Circuit
This circuit shares a higher voltage version of the voltage gain stage from the previous
amplifier. The output stage is beefed up with the output devices being parallelled up, and also a pair acting
as sort of floating halfway power supplies, which follow an attenuated version of the output. This has
two effects, it obviously doubles up the output current available. The series transistors however have the effect
of increasing the safe current handling from 500mA to 5Amps per output device with a 50 volt rail. This
is a good quality for the amplifier, and makes it much more robust, being able to safely deliver 20 times
more current in to a short circuit than the previous amplifier, so much so that a sustained short circuit blows
the power input fuses, rather than the output devices!
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| So what effect does this have on sound quality? This amplifier sounds above all more powerful
and effortless, Even with the little Tandy speakers, with such small cones, you can still hear the power and
grip that this amp has. All the Big speaker boys that tried it loved it too!. Why? Good question. It can only
be down to the output stage. I guess the extra voltage head room is nice, doubling up the output tranies
should half the open loop output impedance which may help. With Bipolar transistors it is typical for the hfe
(current gain) to rise up to a certain current and then start to drop off as the current increases still
further, which in turn starts to stress the driver stage more. Doubling up the output helps out here to,
delaying this effect. The tracking power supplies for the out put drivers can only have a positive effect
providing a more stable environment. I am sure there must be other interesting things going on as well......
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The completed amplifier has 2 output modules and a passive preamp board. This picture is of
one channel. As can be seen it has 8 power transistors, which share the load.