Power Amplifier
The power amp board has remained
unchanged since it was first published in 2002. It certainly isn't broken, so
there's no reason to fix it. The photo below shows a fully assembled board.
Using TIP35/36C transistors, the output stage is deliberately massive overkill.
This ensures reliability under the most arduous stage conditions. No amplifier
can be made immune from everything, but this does come close.
The power amp (like the previous
version) is loosely based on the 60 Watt amp previously published (Project 03),
but it has increased gain to match the preamp. Other modifications include the
short circuit protection - the two little groups of components next to the bias
diodes (D2 and D3). This new version is not massively different from the
original, but has adjustable bias, and is designed to provide a "constant
current" (i.e. high impedance) output to the speakers - this is achieved
using R23 and R26. Note that with this arrangement, the gain will change
depending on the load impedance, with lower impedances giving lower power amp
gain. This is not a problem, so may safely be ignored.
Should the output be shorted, the
constant current output characteristic will provide an initial level of
protection, but is not completely foolproof. The short circuit protection will
limit the output current to a relatively safe level, but a sustained short will
cause the output transistors to fail if the amp is driven hard. The protection
is designed not to operate under normal conditions, but will limit the
peak output current to about 8.5 Amps. Under these conditions, the internal
fuses (or the output transistors) will probably blow if the short is not
detected in time.
Figure shows the power amp PCB
components - except for R26 which does not mount on the board. See figure to
see where this should be physically mounted. The bias current is adjustable,
and should be set for about 25mA quiescent current (more on this later). The
recommendation for power transistors has been changed to higher power devices.
This will give improved reliability under sustained heavy usage.
As shown, the power transistors will
have an easy time driving any load down to 4 ohms. If you don't use the PCB (or
are happy to mount power transistors off the board), you can use TO3
transistors for the output stage. MJ15003/4 transistors are very high power,
and will run cooler because of the TO-3 casing (lower thermal resistance).
Beware of counterfeits though! There are many other high power transistors that
can be used, and the amp is quite tolerant of substitutes (as long as their
ratings are at least equal to the devices shown). The PCB can accommodate
Toshiba or Motorola 150W flat-pack power transistors with relative ease - if you
wanted to go that way. TIP3055/2966 or MJE3055/2955 can also be used for light
or ordinary duty.
At the input end there is provision
for an auxiliary output, and an input. The latter is switched by the jack, so
you can use the "Out" and "In" connections for an external
effects unit. Alternatively, the input jack can be used to connect an external
preamp to the power amp, disconnecting the preamp.
The speaker connections allow up to
two 8 Ohm speaker cabinets (giving 4 Ohms). Do not use less than 4 ohm loads on
this amplifier - it is not designed for it, and will not give reliable service!
All the low value (i.e. 0.1 and 0.22
ohm) resistors must be rated at 5W. The two 0.22 ohm resistors will get quite
warm, so mount them away from other components. Needless to say, I recommend
using the PCB, as this has been designed for optimum performance, and the amp
gives a very good account of itself. So good in fact, that it can also be used
as a hi-fi amp, and it sounds excellent. If you were to use the amp for hi-fi,
the bias current should be increased to 50mA. Ideally, you would use better
(faster / more linear) output transistors as well, but even with those
specified the amp performs very well indeed. This is largely because they are run
at relatively low power, and the severe non-linearity effects one would expect
with only two transistors do not occur because of the parallel output stage.
Make sure that the bias transistor
is attached to one of the drivers (the PCB is laid out to make this easy to
do). A small quantity of heatsink compound and a cable tie will do the job
well. The diodes are there to protect the amp from catastrophic failure should
the bias servo be incorrectly wired (or set for maximum current). All diodes
should be 1N4001 (or 1N400? - anything in the 1N400x range is fine). A heatsink
is not needed for any of the driver transistors.
The life of a guitar amp is a hard
one, and I suggest that you use the largest heatsink you can afford, since it
is very common to have elevated temperatures on stage (mainly due to all the
lighting), and this reduces the safety margin that normally applies for
domestic equipment. The heatsink should be rated at 0.5° C/Watt to allow for
worst case long term operation at up to 40°C (this is not uncommon on stage).
Make sure that the speaker
connectors are isolated from the chassis, to keep the integrity of the earth
isolation components in the power supply, and to ensure that the high impedance
output is maintained.