This amplifier is very simple to build, uses commonly available parts and is stable and reliable, this amplifier most were operated as small PA or instrument amps, but many also found their way into home hi-fi systems. The amp is capable of driving 4 Ohms, but it is starting to push the limits of the transistors, however, even when used at 4 Ohms, very few failures were encountered.
Note:
- The resistor values are not too critical, but if 1/2W metal film resistors are used throughout, this will help to reduce noise.
- The 0.5 Ohm resistors need to be 5W wirewound types.
If you want to build it, see P3A, Layout is not especially critical, and in fact if the components are laid out on a board much as they are seen in the diagram, you should have no problems. 3 Amp fuses should be fitted to each supply rail - these will not prevent output transistors from failing with a shorted speaker lead, but they will prevent further damage (wiring melting, transformer burning out, PCB catching on fire, etc).
The input capacitor should be a polyester type. If an electrolytic is to be used, the positive end goes to the amplifier (there is about +230mV on the bases of the long tailed pair transistors).
When wiring, ensure that the feedback connection is taken from the speaker output terminal, immediately before the inductor. Any track which is carrying half-wave audio from one or the other power transistor resistors will cause distortion of the feedback signal, degrading sound quality.