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Design How-To

Audio amplifier power supply design
- Part 1: Power supply types & transformer
considerations

Douglas
Self

7/21/2010 11:21 AM EDT
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Power-Supply
Technologies
There are three principal ways to power an
amplifier:

1. a simple unregulated power supply consisting of
transformer, rectifiers, and reservoir capacitors;

2. a linear regulated power supply;

3. a switch-mode power supply.

It is immediately obvious that the first and simplest option will
be the most cost-effective, but at first glance it seems likely to
compromise noise and ripple performance, and possibly interchannel
crosstalk. It is therefore worthwhile to examine the pros and cons
of each technology in a little more detail.

I am here dealing only with the main supply for the actual power
amplifier rails. Many amplifiers now have some form of
microcontroller to handle on/off switching by mains relays and other
housekeeping functions; this is usually powered by a separate small
standby transformer, which remains powered when the amplifier supply
is switched off.

The design of this is straightforward – or at least it was until
the introduction of new initiatives to limit the amount of standby
power that a piece of equipment is allowed to consume. The
International Energy Agency is urging a 1 W standby power limit for
all energy-using products.

Simple Unregulated Power Supplies

Advantages

Simple, reliable, and cheap (relatively speaking – the
traditional copper and iron mains transformer will probably be the
most expensive component in the amplifier).

No possibility of instability or HF interference from
switch-mode frequencies.

The amplifier can deliver higher power on transient peaks,
which is just what is required.

Disadvantages

The power into 4 O will not be twice that into 8 O, because
the supply voltage will fall with increased current demand. On the
other hand, the amplifier will always deliver the maximum possible
power it can.

Significant ripple is present on the DC output and so the PSRR
of the amplifier will need careful attention; the problem is,
however, not hard (if you read the second part of this chapter)
and output hum levels below -100 dBu are easily attainable.

The mains transformer will be relatively heavy and bulky.

Transformer primary tappings must be changed for different
countries and mains voltages.

The absence of switch-mode technology does not mean total
silence as regards RF emissions. The bridge rectifier will
generate bursts of RF at a 100 Hz repetition rate as the diodes
turn off. This worsens with increasing current drawn.

Linear Regulated Power Supplies

Advantages

A regulated supply-rail voltage means that the amplifier can
be made to approximate more closely to a perfect voltage source,
which would give twice the power into 4 O than it gives into 8 O.
This is considered to have marketing advantages in some circles,
though it is not clear why you would want to operate an amplifier
on the verge of clipping. There are, however, still load-dependent
losses in the output stage to consider. More on this later.

A regulated supply-rail voltage to a power amplifier gives
absolutely consistent audio power output in the face of mains
voltage variation.

Clipping behavior will be cleaner, as the clipped peaks of the
output waveform are not modulated by the ripple on the supply
rails. Having said that, if your amplifier is clipping regularly
you might consider turning it down a bit.

Can be designed so that virtually no ripple is present on the
DC output (in other words the ripple is below the white noise the
regulator generates) allowing relaxation of amplifier supply-rail
rejection requirements. However, you can only afford to be
careless with the PSRR of the power amp if the regulators can
maintain completely clean supply rails in the face of sudden
current demands. If not, there will be interchannel crosstalk
unless there is a separate regulator for each channel. This means
four for a stereo amplifier, making the overall system very
expensive.

The possibility exists of electronic shutdown in the event of
an amplifier DC fault, so that an output relay can be dispensed
with. However, this adds significant circuitry, and there is no
guarantee that a failed output device will not cause a collateral
failure in the regulators that leaves the speakers still in
jeopardy.

Disadvantages

Complex and therefore potentially less reliable. The overall
amplifier system is at least twice as complicated. The much higher
component count must reduce overall reliability, and getting it
working in the first place will take longer and be more difficult.
For example, consider the circuit put forward by John Linsley-Hood
[1]. To regulate the positive and negative rails for the output
stage, this PSU uses 16 transistors and a good number of further
parts; a further six transistors are used to regulate the supplies
to the small-signal stages. It is without question more complex
and more expensive than most power amplifiers.

If the power amplifier fails, due to an output device failure,
then the regulator devices will probably also be destroyed, as
protecting semiconductors with fuses is a very doubtful business;
in fact it is virtually impossible. The old joke about the
transistors protecting the fuse is not at all funny to
power-amplifier designers, because this is very often precisely
what happens. Electronic overload protection for the regulator
sections is therefore essential to avert the possibility of a
domino-effect failure, and this adds further complications as it
will probably need to be some sort of foldback protection
characteristic if the regulator transistors are to have a
realistic prospect of survival.

Comparatively expensive, requiring at least two more power
semiconductors, with associated control circuitry and over-current
protection. These power devices in turn need heat-sinks and
mounting hardware, checking for shorts in production, etc.

Transformer tappings must still be changed for different mains
voltages.

IC voltage regulators are usually ruled out by the voltage and
current requirements, so it must be a discrete design, and these
are not simple to make bulletproof. Cannot usually be bought in as
an OEM item, except at uneconomically high cost.

May show serious HF instability problems, either alone or in
combination with the amplifiers powered. The regulator output
impedance is likely to rise with frequency, and this can give rise
to some really unpleasant sorts of HF instability. Some of my
worst amplifier experiences have involved (very) conditional
stability in such amplifiers.

The amplifier can no longer deliver higher power on transient
peaks.

The overall power dissipation for a given output is
considerably increased, due to the minimum voltage drop through
the regulator system.

The response to transient current demands is likely to be
slow, affecting slewing behavior.

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re: Audio amplifier power supply design - Part
1: Power supply types & transformer considerations

Nic
Cohen   7/27/2012 7:54:32 AM

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This
is initially only going to be of benefit for the big boys again!!
While the smaller organisations will have to wait for an out the box
equivalent. The only difference is it has a multi-volt flux
capacitor, apologies for the poor attempt to a bad joke!! Nic
kdweb.co.uk

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re: Audio amplifier power supply design - Part
1: Power supply types & transformer considerations

Mike
Turner   9/7/2010 10:34:50 AM

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Hi
Bill Regarding your comment: "It's also worth noting that secondary
(load) current does NOT increase flux density. Only the primary
voltage affects flux density for a given design." It's very
refreshing to hear someone with strong links to the audio community
expressing this simple but so frequently misunderstood truth. Time
and again I wearily see reviewers talking with great approval of
(needlessly) huge power transformers with high current ratings, used
in order "to avoid saturation". The problem doesn't seem to be
confined to the "hi-fi" community either: one close friend who's a
seasoned amplifier designer of some repute was surprised when I put
him right on this point at a recent AES convention. The ignorance of
transformer basics seems so widespread that I'd actually thought of
submitting an AES tutorial paper called "The Misunderstood
Transformer" (or something like that). I've held back to-date not
least because although I've designed plenty of power transformers
(for low-frequency and HF switched-mode use) I've less experience of
detailed design for high quality audio, and there are other
luminaries such as your good self (and dare I mention Brian Sowter,
Per Lundahl...) who have greater knowledge here. Still seems like a
good idea though. Perhaps we could get some T-shirts made bearing
the slogan "The Volts Determine The Flux" !! :-) Kind regards Mike
Turner http://www.aes.org/aes/mike-turner
http://uk.linkedin.com/pub/mike-turner/1/b8a/b7

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re: Audio amplifier power supply design - Part
1: Power supply types & transformer considerations

Guru
of Grounding   7/26/2010
5:41:23 AM

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Although
much is made of the "low" external magnetic field of toroids, they
must be rigorously constructed to live up to that reputation. First,
each winding must COMPLETELY cover the entire core. In most
commercial units, the magnetic field emanates from the egress of
wire leads, where core coverage has a gap. Second, virtually all
power transformers are designed to be as cheap as possible. Less
core and less copper are used in designs that operate very near
magnetic saturation of the core. But the downside is that, as
saturation is approached, radiated magnetic field drastically
increases. It's also worth noting that secondary (load) current does
NOT increase flux density. Only the primary voltage affects flux
density for a given design. Therefore, a design on the verge of
saturation at 115 VAC will likely be a problem at 130 VAC. To make
any power transformer magnetically-friendly, simply operate it at
reduced primary voltage (with reduced secondary voltages, of
course). Another problem with toroids, because they don't have even
the smallest air gaps in the magnetic path, is inrush current when
they're first turned on. In large transformers, this can cause
nuisance breaker tripping. I think the advantages of toroids are
usually over-stated. -- Bill Whitlock, president & chief
engineer, Jensen Transformers, Inc.
www.jensen-transformers.com

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re: Audio amplifier power supply design - Part
1: Power supply types & transformer considerations

Pierre_P
7/23/2010 2:01:14 PM

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While
I agree that using a iron-core, 50/60 Hz transformer makes a lot of
sense for audio amplifier design in the range of 10 -100W of output,
I think that customers wish for smaller box, along with new
requirements for standby power efficiency will push the use of
switching power supplies more and more over time. These switching
power supplies don't suffer the poor power factor of a
bridge-rectified iron transformer with big filtering capacitor, plus
they allow designer to use tricks like variable power rails to save
power when the volume control position is low. These can also
suppply the control voltage along with the power rail. The initial
investment in designing those may pay off for big manufacturers.
This is obviously more difficult for a small shop to design and
certify a line-level switching power supply for a power amp so there
is a definite opportunity here for companies to design specialised
off-the shelf unit for sale to the smaller audio components
designers that can't afford to design them. Pierre
Proulx

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