Audio Amplifier Power Supplies
voltage would dip very little, even when
fully loaded. This can be difficult or costly
to achieve, so some compromise may have
to be made. One solution is to regulate the
T he low power amplifier modules are
mostly aimed at car audio
applications by the manufacturers,
and are therefore designed to work well
supply using an IC or discrete regulator.
The drawback is the extra expense of the
regulator, and a larger supply is required to
counter the power lost in the regulator as
from a 12V D.C. power source. Obviously extra heat. For these reasons a regulator is
a car battery is a good low impedance, low not often used for high power amplifiers,
noise, high current supply, although some however you may consider this option for
care needs to be taken to avoid other noise lower power requirements, especially if
sources in an automotive environment. The mains hum is a problem.
main areas for the introduction of noise are
via the supply and earth leads, and ignition The current requirements for each kit are
radiation. Resistive spark plug leads and/or somewhat flexible. The continuous power
resistive spark plugs are usually sufficient available from the supply must be greater
to reduce ignition noise, and proper supply than the continuous power required from
leads will minimise noise currents. Car the amplifier, plus all the power lost
audio noise filters are readily available, but throughout the circuit, mostly as heat. In
it is often better to provide suitably large general you can not expect much more
supply cables directly from the battery to than 50 – 60% conversion efficiency for an
the amplifier and music source. Pay audio amplifier, so a continuous power
careful attention to grounding, do not output of 3 watts for example from a 12V
assume a wire placed under a nearby source would require about ½ Amp D.C.
bolthead will be sufficient. Make sure the supply, or 1A for stereo. Possibly more
amplifier and source earths are well depending on actual load. However the
connected, especially when the amplifier is peak currents will be higher still, and need
located some distance from the source. to be allowed for. The initial charging
current for large values of capacitance can
These general points still apply when be a major problem. Care is required to
using a mains supply, however there are a make sure the rectifier can handle the short
number of other considerations you need term peak current, as well as the
to consider. A transformer and rectifier transformer and any fuses. However the
combination has a very large ripple average current is usually less of a
voltage. The peak value must be below the problem due to the inherent nature of
absolute maximum voltage stated for the music. The average power necessary to
module. Remember that a transformer drive a speaker will be in the order of
output will be higher than it’s rated AC 1/10th the peak value, if clipping is to be
voltage when it is supplying much less avoided. And clipping Should be avoided
than it’s rated current. Also the rectified at all costs. Not only will the distortion
peak value will be approximately 1.4 times sound bad, but the compression caused can
the specified AC voltage. To reduce the lead to tweeter damage if sustained.
ripple voltage, a suitably large value of Remember that most tweeters can handle
capacitance must be added to the rectifier only a few watts of real continuous power.
output. The no load peak voltage will still Fortunately they are not usually required to
be the same, however the capacitor will handle any more. For these reasons one
maintain the voltage at the point where the must be very careful when testing using
transformer output dips to zero. If the continuous waveforms rather than music.
capacitor was sufficiently large, the
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Audio Amplifier Power Supplies
A suitable low cost power supply for 12V Transformers are often specified as the
modules can be made from a car battery “VA” rating, this is the output volts
charger. These are often available more multiplied by the output current. If the
cheaply than a similar sized transformer above transformer was rated as 100VA,
alone. The common 4A types could power then it’s output would be 50V at 2A for the
a pair of modules like our kit 3143. It is series connection. This will also give us
important to check however that full wave two 25V supplies, at 2A each. The A.C.
rectification is used, or hum will surely be output then needs to be rectified and
evident. It will also be necessary to add a filtered to provide us with the necessary
large enough value of capacitance, as this D.C. supply “rails” our amplifier requires
is unnecessary for battery charging to operate. Because the capacitors charge
purposes, and thus not usually included. to the peak voltage of the rectified sine
The peak no load voltage will certainly be wave, this is approximately 1.4 times the
much greater than 12V, however this specified AC “RMS” value. Thus our 25V
should not be a problem as long as it is A.C. supplies, are now approximately 35V
below the maximum voltage given for the D.C. supplies. This will be under no load
kit you are building. This is usually 18V or conditions, and drop a little under full load
more for most modules designed for car current as we have previously mentioned.
audio applications. Check the data sheets
first and measure your power supply with When dealing with mains transformers, we
a voltmeter if possible, to be sure. also need to be careful of safety
Remember that a car battery will actually requirements. This will usually mean
reach 13.8V during charging, such as when providing a suitable earth to any metal
driving. For this reason power output is case, providing a suitable fuse and switch
usually quoted at 13.8V rather than 12V to the transformer primary windings, and
for car audio. making sure all wires and connections are
properly insulated. When choosing a fuse,
High power amplifiers are usually make sure it is large enough to allow for
designed to work from a positive and surge currents when the amplifier is first
negative supply. One big advantage of this switched on, however it should be small
is that the output will be zero volts DC enough to provide some overload
with respect to ground, and thus no large protection in the event of circuit failure.
DC coupling capacitors are required. Few
extra components are required to provide Make sure all diodes can handle both the
both supplies, however a mains peak voltages and peak currents expected,
transformer with a centre tapped output is not just the normal values.
required. These are very common, but Similarly the capacitors must have a rating
make sure you know what the output higher than the peak no load D.C. voltage
voltage really is when purchasing. A expected. A suitable safety margin is
transformer may be specified as 50V prudent in both cases.
centre tapped, or 25V-0-25V, for example.
The output current may be given for both The circuits shown below should give you
series and parallel connection of the output a good idea of what is required, and how to
windings. If you are using both windings connect the components for both a single
as separate positive and negative supplies, supply, and a “split rail” dual supply.
then the lower current figure applies, i.e.
series connection.
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Audio Amplifier Power Supplies
Standard circuit for a mains derived single rail power supply
.
Switch 1a Fuse 4 * Diodes or Diode Bridge
A
Mains
I/P
+
+ Lage
N
Capacitor
Switch 1b Transformer
E
_
.
Standard circuit for a mains derived split rail power supply
Switch 1a Fuse 4 * Diodes or Diode Bridge
A
Mains
I/P + +
N Gnd
Switch 1b Transformer
+
(centre tapped)
E
_
* All grounds must connect at the same point .
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