Manufacturers Report - "The Importance of the Power Supply" - May, 1997
Electricity and Hydraulics
Lots of people don’t understand electricity, but they do
understand plumbing. Hydraulics provides a good analogy in
understanding basic electrical flow. Wire is a pipe. Water
pressure is voltage. Water flow is electrical current. Lakes and
storage tanks are capacitors. Diodes are one-way valves. Tubes
and transistors are faucets.
The entire power circuitry of an amplifier can be seen as a
community water system. The sun, driving the weather cycle,
causes water to be deposited on the landscape, and it collects in
a lake behind a dam. The community draws water as needed through
pipes. In the winter, the rain collects in the lake, and the
water pressure increases as it fills. In the summer, the water
level falls, and so does the pressure. When the community draws
more water than usual, the level goes down more, and it often
takes more than one season to build it back up.
In an amplifier, your utility, house wiring, power cord, and
transformer provide the rain. The capacitor bank is the
reservoir. The capacitors receive electrical charge every 1/120th
of a second, reflecting two pulses of current from the
transformer for every cycle of the 60 Hz sine wave provided by
the power company.
These pulses are of relatively short duration, and it is up to
the power supply capacitors to store energy during the 6
milliseconds or so electrical drought that occurs between charge
pulses. We want a constant voltage (water level) from our power
supply, and this is usually achieved by the use of large
capacitors which store more charge, and large transformers which
provide as much charge as is needed. You get the idea.
Since we are not designing amplifiers here, but rather trying to
get a handle on what constitutes quality in a market full of
hype, I want to talk about some general ideas and comment on some
of the common approaches used by manufacturers. Understand that
we simply want a constant, noise-free, voltage to be available
from a power supply, regardless of how much demand we place on
it. Click here to see a generic schematic diagram of a power
Bigger and heavier is better. Bigger transformers and wires load
down less. Big capacitors hold more charge.
Is there such a thing as too big? Certainly there are diminishing
returns as we get bigger. When a transformer is delivering 1 watt
to a preamp circuit, going from a one kilowatt rating to two
kilowatts isn’t going to buy you much improvement. This
consideration is not much of a deterrent to the average
In case you don't already know, the power transformer provides
the essential function of "transforming" the AC voltage
and current available from the wall outlet into the voltage and
current best suited to the power amplifier circuit. Usually the
power amplifier does not want 120 volts, but rather something
like two sources of 40 volts in order to provide the values for a
direct coupled 200 watt amplifier.
The transformer is a big magnetic system, with a primary winding
which is driven by the AC power from the wall, and one or more
secondary windings which couple to the amplifier circuits through
rectifiers and power supply storage capacitors. The current
flowing through the primary creates a magnetic field in the iron
core, which induces voltage and current in the secondary
windings, transferring the energy to them.
Another important function of the power transformer is isolation
from the AC line. As you know, you can get a sizable shock from
the wall outlet, and the power transformer delivers power from
the wall to the amplifier without a direct connection between the
two, which creates a much safer situation for the consumer.
It is possible to build amplifiers without any transformer, and I
have done so. The result, displayed at CES about 10 years ago,
rectified the AC line directly, resulting in two 170 volt DC
rails which provided about 10,000 watt peaks into 4 ohms through
balanced power circuitry. The result was fairly scary, and
everyone was afraid to turn up the volume control.
The best power transformers are toroids, with donut shaped
magnetic cores. They pack the most power for weight and size, and
they make less noise. Toroidal transformers have to be rated at a
minimum of several times the intended wattage (power output of
the amplifier) because the power is delivered in short pulses to
Typically, a Class AB stereo amplifier rated at 200 watts per
channel continuously should be capable of delivering 700 watts or
so (350 per channel), and this means a transformer rating of
about 2000 watts. Anything less means non-continuous operation.
This might be alright for a class AB amplifier where maximum
continuous operation is not required.
If the stereo amplifier is rated 200 watts per channel pure Class
A, it will draw about 1000 watts all the time, meaning that about
3000 watts of power transformer is called for, no less.
Now a toroidal transformer delivers about 30 watts per pound, so
a 3000 watt toroid will weigh about 100 lbs, maybe more. The rest
of such an amplifier will probably weigh about as much, so if you
are looking at a 200 watt per channel stereo Class A amplifier,
you will want to see if it weighs at least 200 lbs.
One pound of amplifier weight for every 2 watts is a good litmus
test for evaluating Class A amplifiers. An amplifier weighing
less might not be pure Class A. It might be almost Class A, or it
might be one of the many products which achieve a Class A
designation through trick circuitry.
To lower noise still further, toroids are sometimes encapsulated
in metal cans. To reduce magnetic radiation, these cans are
usually, but not always, made of steel. This is good, but be
aware that in the past, at least one company has used a small
transformer in a big can, and made up the difference with sand.
Because of the high capacitance values required, power supply
capacitors are almost invariably electrolytic in construction.
The capacitors you see in power amplifiers are rated in terms of
capacitance in micro-farads, voltage, and current. A typical
value for capacitance of one of the big cans is 25,000
micro-farads, or 0.025 farads. A farad is a big thing. One farad
of capacitance will lose 1 volt after delivering 1 amp for 1
second. In a power amplifier drawing an 8 amp bias, like our 200
watt stereo Class A example, this means a power supply ripple of
about 0.06 volt rms.
Most of the time, you want to see a total of at least 100,000
micro-farads in the power supply, which for our example gives a
ripple of about 0.6 volts. This is pretty good, representing
about 1% of the total supply voltage. Smaller amplifiers can get
by with less, big amps require more.
Big electrolytic capacitors have a small amount of inductance, or
"coilness", in their makeup, a result of the spiral
winding of the capacitive film. To reduce the effect of this
inductance, film capacitors which have low inductance are often
placed in parallel, so that at high frequencies the current flows
a little more easily.
An examination of the numbers will provide some insight here. It
is common for the inductance of a large electrolytic capacitor to
cause its impedance to begin increasing at about 10 kHz so that
its impedance is a large fraction of an ohm at 100 KHz. Placing a
film cap in parallel will keep the impedance to 0.1 ohm or so
above this frequency.
Is this important because audio has real power at these
frequencies? No. Audio has power which declines at about 12
dB/octave above 5 kHz, and real musical slew rate figures are a
fraction of a volt per microsecond, meaning that practically no
power is needed at 100 kHz.
However, high frequency impedance can be important to the
stability of the amplifier, particularly with more complex
circuits, as the source impedance of the power supply starts
figuring into the feedback at frequencies of 1 MHz or so.
Interestingly, some designers have depended on a particular
source impedance of the supply at these frequencies for
stability, thus it is possible to destabilize the amplifier
circuit by paralleling film capacitors across the electrolytics.
In general, however, film caps in the power supplies are a good
sign from the consumer’s standpoint.
As much as we often try to eliminate inductance in capacitors and
wiring, inductors in the form of coils can be used to improve the
performance of power supplies. Placing inductors and capacitors
on the AC line to form filters will reduce both incoming and
outgoing high frequency noise. Large inductors in series with the
transformer primaries and secondaries can be used to stretch the
duration of the charge pulse to the power supply capacitors,
improving regulation and reducing noise. Large inductors in
combination with multiple power supply capacitors can form
"pi" filters to reduce the noise on the supply lines.
Inductors are very useful, but they cost money. Their use in
power amplifier power supplies is an indication that the
manufacturer has an unusually strong commitment to performance.
Audiophiles love wire. Perhaps the appeal lies in the
accessibility to understanding. Perhaps not. In any case, I like
my wire thick and short, and made out of pure soft metals such as
copper or silver. I like it terminated tightly and soldered where
Yeah, sure, rectifiers are important, after all, the AC has to
get converted to DC, but I don’t like the fast recovery
types that some audiophiles have raved about. Fast recovery means
that they withstand many amps and volts in a tenth of a few
nano-seconds, something we don’t see very often on the old
60 Hz AC line. They are essential element in switching power
supplies, but for regular "linear" power supplies, I
much prefer SLOW diodes, and we create them by placing small
capacitor circuits across the diodes, which greatly reduces
Active linear regulation is a great way to make the supply
voltage constant. Unfortunately it is not usually done properly.
In the past, some amplifiers using active regulation have been
criticized for a lack of apparent dynamics, and this has given
the technique a lesser reputation than it deserves.
Properly done, linear regulation has to go beyond the cursory
requirements of the amplifier ratings. The regulator should be
capable of ten times the current of the continuous output of the
amplifier channel. The regulator should be preceded and followed
by large capacitances with values comparable to those needed for
unregulated circuits. The transformer size still needs to be as
big as that used in an unregulated circuit.
Approached in this manner, linear active regulation delivers the
A far less expensive approach achieves some of the regulation
goals, and that is to regulate or otherwise isolate the low power
front end of the amplifier, leaving the output stage looking at
an unregulated supply. This can be accomplished with entirely
separate supplies, active regulation, or with as little as two
resistors and two capacitors.
Another way to regulate is by using constant current sources,
which feed the circuit a constant current that does not fluctuate
with supply voltage. A good constant current source can improve
regulation for low power front end circuits by a factor of 100,
and combined with supply voltage regulation, gives really
excellent performance at little cost.
You can also bias the output stage with a constant high current
source to create a single- ended Class A amplifier. I’m not
The advantages of switching supplies revolve around low weight,
low material cost, and their ability to actively regulate at no
additional cost. Noise is a potential problem with switching
supplies, but is solvable by physically isolating and filtering
the supply, in other words, by spending money.
Simply put, switching supplies are a lot like conventional
supplies, with a power transformer, rectifiers, and power supply
capacitors. Switchers take advantage of the fact that the
requirements for transformer iron and supply capacitance are
inversely proportional to the frequency they are driven by.
Instead of the 60 Hz provided by the AC line, switching supplies
drive the transformer above 20 kHz, dramatically reducing the
necessary size of the transformer.
This can be a deep subject, but suffice it to say that I believe
that some of the same caveats apply to switching supplies as
linear regulators. Again, they should be rated far beyond the
nominal current requirements of the amplifier circuit,
particularly as the switchers I have seen usually degrade badly
beyond their ratings. Also, it helps if the power supply
capacitors before and after the switcher are very substantial.
This is typically not the case, since one of the primary
motivations to use switchers is to save money.
More sophisticated use of switching circuitry, such as Bob Carver’s
is more than I would care to tackle here, but you can certainly
get a lucid explanation from him.
We all know what Mono means, which is a one channel amplifier. Of
course, for a channel which does not have to share power
resources, it means an improvement, since in a given size box it
can have twice as much transformer and capacitor bank. The other
intent is to physically and electrically isolate each power
amplifier channel from every other, meeting only at the AC line,
and sometimes not even there. This way, whatever is happening on
one channel has minimal influence on the others.
Mono operation is very desirable in high end systems, but of
course it is expensive. A modest compromise is offered by
"dual-mono" operation, in which two channels share the
same chassis and power cord, but have separate transformers and
supply capacitors. This achieves much of the isolation desired at
Just about total isolation. Near zero noise. Costs a mint.
So what have we learned here? In general it takes big money to
buy the big hardware to make really good power amplifier
Some of the approaches commented on here result in only marginal
improvements, but they are measurable. It is not necessary when
contemplating these aspects of power supply design to get into a
debate of objective versus subjective performance. There is only
the issue of how much you are willing to invest in diminishing
Engineering being the science of compromise, each manufacturer
draws their own cost/benefit line, and it has been my experience
that most manufacturers are quite conscientious about this. The
degree of sophistication and massiveness of a supply has the
context of the price of the product, and your expectations should
be scaled accordingly.
As a consumer, you want the best sound you can get. You can
accomplish that through critical listening. As a secondary goal,
we all like to get what seems to be good hardware value, and we
want to know that that the manufacturer has actually put some
real money into the product which costs a small fortune. If you
can read the specs or look under the hood, the power supply,
being one of the most expensive parts of the amp, usually is a
good indicator. It should be the biggest and heaviest part of the
What if you don’t want to go through the trouble but still
want your money’s worth? Get at least 15 pounds of amplifier
for each thousand dollars spent.