If you have been working with digital circuits and
processors, you are probably used to working with five
volt power supplies and know the basic precautions to
take. This amplifier will have about 35 volts from heatsink
to heatsink — enough for a fair shock if you have wet or
sweaty hands. So, don’t touch both heatsinks at once, in
particular one with each hand!
Also, don’t work with a ring or metal wristband on.
Filter capacitors hold considerable energy and getting a
metal band across one might cause considerable heating.
Always wrap the fuse and power switch connections with
tape or use heat shrink tubing on them before applying
any power. This at least limits your exposure to only the
AC from the transformer and the DC power supply output
Parts And Assembly
You may question the use of carbon film resistors
since metal film ones are quieter. This amplifier checks out
at 105 dB signal to noise. To obtain 12 watts out, we will
use a 25 volt CT transformer rated at two amps. You can
increase the power output to 30 watts by using a higher
voltage and current power transformer. We’ll discuss this
later. The amplifier in Figure 1 is a 30 watt version.
The power transistors — leads down, label facing you
— have the base, collector, and emitter in that order. The
small signal transistors — flat facing you and leads down —
are emitter, base, and collector in that order.
Heatsinks might come with a graphite impregnated
fiberglass backing. This is a heat conducting layer that
conforms to the transistor mounting surface and is NOT
an insulator. With it, you don’t need heatsink compound.
Be sure to debur the hole at the mounting
Sanken transistors have a mounting hole
that will clear a 4-40 screw or an M2.5
metric. The plastic case is extremely hard (I
suspect it might be ceramic). I tried to drill
out the hole just a little for a 6-32 screw and
the case cracked; there was no damage to
the working parts, it just doesn’t look very
nice. I used a separate heatsink for each of
the two output transistors.
The bias adjust network has the sensing
transistor connected on leads as short as
possible and the transistor — a Phillips BD139
or a 2SD669 — is mounted stacked with one
of the output transistors, with the mounting
surface to the face of the output transistors.
Point the leads in the same direction as
those of the power transistor. Choose the one
that allows the shortest leads. Heatsink
compound between the two is desirable. Use sleeving on
the three leads of the transistor.
■ FIGURE 1. Power amplifier in wood case.
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