Discrete Audio Amp from Short Circuits Volume 2
This discrete transistor audio amp is from the 2nd volume of Short Circuits, project 9, page 64:
The Short Circuits books were mentioned before: https://wb9kzy.blogspot.com/2025/12/short-circuits.html
Volume 2 pdf: https://assets.contentstack.io/v3/assets/blt7eae2e8ef79b5522/bltd61d4842c9bc12a7/BJ8504-ShortCircuits-2-310316.pdf?branch=prod
This project caught my eye as a potential LM386 killer:
It's shown operating from a 9V battery
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| the board is nice but not not required |
Presumably this means that the standby current is low although it's not mentioned - the LM386 is spec'd at 8 mA @ 6 volt supply, 4 mA typical. The builder just adjusts a bias pot by ear to reduce distortion. The voltage gain is quoted as 32 which is a little higher than an unboosted LM386 (gain of 20) which with the feedback bypass can deliver a gain of 200.
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| more complicated than an LM386 circuit but not that bad |
A heatsink is recommended for running at 12 volts, it appears it's more about biasing than heat disapation. The "amplified diode" bias transistor, Q3 is clamped to the heat sink to allow it to be at roughly the same temperature as the two output transistors.
I was thinking that such a clamping arrangement might also work for something like a discrete NE602 mixer circuit. One big virtue of an IC bipolar transistor array is that the gains of the transistors are matched. BUT with that IC, all of the transistors are at roughly the same temperature - for a discrete version with T0-92 transistors the clamped type heat sink might help to insure that the discrete transistors are at roughly the same temperature.
I will have to see if I've got all the parts for this amp to try it out. I suppose one of the "cool kids" would simulate the circuit first but what fun is that ? No one has ever let out the smoke or burned a finger on a simulated circuit in LTSpice !
Best Regards,
Chuck, WB9KZY
http://wb9kzy.com/ham.htm
