Ground Loops.

Updated: 9 Oct
2001

.C O N T E N T S . O F . T H I S . P A G E.

1:
Mains grounding currents

2:
Transformer stray magnetic fields

3:
Transformer stray capacitance

4:
Ground currents inside equipment

5:
Balanced mains power. NEW!

Ground loops: how they work and how to deal with them.

A ground-loop is created whenever two or more pieces of mains-powered
equipment are connected together, so that mains-derived AC flows through shields
and ground conductors, degrading the noise floor of the system. The effect is
worst when two or more units are connected through mains ground as well as audio
cabling, and this situation is what is normally meant by the term "ground-loop".
However, ground currents can also flow in systems that are not galvanically
grounded; they are of lower magnitude but can still degrade the noise floor, so
this scenario is also considered here.
The ground currents may either be
inherent in the mains supply wiring (see "1: Mains grounding currents") or
generated by one or more of the pieces of equipment that make up the audio
system.(see "2: Transformer stray magnetic fields" and "3: Transformer stray
capacitance")
Once flowing in the ground wiring, these currents will give
rise to voltage-drops that introduce hum and buzzing noises. This may occur
either in the audio interconnects, or inside the equipment itself if it is not
well-designed. See Section 4.

Here I have used the word "ground" for conductors and so on, while "earth" is
reserved for the damp crumbly stuff into which copper rods are thrust.

HUM INJECTION BY MAINS GROUNDING CURRENTS.
Fig 1 shows
what happens when a so-called "technical ground" like a buried copper rod is
attached to a grounding system which is already connected to "mains ground" at
the power distribution board. The latter is mandatory both legally and
technically, so one might as well accept this and denote as the reference
ground. In many cases this "mains ground" is actually the neutral conductor,
which is only grounded at the remote transformer substation. AB is the cable
from substation to consumer, which serves many houses from connections tapped
off along its length. There is substantial current flowing down the N+E
conductor, so point B is often 1 Volts rms or more above earth. From B onwards,
in the internal house wiring, neutral and ground are always separate. (In the
UK, anyway)

Two pieces of audio equipment are connected to this mains
wiring at C and D, and joined to each other through an unbalanced cable F-G.
Then an ill-advised connection is made to earth at D; the 1V rms is now
impressed on the path B-C-D, and substantial current is likely to flow through
it, depending on the total resistance of this path. There will be a voltage-drop
from C to D, its magnitude depending on what fraction of the total BCDE
resistance is made up by the section C-D. The earth wire C-D will be of at least
1.5 sqmm cross-section, and so the extra connection FG down the audio cable is
unlikely to reduce the intefering voltage much.
To get a feel for the
magnitudes involved, take a plausible ground current of 1 Amp. The 1.5 sqmm
ground conductor will have a resistance of 0.012 Ohms/metre, so if the mains
sockets at C and D are one metre apart, the voltage C-D will be 12 mV rms.
Almost all of this will appear between F and G, and will be indistinguishable
from wanted signal to the input stage of Unit 2, so the hum will be severe,
probably only 30 dB below the nominal signal level.

The best way to solve this problem is not to create it in
the first place. If some ground current is unavoidable then the use of balanced
inputs (or ground-cancel outputs- it is not necessary to use both) should give
at least 40 dB of rejection at audio frequencies.

Fig 1 also shows a third earthing point, which fortunately
does not complicate situation. Metal water pipes are bonded to the incoming
mains ground for safety reasons, and since they are usually electrically
connected to an incoming water supply current flows through B-W in the same way
as it does through the copper rod link D-E. This water-pipe current does not
however flow through C-D and cannot cause a ground-loop problem. It may however
cause the pipes to generate an AC magnetic field which is picked up by other
wiring.

HUM INJECTION BY TRANSFORMER STRAY FIELDS.
Fig 2 shows a
thoroughly bad piece of physical layout which will cause ground currents to flow
even if the system is correctly grounded to just one point.

Here Unit 1 has an external DC power-supply; this makes it
possible to use an inexpensive frame-type transformer which will have a large
stray field. But... note that the wire in the PSU which connects mains ground to
the outgoing 0V takes a half-turn around the transformer, and significant
current will be induced into it, which will flow round the loop C-F-G-D, and
give an unwanted voltage-drop between F and G. In this case reinforcing the
ground of the audio interconnection is likely to be of some help, as it directly
reduces the fraction of the total loop voltage which is dropped between F and
G.
It is difficult to put any magnitudes to this effect because it depends on
many imponderables such as the build quality of the transformer and the exact
physical arrangement of the ground cable in the PSU. If this cable is rerouted
to the dotted position in the diagram, the transformer is no longer enclosed in
a half-turn, and the effect will be much smaller.

HUM INJECTION BY TRANSFORMER STRAY CAPACITANCE.
It seems at
first sight that the adoption of Class-II (double-insulated) equipment
throughout an audio system will give inherent immunity to ground-loop problems.
Life is not so simple, though it has to be said that when such problems do occur
they are likely to be much less severe.
This problem afflicts all Class-II
equipment to a certain extent.

Fig 3 shows two Class-II units connected together by an
unbalanced audio cable. The two mains transformers in the units have stray
capacitance from both live and neutral to the secondary. If these capacitances
were all identical no current would flow, but in practice they are not, so 50 Hz
currents are injected into the internal 0V rail and flow through the resistance
of F-G, adding hum to the signal.
A balanced input or ground-cancelling
output will remove or render negligible the ill-effects. Reducing the resistance
of the interconnect ground path is also useful- more so than with other types of
ground loop, because the ground current is essentially fixed by the small stray
capacitances, and so halving the resistance F-G will dependably halve the
interfering voltage. There are limits to how far you can take this- while a
simple balanced input will give 40dB of rejection at low cost, increasing the
cross-sectional area of copper in the ground of an audio cable by a factor of
100 times is not going to be either easy or cheap.
Fig 3 shows equipment with
metal chassis connected to the 0V; (this is quite acceptable for safety
approvals- what counts is the isolation between mains and everything else, not
between low-voltage circuitry and touchable metalwork) note the chassis
connection however has no relevance to the basic effect, which would still occur
even if the equipment enclosure was completely non-conducting.

The magnitude of ground current varies with the details of
transformer construction, and increases as the size of the transformer grows.
Therefore the more power a unit draws, the larger the ground current it can
sustain. This is why many systems are subjectively hum-free until the connection
of a powered subwoofer, which is likely to have a larger transformer than other
components of the system.

Equipment type

Power consumption

Ground current

Turntable, CD, cassette deck

20W or less

5 uA

Tuners, amplifiers, small TVs

20-100W

100 uA

Big amplifiers, subwoofers, large TVs

More than 100W

1mA

GROUND CURRENTS INSIDE EQUIPMENT.
Once ground currents
have been set flowing, they can degrade system performance in two locations:
outside the system units, by flowing in the interconnect grounds, or inside the
units, by flowing through internal PCB tracks, etc. The first problem can dealt
with effectively by the use of balanced inputs, but the internal effects of
ground currents can be much more severe if the equipment is poorly designed.

Fig 4 shows the situation. There is, for whatever reason,
ground current flowing through the ground conductor CD, causing an interfering
current to flow round the loop CFGD as before. Now, however, the internal design
of Unit 2 is such that the ground current flowing through FG also flows through
G-G' before it encounters the ground wire going to point D. G-G' is almost
certain to be a PCB track with higher resistance than any of the cabling, and so
the voltage drop across it can be relatively large, and the hum performance
correspondingly poor. Exactly similiar effects can occur at signal outputs; in
this case the ground current is flowing through F-F'.
Balanced inputs will
have no effect on this; they can cancel out the voltage-drop along F-G, but if
internal hum is introduced further down the internal signal path, there is
nothing they can do about it.

The correct method of handling this is shown in Fig 5. The
connection to mains ground is made right where the signal grounds leave and
enter the units, and are made as solidly as possible. The ground current no
longer flows through the internal circuitry. It does however still flow through
the interconnection at FG, so either a balanced input or a ground-cancelling
output will be required to deal with this.

BALANCED MAINS POWER.
There has been speculation in recent
times as to whether a balanced mains supply is a good idea. This means that
instead of Live and Neutral (230V and 0V) you have Live and The Other Live
(115V-0-115V) created by a centre-tapped transformer with the tap connected to
Neutral. See Fig 6 below.

It has been suggested that balanced mains has miraculous
effects on sound quality, makes the sound-stage ten-dimensional, etc,etc. This
is obviously rubbish. If a piece of gear is that fussy about its mains (and I
don't believe any such gear exists) then drop it in the river.

If there is severe RFI on the mains, an extra transformer in
the path may tend to filter it out. However, a proper mains RFI filter will
almost certainly be more effective- it is designed for the job, after all- and
will definitely be much cheaper.

Where you might gain a real benefit is in a Class-II (ie
double-insulated) system with very feeble ground connections. Balanced mains
would tend to cancel out the ground-currents caused by transformer capacitance
(see Fig 3 and above for more details on this) and so reduce hum. The
effectiveness of this will depend on C1 being equal to C2 in Fig 6 above, which
is determined by the details of transformer construction in the unit being
powered. I think that the effect would be small with well-designed equipment and
reasonably heavy ground conductors in interconnects. Balanced audio connections
are a much cheaper and better way of handling this problem, but if none of the
equipment has them then beefing up the ground conductors should give an
improvement. If the results are not good enough then as a last resort, balanced
mains may be worth considering.

Finally, bear in mind that any transformer you add must be able
to handle the maximum power drawn by the audio system at full throttle. This can
mean a large and expensive component.

I wouldn't be certain about the whole of Europe, but to the
best of my knowledge it's the same as the UK, ie not balanced. The Neutral line
is at earth potential, give or take a Volt, and the Live is 230V above this. The
3-phase 11kV distribution to substations is often described as "balanced" but
this just means that the power delivered by each phase is kept as near equal as
possible for the most efficient use of the cables.

It has often occurred to me that balanced mains 115V-0-115V
would be a lot safer. Since I am one of those people that put their hands inside
live equipment a lot, I do have a kind of personal interest here...

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