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Design How-To

Audio amplifier power supply design
- Part 3: Power-supply rail rejection

Douglas
Self

9/21/2010 01:42 PM EDT
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[Part
1 looks at advantages and disadvantages of
different power supply technologies as well as the design
considerations involved in choosing and evaluating a mains
transformer. Part
2 considers the pros and cons of external
supplies, inrush current control, RF emissions from bridge
rectifiers, and relay supplies.]

Power-Supply Rail Rejection in Amplifiers
The
literature on power amplifiers frequently discusses the importance
of power-supply rejection in audio amplifiers, particularly in
reference to its possible effects on distortion [4]!

I have (I hope) shown in earlier chapters that regulated power
supplies are just not necessary for an exemplary THD performance. I
want to confirm this by examining just how supply-rail disturbances
insinuate themselves into an amplifier output, and the ways in which
this rail injection can be effectively eliminated. My aim is not
just the production of hum-free amplifiers, but also to show that
there is nothing inherently mysterious in power-supply effects, no
matter what subjectivists may say on the subject.

The effects of inadequate power-supply rejection ratio (PSRR) in
a typical Class-B power amplifier with a simple unregulated supply
may be twofold:

A proportion of the 100 Hz ripple on the rails will appear at
the output, degrading the noise/ hum performance. Most people find
this much more disturbing than the equivalent amount of
distortion.

The rails also carry a signal-related component, due to their
finite impedance. In a Class-B amplifier this will be in the form
of half-wave pulses, as the output current is drawn from the two
supply rails alternately; if this enters the signal path it will
degrade the THD seriously.

The second possibility, the
intrusion of distortion by supply-rail injection, can be eliminated
in practice, at least in the conventional amplifier architecture so
far examined. The most common defect seems to be misconnected rail
bypass capacitors, which add copious ripple and distortion into the
signal if their return lines share the signal ground; this was
denoted Distortion 5 (rail decoupling distortion) on my list of
distortion mechanisms in Chapter 3.

This must not be confused with distortion caused by
inductive coupling of half-wave supply currents into the
signal path – this effect is wholly unrelated and is completely
determined by the care put into physical layout; I labeled this
Distortion 6 (induction distortion).

Assuming the rail bypass capacitors are connected correctly, with
a separate ground return, ripple and distortion can only enter the
amplifier directly through the circuitry. It is my experience that
if the amplifier is made ripple-proof under load, then it is proof
against distortion components from the rails as well. This bold
statement does, however, require a couple of qualifications.

Firstly, the output must be ripple-free under load, i.e.
with a substantial ripple amplitude on the rails. If a Class-B
amplifier is measured for ripple output when quiescent, there will
be a very low amplitude on the supply rails and the measurement may
be very good, but this gives no assurance that hum will not be added
to the signal when the amplifier is operating and drawing
significant current from the reservoir capacitors.

Spectrum analysis could be used to sort the ripple from the
signal under drive, but it is simpler to leave the amplifier
undriven and artificially provoke ripple on the HT rails by loading
them with a sizeable power resistor; in my work I have standardized
on drawing 1 A. Thus one rail at a time can be loaded; since the
rail rejection mechanisms are quite different for V+ and V-, this is
a great advantage. Drawing 1 A from the V- rail of the typical power
amplifier in Figure 9.7 degraded the measured ripple output from -88
dBu (mostly noise) to -80 dBu.

Figure 9.7: Diagram of a generic power amplifier,
with diode biasing for input tail and VAS source

Secondly, I assume that any rail filtering arrangements will work
with constant or increasing effectiveness as frequency increases;
this is clearly true for resistor-capacitor (RC) filtering, but is
by no means certain for electronic decoupling such as the
NFB current-source biasing used in the design in Chapter 7. (These
will show declining effectiveness with frequency as internal loop
gains fall.) Thus, if 100 Hz components are below the noise in the
THD residual, it can usually be assumed that disturbances at higher
frequencies will also be invisible, and not contributing to the
total distortion.

To start with some hard experimental facts, I took a power
amplifier – similar to Figure 9.7 – powered by an unregulated supply
on the same PCB (the significance of this proximity will become
clear in a moment) driving 140 W rms into 8 Ω at 1 kHz. The PSU was
a conventional bridge rectifier feeding 10,000 µF reservoir capacity
per rail.

The 100 Hz rail ripple under these conditions was 1 V peak to
peak. Superimposed on this were the expected half-wave pulses at
signal frequency; measured at the PCB track just before the HT fuse,
their amplitude was about 100 mV peak to peak. This doubled to 200
mV on the downstream side of the fuse – the small resistance of a
6.3 A slow-blow fuse is sufficient to double this aspect of the PSRR
problem, and so the fine details of PCB layout and PSU wiring could
well have a major effect. (The 100 Hz ripple amplitude is of course
unchanged by the fuse resistance.)

It is thus clear that improving the transmitting end of
the problem is likely to be difficult and expensive, requiring
extra-heavy wire, etc., to minimize the resistance between the
reservoirs and the amplifier. It is much cheaper and easier to
attack the receiving end, by improving the poweramp's PSRR.
The same applies to 100 Hz ripple; the only way to reduce its
amplitude is to increase reservoir capacity, and this is expensive.

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re: Audio amplifier power supply design - Part
3: Power-supply rail rejection
davi_viotto
9/24/2010 2:59:46 AM

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There's
literature such as RCA Designer's Handbook printed in 1971 which
explains the operation of audio amplifiers with bipolar transistors.
This shows that the future needs of the best books for better
projects, but no shows for free when it's worth money, would rather
sell. Before, we had scientific information on new things, not
today, tell things 30 years ago.

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