 1 device
can be anywhere in the system – an input or an output. The ground loop problem
can be hard to diagnose. For example, say you had a system comprised of all SLB
devices except for a single Pin 1 Problem device. There is no problem because
the SLB isolation is intact and the ground loop is broken. Now, you buy a new
component that happens to be a Class 1 device and install it in the system. Now
you have hum that you didn’t have before. You take the new component out of the
system and the hum goes away. You would have reason to suspect that something
was wrong with your new component, while the problem is really caused by that
Pin 1 Problem device that had been in your system all along. Bill Whitlock’s
article Understanding, Finding, & Eliminating Ground Loops in Audio
& Video Systems contains some great techniques that you will need to
isolate the problem device.

Thinking that if one Safety Loop Grounding
circuit is good, two would be better, some designers separately isolate the
component input and output, each with its own SLB circuit. This does isolate the
input from the output but unfortunately comes with the cost of increasing the
ground noise within the component. We’ll see how this happens in the section on
Ground Isolation.

Often the preamplifier is the central component in the
audio system, with a single output and many inputs. If both the preamplifier and
amplifier are SLB devices, you can ensure that there can be only one Class 1 or
Pin 1 Problem device in the system at any time by switching the input grounds
along with their associated signals.

Other than that, we are back to
galvanically isolating the problem component with a transformer.

I know
that you have been wondering about the forth scheme and thinking that there are
three kinds of people – those that can count and those that can’t count. Well
there really is a forth system and it is the ultimate solution for system
grounding. This solution, which is shown in figure 3.7-11, is centered on a
preamplifier that has all of its input and outputs galvanically isolated. Since
everything is galvanically isolated, it doesn’t matter what is connected to the
preamplifier. You can mix Class 1, Class 2, SLB, Pin 1 Problems, and computers –
none of the grounds are interconnected. The preamplifier may be a Class 1 or a
Class 2 or an SLB device, although it really doesn’t make any sense to make it
an SLB device because there is no reason to break the safety ground circuit.
Notice that the computer is plugged into a separate branch of the mains power to
reduce the earth noise in the system.

Figure 3.7-11, A
grounding scheme centered on an isolated preamplifier.

Did I say
that there were four schemes? Well there is really a fifth. That is a scheme
where the isolated preamplifier scheme is turned inside-out with all of the
attaching components having galvanically isolated interfaces. In this case, the
preamplifier does not need to have isolated interfaces. The attaching devices
may be Class 1, Class 2, or computers. They cannot have a Pin 1 Problem because
of their galvanic isolation. Again, it doesn’t make any sense to have a
galvanically isolated device further isolated with a Safety Loop Breaker
Circuit. I currently use this grounding scheme in my personal audio system. This
scheme is shown in figure 3.7-12.

Figure 3.7-12, A
grounding scheme involving isolated devices.

3.8 Cables

Cables are a science of themselves and to get an idea of
some of the complexity involved, you can read Pin 1 Revisited by Jim
Brown, SCIN, also
by Jim Brown and chapter 9 of Morrison [1]. However the good news is that in the
relatively benign environment of the home and with short cables used in home
audio installations, cables do not need to be a critical factor. I will touch on
a few important factors though. Cables both radiate energy and are affected by
EMI from other devices. EMI has two components, an electric field and a magnetic
field, and there are different aspects of a cable that will effectively
counteract both of these fields. Electric fields are blocked by the cable shield
and a good shield for that is the woven copper type - the tighter the weave, the
better. A twisted pair of wires is relatively immune from a magnetic field, so
you will want to use shielded twisted pair for all of your cables. This is even
good for the power cable, although you will want heaver wire for this
application. Another important factor is the DC resistance, or low-frequency
impedance of the ground wires – we want that as low as possible. We saw why this
is important back in chapter 3.2 when we interconnected the components.

3.9 - Other Interconnections

My mother used to say, gesturing at
something I had found, “Don’t touch that, you don’t know where it has been!”
Digital audio signals are kind of like that – you don’t know where they have
been. Often the source of the signal will have a switching power supply that is
dumping tons of noise into the ground system. Examples would be a computer or
video system. For best results, the digital system should be isolated from the
audio system as much as possible for both power and signal. Each should be
powered from separate power line branches. The isolation for the signal will
depend on what the interface is. As poor as a Toslink is for many reasons, it
does provide galvanic isolation. An SPDIF or AES interface should be isolated
with a pulse transformer. A USB interface is a little tougher. If you are
designing your own interface, you can use opto-isolators or GMR isolators, or
convert the interface to SPDIF and use a pulse transformer. I haven’t used any,
but there are plenty of external USB isolation devices available. Thomas
Kugelstadt’s article, “When good grounds turn bad!” is a good overview of the problem
of interfacing with digital devices and what you can do to alleviate the
problem. Although the article uses an RS-485 data link for an example, the
concepts are valid for any digital interconnection scheme.

Even the
analog signals have problems: according to Jim Brown in his RFI article
“Virtually all computer sound cards have Pin 1 Problems.”

Cable TV is
another problem area for causing hum in an audio system. Almost all cable
grounds are at a different level than the mains safety ground and need to be
isolated with an RF isolator. Satellite TV systems can also cause a problem and
need to be isolated, however these need a different type of isolator from the
CATV isolator. A search of the internet will turn up lots of options.

3.10 - Interconnection Summary

Provide a dedicated branch power line, or at least a benevolently loaded
one, for audio components.

Plug all audio components into the same power strip or power outlet.

Provide a separate branch power line for computers, TVs or any other
devices having switching power supplies.

Provide isolated interfaces for connections between:
a. The audio
system and other devices, like computers or TVs.
b. Pin 1 Problem devices
and any other audio component.
c. Class 1 component and a class 2
component. Isolation may not be needed for this case.
d. Class 1
component and an SLB isolated component. Isolation may not be needed
for this case.

Loops aren’t bad – it depends on what is on the loop.

Use shielded heavy gauge twisted pair interconnect cables.

Make a map of your system grounds.

Understanding, Finding, & Eliminating Ground Loops in Audio
& Video Systems by Bill Whitlock has a lot of great information on
solving interconnection problems in audio systems.

Chapter 4 – Ground Structure within a Component

So far, for
simplicity I have included only a single signal reference and power supply in
each device. However, most components are comprised of several circuit boards,
each with its own grounding scheme incorporating busses or stars on them. These
are then interconnected, together with one or more power supplies and the
chassis. This can get quite complex and it is a good idea to make a map of the
ground structure and power structure when designing a piece of audio equipment.
Individual grounds and power lines should be routed as carefully and purposely
as the audio signals.

In a nutshell, a problem can occur when two
grounds share the same conductor (perhaps by necessity) or something is
connected to the wrong ground. System grounding may be established using a star
structure, a star-of-stars, a buss arraignment or a combination of those. Many
problems are eliminated when every ground is connected to a single point – star
grounding. However, this is often impracticable: The best we can hope for is a
star-of-stars approach with individual stars connected.

It’s all about
controlling the paths – ground current (of any kind) should go only where it is
needed. Likewise, power current should go only where it is needed.

4.1 - Grounding Rules

Here are some rules to help you plan your
grounding structure. The first four rules are from what we learned about
interconnecting equipment.

Rule 1: Each of the following
must be connected to the system star ground by one and only one
route.

All signal references

All power commons

Shields of non-galvanically isolated single-ended inputs and outputs

Safety ground and chassis. The safety ground and chassis should be thought
of as a single entity.

The connection may be direct, or indirect through a star-of-stars
or buss. This is expanded upon below.

The safety ground and chassis may be connected to the system star
ground through a Safety Loop Breaker Circuit.

The “one and only one” part of this rule precludes ground loops. There is
no excuse for a ground loop within a single component.

Rule 2:
The shield of a balanced input or output (XLR pin 1) must be
connected to the chassis at or as close as is possible to the
connector.

Rule 3: The shield of a single-ended input or
output that is not galvanically isolated must be directly connected to
the system star ground.

The shield is the signal reference in the
cable.

Rule 4: Any circuit associated with an input or output
that is not galvanically isolated must have its signal reference
directly connected to the system star ground.

Rule 5:
The mains safety ground must be directly connected to the chassis.

From IEC 60950, “The wire is terminated with a closed loop
connector which is fixed to the earthing stud or screw with a star or lock
washer and a nut. Other parts of the product that need to be earthed are
connected by closed loop connectors to the same stud and locked with an
additional nut. It is important that the earth wire from the power supply cord
is located at the bottom of the stud and locked with its own nut. The earthing
stud must not be used for any purpose other than earthing. It cannot be used,
for example, for the mechanical fixing of parts other than the earth conductors.
Its mechanical structure must also be such that it cannot be loosened from
outside the device. For example, it cannot be a post fixed with a screw from
outside the product.”

Rule 6: Each signal reference must
be directly connected to its power reference.

That is, no circuit
may have its signal reference connected to its power common through another
circuit’s signal reference or power common. This rule allows for a star-of-stars
with the signal reference and power common directly connected together in a star
and that star connected to the system star (either directly or through a
buss).

Rule 7: Circuits may be grouped together with
their signal references forming a buss.

The order of the grouping is
not arbitrary. Just as the signal is routed along, stage to stage, the
associated signal reference can be routed with the signal between stages. Keep
the signal and its associated signal reference electrically close together; they
should be treated as a pair. This minimizes the risk of noise being injected
into the signal reference.

One end of the buss should be connected to the
system star ground, either directly or by a star of stars.

4.2 - Grounding Examples

Now let’s take a look at some
examples.

Figure
4.2-1, An example of proper ground routing with a ground
buss.

Figure 4.2-1 shows a Digital to Analog Converter (DAC) with
an SPDIF input and single-ended analog output. Each of the power supplies has a
single power common output and each of the circuits has a single signal
reference output; all connected to a ground buss. Thus we have a buss of stars
which in turn is connected to the system star ground. The order to which each
attachment is made to the buss is important, flowing along with the signal from
input to output. Notice that none of the Grounding Rules are violated. In an
implementation of this example, there may not be a separate physical buss that
can be identified. Rather, the buss is formed from ordinary printed circuit
traces and regular hookup wire between PC boards. It is the structure that
creates the buss.

Figure 4.2-1 also shows separate power supplies for the
DAC and the amplifier. The DAC is mostly digital electronics that tends to
reflect digital switching noise back into its power supply, particularly on the
power common. Therefore it is a good idea to keep this power common separate
from analog signal references or the power common of analog supplies.

Now let’s change one thing and see what happens. I am not an advocate
for an SPDIF interface without a pulse transformer, but there are plenty of them
out there so for this example I will remove the transformer and bring the SPDIF
interface directly into the DAC.

Figure
4.2-2, A poorly-chosen grounding scheme.

I’m sure that you see
that we now have a Pin 1 Problem. Rule 3 is broken by connecting the input
shield to the buss rather than to the system star ground. Rule 4 is broken by
connecting the DAC circuit signal reference to the buss rather than directly to
the system star ground. Okay, let’s fix the grounding so that those rules are
not violated.

Figure
4.2-3, A well-chosen grounding scheme.

In fixing those two things
we have transformed the grounding structure into a star-of-stars. This
illustrates an important concept – any device that has both single-ended inputs
and outputs (that are not galvanically isolated) cannot use a ground buss
structure. This is because the signal references for both the input circuit and
output circuit must be directly connected to the system star ground and adding a
ground buss between them would form a ground loop. Another example of such a
component is the ubiquitous single-ended preamplifier. Grounding problems with a
preamplifier may well be the source of the myth that a star-of-stars is the only
acceptable grounding scheme.

Let’s take a look at another example, this
time a power amplifier. Using a ground buss structure in a power amplifier has
been around for a long time and is popular with Japanese amplifier constructors.

Figure
4.2-4, a power amplifier using a ground buss.

The first thing to
notice is that the shell of the input RCA jack is not isolated from the chassis.
Rather, it is effectively the system star ground. The order of attachment to the
buss is important with the buss starting at the input jack and ending at the
negative speaker jack. The quietest circuits are connected closest to the input
jack, with the progressively noisier circuits towards the speaker jack. An
implementation of this example will have a readily identifiable buss, usually a
very thick bare solid copper wire. An example of this technique can be seen here.

All of this is not to argue that a ground buss is superior to a star
ground or vice versa, rather both are tools that can be effectively used in an
appropriate situation. Stars are easier to implement because you don’t have to
worry about the order of connections like you must on a buss. Further
information on implementing star grounds and ground busses is in Randall Aiken’s
article here.

4.3 - Power Supply Chassis

When should you put the power supply and
amplifier in separate chassis? First of course is if the unit is physically too
large for a single chassis. The second reason is for noise immunity - for
example, a phono preamplifier with a sensitive input stage. Or perhaps you have
a huge power transformer that is radiating a large magnetic field and you need
to physically separate it from the amplifier. There is no grounding reason to
have separate chassis.

The next question is – how should they be
separated? The answer is simple; and obvious if you think about it. Even though
the two devices are physically in two chassis, they should be thought of as two
parts of a single chassis interconnected with a cable. Design your grounding and
make a map of the grounding and power on a piece of paper. Then draw a line
around what you want in the power supply chassis (or vice verse, what you want
in the amplifier chassis). Anything that crosses the line is in the cable. This
is shown in figure 4.3-1. The cable shield extends the chassis shield between
the two chassis and should be connected to each chassis at or as close as
possible to the connector. The safety ground wire in the cable is connected to
each chassis at its system star ground, thereby extending the safety ground
function to the second chassis. Be careful of the routing of the safety ground
wire that connects the two chassis – it should take as direct a route as
possible and lie close to the chassis. Use a connector with a pin that makes
first and breaks last for the safety ground. It would be a good idea to use a
twisted pair for the wires containing power and power common to cut down the
susceptibility to radiated magnetic EMI.

Figure
4.3-1 A separate power supply chassis.

The power supply chassis may
contain more than one power supply. Each power supply should have individual
power and power common lines with each power common connected to its destination
in the amplifier chassis. The destination may be a star ground, a ground buss or
an individual signal reference. The alternative of connecting all of the power
common lines to the star ground in the power supply chassis and then running a
single shared power common wire to the amplifier suffers from Common Impedance
Coupling. Also, you may want the different power common wires to go to separate
places in the amplifier.

A power supply chassis can service more than
one amplifier chassis. In this case a cable with the complete set of power,
power common, safety ground and shield, should run between the power supply
chassis and each amplifier chassis. If a power supply in the power supply
chassis serves two (or more) amplifier chassis, its power common should be
connected to the destination in each amplifier chassis.

4.4 – Input Switching

There was a time when it was the vogue to
switch both wires, signal and ground, of single-ended inputs. I don’t know where
this came from – perhaps it was just a copy-cat of switching balanced inputs. Or
perhaps it was an attempt to keep a myriad of dirty grounds from corrupting the
signal reference. In any event, now that we have cleaned up the grounds, it may
not necessary to switch the grounds. However there still are a couple of cases
where switching both the ground and signal may help:

Switching the
grounds helps with cross-talk and bleed-through between the inputs. Cleaning up
the grounds should help this. If there is still a problem you can connect (with
a relay) a 50 Ohm resistor between the signal and signal reference of all inputs
except that which is selected.

AC leakage current flowing through the
interconnect shield may pose a problem, particularly in Class 2 and SLB devices.
Remember, when the interconnect shields are not switched, all of the grounds in
the system are connected together in a mesh.

4.5 – Input Jacks

The RCA jack of a single-ended connection is
usually isolated from the chassis and the shield is connected to the star
ground. It may be beneficial to connect a 10nF ceramic capacitor from the shell
of the RCA connector (shield) to the chassis. This provides a direct path for RF
noise from the shield to the chassis right where the shield enters the
chassis.

4.6 – Volume Controls

Volume controls are potentially a source of
hum and noise from two causes: first, by radiated coupling into the resistive
element, and second, by inappropriate choice of circuit connections. The
radiated coupling is easily solved by ensuring that the body of the control is
grounded to the chassis through its metal mounting bushing.

Volume
controls on non-galvanically isolated interfaces are unique in that they are in
the output circuit of one component and also in the input circuit of another
component. They work by common impedance coupling between the two circuits.
Therefore keep in mind that there are two separate signal loops, an input signal
loop and an output signal loop. It is important to keep the two loops separate
in the grounding system. This is best accomplished by connecting the reference
end of the volume control potentiometer to the system star ground.

Figure
4.6-1, Volume controls on a non-isolated input.

When the input is
galvanically isolated it is no longer necessary to connect the volume control to
the system star ground. It should be considered as part of the amplifier circuit
and referenced to the signal reference of that amplifier.

Figure
4.6-2, Volume controls on isolated inputs.

4.7 - Grounding Transformers

Different kinds of transformers have
different grounding requirements. Sometimes a transformer will have an internal
shield between the windings and this will give you a hint how the transformer
should be grounded. The shield is to inhibit capacitively coupled AC leakage
current between windings and between windings and the frame. Sometimes the
shield is connected to the frame and other times it has a separate wire. What is
on the winding being shielded will tell you where to ground the frame and the
shield. The idea is to get the AC leakage current noise back to where it came
from in as short a path as possible. So, for example, the AC leakage current
from the primary of a power transformer came from the mains power line and
should be returned to earth, so the transformer frame should be connected to the
safety ground.

The frame of the power transformer in a class 1 component
(with chassis connected to safety ground) must be connected to safety ground -
just make sure that there is a good contact between the transformer frame and
the chassis. It doesn’t make sense to ground the power transformers in a class 2
component because since there is no connection to safety ground, there is
nowhere to bleed the AC leakage current. Also, perhaps the transformer is not
intended to be grounded as part of the isolation design. Therefore, grounded or
ungrounded, the power transformer in a class 2 device should be left as it is.

Some audio output transformer circuits provide better performance with
the transformer frame grounded while others work better with the transformer
frame ungrounded so you will need to experiment to determine which is better for
a particular case. The leakage current in a power amplifier output transformer
came from the output stage power supply and thus, if the transformer is to be
grounded, the frame should be isolated from the chassis and a wire attached to
the frame should be returned to the power common of that output
stage.

Speaking of output transformers, the negative terminal of the
output should be connected to the power common of the output stage.

The
output windings of line output transformers and the input windings of line input
transformers don’t need to be grounded. Looking back on the chapter on
transformer coupled interfaces; you will remember that grounding one side of the
interface creates a single-ended interface and the grounding schemes are covered
there.

Some line interface transformers have a center tap which may be
grounded for a balanced interface. I would recommend grounding the center tap of
only the sending end – grounding both ends may create a ground loop. Even though
the primary winding is from the output driver, I would not recommend grounding
the center tap to the output drive power common because this would allow the
possibility of a Pin 1 Problem if the other end were also grounded. Rather, I
would recommend using the system star ground in this case.

4.8 – Ground Isolation

Some people believe that it is necessary to
isolate the system star ground from the chassis and safety ground in order to
have a hum-free audio system. However, if all of the components in the system
have their grounding implemented properly, there is absolutely no need for
ground isolation, Although isolating the grounds may eliminate a ground loop, it
does come with two penalties: First, since the signal reference is not directly
connected to the chassis, the chassis is not an effective shield for the
electronics. Second, since the power common is isolated from the safety ground
and connected to the signal reference, any AC leakage current from the power
supply may flow through the signal reference to get to the safety ground in
another component.

If you must isolate the grounds; never, ever, for any
reason, disconnect a safety ground or fail to provide a safety ground in any
equipment that you build. First, it is unsafe and second, there are equally
effective methods of isolating grounds that do not come with the safety hazard.
Figure 4.8-1 shows two such methods.

Figure
4.8-1, Ground isolation

First is to provide a “ground lift” switch
between the two grounds to be isolated. This does such a good job of isolating
the grounds that it precludes current from an electronic short circuit from
blowing a fuse. This is illustrated in figure 1-3.

A better solution is
to provide a Safety Loop Breaker Circuit (SLB). This circuit will allow the
current from a fault to flow to the chassis and also provide ground isolation
under normal, non-fault conditions. In his article, Rod Elliott states that a
Safety Loop Breaker Circuit may not be legal in some places so you should check
this out before you use this circuit.

The best solution for ground
isolation is to employ galvanic isolation on problem interconnections.

A
component with a properly designed grounding system will not have any internal
ground loops. Therefore, there is never any reason to isolate grounds within a
component.

Let’s take a closer look at how the Safety Loop Breaker
circuit works to see why I say it is a third choice, behind fixing the ground
loop problem or galvanically isolating the ground loop with a transformer. I’ll
start with dissecting the problem so as to better understand what is going on.

Ground Loop Suppression

Figure
4.8-2

Figure 4.8-2, The ideal, as shown in the left hand circuit,
is to have the signal transferred unchanged from the sending circuit to the
receiving circuit. However, if there is noise in the system, as shown in the
right hand circuit, the signal reference rides on the noise and the sum of the
signal plus noise is presented to the receiving circuit.

Figure
4.8-3, Showing how the noise is added to the signal by common impedance
coupling.

Let’s say, for example, that the interconnect shield
connecting the two signal references has an impedance of 1 Ohm and the level of
the noise is 1 Volt. The 1 Volt across 1 Ohm will cause a noise current of 1 Amp
to flow, and the 1 Volt of noise will be presented to the receiving circuit.
Note that I refer to the impedance in the loop rather than the resistance in the
loop. This is because the noise spectrum is spread across a wide frequency range
and the impedance at higher frequencies is often more significant than the
resistance. For simplicity, I assume for all of these examples that the noise
source has an internal impedance of zero Ohms. Of course it will have a (very
low) finite impedance of some internal ground wiring. Refer to Chapter 3.1 for
an example of a noise generator.

Figure
4.8-4, Adding impedance to the noise loop.

In this example I have
added 9 Ohms of impedance to the noise loop for a total of 10 Ohms of impedance.
The impedance may be added in the component with the noise or in the attaching
component. The noise voltage is still 1 Volt; however the 1 Volt across 10 Ohms
of impedance causes only 0.1 Amp of noise current in the loop. The 0.1 Amp of
current through the 1 Ohm impedance of the interconnect shield develops only 0.1
Volt of noise to be presented to the receiving circuit. We have thus reduced the
noise that the receiving circuit sees by a factor of ten. Note that none of the
noise has disappeared; the remaining 0.9 Volts of noise is dropped across the
added 9 Ohms of resistance. There is an important difference between the two
circuits in figure 4.8-4. In the left hand circuit, the added impedance is under
the noise source so that signal references are close to safety ground; therefore
the noise voltage on the ground system is low. One signal reference (being a
class 1 device) is at ground while the other is 0.1 Volts above ground. In the
right hand circuit, the added impedance is in the attaching component so that
the signal references are raised above ground. One signal reference is 1 Volt
above ground while the other is 0.9 Volts above ground. In both cases there is
0.1 Volts between the signal references but the left hand circuit has a much
lower ground noise. The noise on the ground system is not important to this one
signal because it is common mode, but noise on the ground system may become a
problem if other connections in the system allow this noise to flow to other
areas. We’ll see some examples of this later. Although it is a trade off, it is
best if noise on the individual signal as well as noise on the ground system are
both kept as low as possible. Therefore, when there is a choice, it is better to
install the added impedance in the circuit containing the noise source. We have
been talking about isolating grounds; however we really are isolating the noise
generator – the ground isolation is an artifact of that.

Thus the
conclusion is that the added impedance helps reduce the noise by inhibiting the
noise current in a ground loop; however it does not eliminate the problem as
would galvanic isolation or fixing the ground loop problem. None of the noise is
eliminated – it is just moved to a part of the circuit where it will have less
of an impact on signal.

Well, if 9 Ohms is good, why not add 90 Ohms or
900 Ohms of impedance? This would further reduce the noise level; however there
is another, more important, factor to consider – that of safety. If an
electrical fault occurs there can be a very high voltage present across the
impedance. We need to keep the ground impedance low so that most of the fault
current will flow through the ground circuit rather than through an external
route, for example, a person.

Safety Loop Breaker Circuit

The Safety Loop Breaker circuit
presented in Rod Elliott’s article addresses this issue of ground noise differently.
Instead of adding impedance to the ground loop, Rod’s circuit subtracts voltage
from the point in the circuit where it is inserted. This is accomplished with a
pair of diodes (inside a bridge rectifier) in parallel opposition; that is, the
anode of each is connected to the cathode of the other. These diodes prevent the
voltage across the Safety Loop Breaker circuit from being greater than 1 diode
voltage drop (0.6 Volts). As with the simple resister example shown above, none
of the noise has disappeared: the diodes will drop 0.6 Volts and any remaining
noise (over the 0.6 Volts) is distributed across the impedance in the circuit.
If the voltage across the circuit is less than 0.6 Volts, then the diodes will
not conduct and the circuit works by adding impedance to the loop with a 10 Ohm
resistor across the circuit. This resistor also limits the impedance between
safety ground and the internal ground buss. Also, In the event of an electrical
fault that places a high voltage on the internal ground buss, the diodes will
keep the voltage on the buss from exceeding 0.6 Volts. There is a 100nF
capacitor across the circuit to reduce RFI problems. Let’s take a look at how
all this works.

Figure
4.8-5

Figure 4.8-5, We need to think in terms of voltages here. The
Safety Loop Breaker circuit drops 0.6 Volts of the 1 Volt noise voltage, leaving
0.4 Volts to be dropped across the impedance in the circuit – the interconnect
shield. In the left hand circuit, the Safety Loop Breaker circuit is under the
noise source so the Safety Loop Breaker circuit drops 0.6 Volts, leaving 0.4
Volts on the signal reference. The other signal reference is at 0 Volts
(ground). In the right hand circuit, the Safety Loop Breaker circuit is in the
attaching component so that the signal reference has the 0.6 Volts of noise that
is dropped by the circuit. The other signal reference has 1 Volt of noise on it.
In both cases there is 0.4 Volts between the signal references but the left hand
circuit has a much lower ground noise.

What would happen if we installed
a Safety Loop Breaker circuit in both the component with the noise as well as
the attaching component?

Figure
4.8-6

Figure 4.8-6, Here is an example of there being not enough
voltage around the loop for the Safety Loop Breaker circuits to drop 0.6 Volts
each. The voltages and current are now established by the impedances in the
loop. The noise on the interconnect shield between the two components has been
reduced by almost a factor of ten and the noise on the ground system, 0.48 Volts
on the right and 0.52 Volts on the left, is about half-way between the two cases
shown above that each have a single Safety Loop Breaker circuit.

These
examples with a single noise source and only two components are pretty simple.
More complex cases with multiple components, multiple noise sources and isolated
Safety Loop Breaker circuits are shown below.

Multiple Ground Loops

Now, strange things happen when we add
additional components to the system. This is where we see the effect of ground
noise in the system.

Figure
4.8-7

Figure 4.8-7, A third component with a Safety Loop Breaker
circuit added to the system. This example is an extension of that shown above in
figure 4.8-6. Component 1 and component 2 are exactly the same with 1 Volt of
noise being generated in component 1. The difference is that a third component,
exactly the same as component 2 is added to the system. The Safety Loop Breaker
circuits in components 2 and 3 are in parallel, effectively lowering the
impedance in the loop that is presented to the noise generator in component 1.
The result is poorer noise reduction. Not only has the noise on the shield of
the interconnect cable between components 1 and 2 increased, but the noise on
the signal reference of component 2 is passed along on the shield of the
interconnect cable to component 3. Note that the total noise impacting the
signal across the system is the sum of the noise between components 1 and 2 plus
the noise between components 2 and 3. So the total noise is 0.09 Volts (0.06 +
0.03) – almost twice the amount present without component 3. Because this noise
is on the ground system, it is always present everywhere in the system even if a
different signal is selected to be active (for example in a preamp). Okay, so
what would happen if the added device were a class 1 device with the reference
directly connected to the safety ground?

Figure
4.8-8

Figure 4.8-8, A third component, which is a class 1 device,
attached to the system. This example is the same as that in figure 4.8-7 except
that component 3 is a class 1 device. The noise performance in every area is
worse than if component 3 had a Safety Loop Breaker circuit. This is because the
class 1 device effectively nullifies the Safety Loop Breaker circuit in
component 2. Now, an interesting thing happens if we swap component 2 with
component 3. The components are the same, just their position has changed.

Figure
4.8-9

Figure 4.8-9, The noise on the shield of the interconnect
cable between components 1 and 2 is about twice that as in the case shown in
figure 4.8-8 but the total noise as seen by component 3 is about the same (0.4
Volt + 0 Volt versus 0.21 Volt + 0.19 Volt.) The important difference is that
now the ground system is clean. Note that the Safety Loop breaker circuit in
component 3 is redundant and it doesn’t make any difference if it is there or
not. We are back to where we were in figure 4.8-5.

The conclusion from
the above is that adding a second Safety Loop Breaker circuit in the attaching
component can help the noise on an individual interface, but at the expense of
noise on the ground system. Adding additional Safety Loop Breakers circuits to
the system may actually be detrimental. Including a class 1 device in the system
provides a clean ground system at the expense of increasing the noise on the
individual interface.

Multiple Noise Sources

Let’s now take a look at what happens when
there is multiple noise sources in the system.

Figure
4.8-10

Figure 4.8-10, Two interconnected noise sources. Because the
two noise sources are similar but not identical, the noise voltages will
sometimes add and sometimes subtract. The power of the two noise signals will
directly add, while the voltages will add as the square root of the sum of the
squares of the two noise voltages. The result is that each noise source
contributes 0.7 Amp of noise current and the combined effect produces 1.414
Volts of noise across the 1 Ohm resistor.

Figure
4.8-11

Figure 4.8-11, Nothing unusual here; one Safety Loop Breaker
circuit helps a little and two circuits help a lot more. As with a single noise
source, things get interesting as we add additional components to the system.

Figure
4.8-12

Figure 4.8-12, A single Safety Loop Breaker circuit with two
noise sources. This example, as well as the following example shown in figure
4.8-13, is the same as two circuits shown in figure 4.8-5 back-to-back. In this
example both components 1 and 3 have 1 Volt noise sources and component 2 has a
Safety Loop Breaker circuit inserted into both ground loops. The diodes in this
circuit clamp the voltage across it to 0.6 Volts; therefore each cable shield
has 0.4 Volts of noise. Note that the two noise voltages do not add in this case
because the total voltage is limited to 0.6 Volts.

What if the Safety
Loop Breaker circuits were in the components with the noise rather than in the
attaching component?

Figure
4.8-13

Figure 4.8-13, Safety Loop Breaker circuits in the noisy
components. The difference between this example and the previous is that
component 2 is now a class 1 component with its signal references connected to
safety ground. The signal references of components 1 and 3 are 0.4 Volts away
from this ground. Thus, the ground system in this example is a lot quieter than
that of figure 4.8-12. This is most important if component 2 were a preamplifier
with other components attached. The noise on the ground system is seen by and
thus affects all components in the system.

Okay, but what if component 2
also had a Safety Loop Breaker circuit; wouldn’t that be even better?

Figure
4.8-14

Figure 4.8-14, An example of the interaction of three Safety
Loop Breaker circuits. The analysis of this gets messy because the two noise
sources interact through all three of the Safety Loop Breaker circuits. The
result is that you trade off a little noise on the interfaces for a lot more
noise on the ground system.

You can add or remove Safety Loop Breaker
circuits at different points in the system but the problem is that both noise
sources contribute to each other through shared impedances and need to be
isolated from each other so that there are no shared impedances.

The
conclusion from the above is pretty much the same as with a single noise source:
Adding a Safety Loop Breaker circuit in the attaching component in addition to
that in the noisy component can help the noise on an individual interface, but
at the expense of noise on the ground system. Including a class 1 device in the
system provides a clean ground system at the expense of increasing the noise on
the individual interface.

Okay then, would it help to put a separate
Safety Loop Breaker circuit on each interface?

Figure
4.8-15

Figure 4.8-15, Separate Safety Loop Breaker circuits. At
first glance this looks pretty good – each cable shield has only 0.05 Volts of
noise just like the circuit in figure 4.8-6. However there is a problem lurking
internally in component 2 that will become obvious if we re-draw that portion of
the circuit.

Figure
4.8-16

Figure 4.8-16, Internal ground structure of component 2. We
see that although the noise voltage on each signal reference is pretty good,
that we have 0.68 Volts (0.48 Volts x 1.414 noise addition factor) of noise
between the two signal references. This is far worse than with a single Safety
Loop Breaker circuit and is almost the same as with no Safety Loop Breaker
circuit at all! By the way, this example shows what the result can be when
violating Grounding Rule 4 – the signal references must both be connected to the
system star ground. Okay, let’s just connect the two signal references together
– that should fix the problem. Nope – what we now have is the same as the
circuit in figure 4.8-14 except that the two 10 Ohm impedances are now in
parallel making 5 Ohms. This reduces the effectiveness of the Safety Loop
Breaker circuit such that the combined noise voltage on the references is now
0.51 Volts and the noise voltage across each interconnect shield impedance is
0.28 Volts – just about the amount of a single Safety Loop Breaker circuit.

Thus the conclusion is that you can do no better than with a single
Safety Loop Breaker circuit in a component.

The conclusions about a
Safety Loop Breaker circuit are:

The circuit does not eliminate, or even reduce, noise – it moves the
noise. The noise may be moved to a place where it does not affect the signal.
However, the noise may be moved to a place where it does affect the signal;
where the noise is moved depends on where the circuit is located.

A circuit located in a device causing the noise always helps.

In a system containing only two components, the device causing the noise
and the attaching device, adding a circuit to the attaching device also helps.

In a system containing more than two components, adding a circuit to the
attaching device hurts the noise performance. This is because some of the
noise is moved to the ground system.

It hurts the noise performance to have more than one circuit in a device.
This is because the noise will be moved to the internal signal reference in
that device.

Thus the conclusion is that the Safety Loop Breaker circuit helps reduce
the noise by inhibiting the noise current in a ground loop; however it does
not eliminate the problem as would galvanic isolation or fixing the ground
loop problem. None of the noise is eliminated – it is just moved to a part of
the circuit where it will have less of an impact on signal.

4.9 - Construction Summary

Follow the grounding rules in section 4.1.

Make a ground map and carefully design the ground structure using stars
and busses.

Power supplies are two-terminal devices. Do not tap into the internal
circuitry.

Use Safety Loop Breaker circuits only where they are needed.

Chapter 5 - Conclusions

Audio Component Grounding

Most grounding problems are caused by
something being connected to the wrong ground or ground current flowing where it
is not needed. “Wrong ground” implies that there is more than one kind of ground
and I differentiate grounds as follows:

Safety ground – this is the separate (green or green/yellow) wire in the
power line going back to the circuit breaker panel. It is connected to earth
at the panel.

Chassis and cable shields – These provide protection from electrostatic
fields.

Power common – This is the 0 Volt reference from the power supply.

Signal reference – This provides a point of reference for the signal in a
circuit.

These grounds need to be connected together in a very
specific manner as part of the design of a component grounding structure.
Grounding should be designed as carefully as any other part of the component and
I encourage you to make a map of the grounding structure to show up any
potential problems. Here are some rules to help you plan your grounding
structure.

Rule 1: Each of the following must be
connected to the system star ground by one and only one route.

All signal references

All power commons

Shields of non-galvanically isolated single-ended inputs and outputs

Safety ground and chassis. The safety ground and chassis should be thought
of as a single entity.

The connection may be direct, or indirect through a star-of-stars
or buss. This is expanded upon below.

The safety ground and chassis may be connected to the system star
ground through a Safety Loop Breaker Circuit.

The “one and only one” part of this rule precludes ground loops. There is
no excuse for a ground loop within a single component.

Rule 2:
The shield of a balanced input or output (XLR pin 1) must be
connected to the chassis at or as close as is possible to the
connector.

Rule 3: The shield of a single-ended input or
output that is not galvanically isolated must be directly connected to
the system star ground.

The shield is the signal reference in the
cable

Rule 4: Any circuit associated with an input or output
that is not galvanically isolated must have its signal reference
directly connected to the system star ground.

Rule 5:
The mains safety ground must be directly connected to the chassis.

From IEC 60950, “The wire is terminated with a closed loop
connector which is fixed to the earthing stud or screw with a star or lock
washer and a nut. Other parts of the product that need to be earthed are
connected by closed loop connectors to the same stud and locked with an
additional nut. It is important that the earth wire from the power supply cord
is located at the bottom of the stud and locked with its own nut. The earthing
stud must not be used for any purpose other than earthing. It cannot be used,
for example, for the mechanical fixing of parts other than the earth conductors.
Its mechanical structure must also be such that it cannot be loosened from
outside the device. For example, it cannot be a post fixed with a screw from
outside the product.”

Rule 6: Each signal reference must
be directly connected to its power reference.

That is, no circuit
may have its signal reference connected to its power common through
another circuit’s signal reference or power common. This rule allows for a
star-of-stars with the signal reference and power common directly connected
together in a star and that star connected to the system star (either directly
or through a buss).

Rule 7: Circuits may be grouped
together with their signal references forming a buss.

The order of
the grouping is not arbitrary. Just as the signal is routed along, stage to
stage, the associated signal reference can be routed with the signal between
stages. Keep the signal and its associated signal reference electrically close
together; they should be treated as a pair. This minimizes the risk of noise
being injected into the signal reference.

One end of the buss should be
connected to the system star ground, either directly or by a star of stars.

Audio Component Interconnection

Provide a dedicated branch power line, or at least a benevolently loaded
one, for audio components.

Plug all audio components into the same power strip or power outlet.

Provide a separate branch power line for computers, TVs or any other
devices having switching power supplies.

Provide isolated interfaces for connections between:

a. The audio
system and other devices, like computers or TVs.

b. Pin 1 Problem
devices and any other audio component.

c. Class 1 component and a class
2 component. Isolation may not be needed for this case.

d. Class
1 component and an SLB isolated component. Isolation may not be needed
for this case.

Loops aren’t bad – it depends on what is on the loop.

Use shielded heavy gauge twisted pair interconnect cables.

Understanding, Finding, & Eliminating Ground Loops in Audio
& Video Systems by Bill Whitlock has a lot of great information on
solving interconnection problems in audio systems.

Bibliography and References

Books

[1] Grounding and Shielding Techniques in Instrumentation,
Ralph Morrison.

This book is complete and explicit with lots of
examples. It is very technical and not a casual read but is essential to
thoroughly understand the subject. It is now in its 5th edition and is
expensive, although earlier editions are available used. Page references in this
article are from the 3rd edition.

[2] Solving Interference Problems in
Electronics, Ralph Morrison.

Page references in this article are from the
1st edition.

[3] Noise Reduction Techniques in Electronic Systems, Henry
W. Ott.

Like Morrison, this book is an excellent reference for the
subject material. Page references in this article are from the 1st
edition.

[4] Electromagnetic Compatibility Engineering, Henry W. Ott

A significant update and expansion of [3] Get this book even if you have
his earlier one.

[5] through [10] Reserved for future entries

Publications

[11] Journal of the Audio Engineering Society,
Volume 43, Number 6, June 1995. This whole issue is on shields and
grounds.

[12] Susceptibility in Analog and Digital Signal Processing
Systems, Neil A. Muncy JAES, Volume 43, Number 6, June 1995 Pages 435-453. Muncy
introduced the “Pin 1 Problem” in this paper

[13] Balanced Lines in Audio
Systems: Fact, Fiction, and Transformers, Bill Whitlock. JAES, Volume 43, Number
6, June 1995 Pages 454-464.

[14] AES48-2005 AES standard on
interconnections – Grounding and EMC practices – Shields of connectors in audio
equipment containing active circuitry.

[15] Electro-magnetic
compatibility – “The art of grounding” , Brent Hertz
AES preprint 3041 (G-1)
February, 1991. Grounding in the studio environment.

[16] EN 60065 -
Audio, Video and Similar Electronic Apparatus, Safety Requirements

[17]
through [20] - Reserved for future entries

Articles on the Internet

Interconnection of Balanced and Unbalanced Equipment Bill
Whitlock

Hum
& Buzz in Unbalanced Interconnect Systems, Jensen AN-004, Bill
Whitlock

A
New Balanced Audio Input Circuit for Maximum Common-Mode Rejection in Real
World Environments Bill Whitlock

Understanding, Finding, & Eliminating Ground Loops in Audio
& Video Systems Bill Whitlock

Subtleties Count in Wide-Dynamic-Range Analog Interfaces
Bill Whitlock

Sound System
Interconnection, Rane Corp. Technical Staff

Grounding and
Shielding Audio Devices, RaneNote 151, Stephen Macatee

Considerations in Grounding and Shielding Computer-Controlled
Audio Devices, Stephen Macatee

Pin 1
Revisited, Jim Brown

SCIN: Shield
Current Induced Noise, Jim Brown

A Ham’s
Guide to RFI, Ferrites, Baluns, and Audio Interfacing, Jim Brown. A
different perspective on the problem. A particularly good coverage of
filtering RFI.

Power and Grounding for Audio and Video Systems: A White Paper
for the Real World – International Version, Jim Brown. An excellent
introduction to the subject, from the professional audio perspective, but a
lot applicable for the advanced hobbyist.

Ground Loops, D. Self. Ground loops – how they work and how
to deal with them.

Star
Grounding, Randall Aiken

Common Mode Choke Theory

The
Two-Point Earth Method. Inside the Sakuma Amplifier.

Earthing
Your Hi-Fi – Tricks and Techniques, Rod Elliott. A good description of the
Safety Loop Breaker Circuit.

Blocking Mains DC Offset, Rod Elliott

Earthing. Short introduction to the requirements for a Class
1 device.

Ground Loop Problems and How To Get Rid of Them, Tomi
Engdahl

Ground and (safety) Earth, Eric Juaneda. Good coverage of
parasitic currents.

International Safety
Standard for Information Technology Equipment, IEC 60950

When good grounds turn bad – isolate! Thomas Kugelstadt.
Analog Applications Journal, 3Q 2008, page 11.

So You Thought Your Amplifier Was Balanced? Andy Grove and
Peter Qvortrup. Good introduction to the advantages to balanced circuitry, and
particularly transformer interfaces.

Revision History

May 29, 2009 - 1.0
Base

June 1, 2009 - 1.1
Expanded on computer attachment In Effective
Interconnection Schemes.

June 13, 2009 - 1.2
Re-structured the Effective Interconnection
Schemes section.

June 29, 2009 - 1.3
Minor editorial changes
Added Conclusions
chapter

July 3, 2009 - 1.4
Changed Input Switching section.

July 9, 2009 - 1.5
Added section on Safety Loop Breaker circuit

July 20, 2009 - 1.6
Restructure chapter 3 for better readability

July 30, 2009 - 1.7
Add reference to t.i. Analog Applications
Journal.

September 7, 2009 - 1.8
Correct typo

April 23, 2010 - 1.9
Restructure chapter 3 to eliminate duplicate
heading number

May 1, 2010 - 1.10
General cleanup
fix typographical
errors
fix several figures
update references

Comments on the article "Audio
Component Grounding and Interconnection"

21st April 2010

jlsem

diyAudio Member

Nice article. It's good to
see applications of Morrison's lessons to audio.

John

21st April 2010

Gordy

diyAudio Member

Good work DD. Very
comprehensive indeed.

22nd April 2010

Akita

diyAudio Member

Well done! A compilation of
knowledge I have been search for few years.

22nd April 2010

waltzingbear

diyAudio Member

Nicely done

except
for one "trivial" but important point on your diagram of the service entrance.
You show the safety ground going directly to the ground rod, that is incorrect,
in the US it is bonded at the panel for safety, this bonding point is the actual
earth reference point.

Especially in lightning prone areas, this is an
important consideration. If this is incorrectly installed, it may mean the
system is much more prone to equipment damage, the potential difference between
the neutral and safety can rise to dangerous levels and cause damage within the
equipment.

I would also object to your blanket rejection of switching
supplies. While it is true it is more difficult to design with switching
supplies, it is not true that this kind of design cannot be designed well. There
are potential benefits from switching supplies that should not be dismissed
without better consideration.

Alan

22nd April 2010

Omega_Void

diyAudio Member

In section 4.7 you
say:

"So, for example, the AC leakage current from the primary of a power
transformer came from the mains power line and should be returned to earth, so
the transformer frame should be connected to the safety ground."

Surely
if the current came from live it should be returned to neutral, not to the
safety earth?

Have I misinterpreted something?

22nd April 2010

gary
h

diyAudio Member

What a useful
compendium,

having tracked down various subjects covered in this article
individually, it is pleasing to find them intelligently and comprehensively
assembled using language that is accessible to an inexperienced beginner, (like
me.)

I would assume this is required reading for anyone starting out in
DIY audio or plagued by the ubiquitous treachery of ground related
noises.

Many thanks.

gary h

22nd April 2010

zdavesf

diyAudio Member

Excellent, Excellent
Article. I love the idea of drawing out a ground diagram, we do it for signal
why not for ground, great! I am going to print this off, and put it in my binder
Labeled "Super Awesome Audio Design Refernece Guides"

Thank you ever so
much.

Dave

we all have to start somewhere...

22nd April 2010

SY

diyAudio Moderator

Ditto- incredibly useful
and really well-written article. Nothing tortures novice and intermediate
constructors so much as grounding. Even us experienced guys screw it up now and
then.

“Committees for the betterment of any society are
baskets of loose knobs jostling for doors to close.”

22nd April 2010

HiFiNutNut

diyAudio Member

This is the most helpful
and educational article on grounding I have read out of dozens. I wish it was
produced a couple of years ago and it would save me a lot of time.

One
thing still puzzles me. Legal or not, isolating the chassis from the earth with
a 600V 35A bridge wiring up the diodes in both ways would solve many of the
problems without compromise of safety and without the inherent problems of a
loopbreaker. The idea is so simple but why nobody has recommended it?

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