This is a very simple, low cost, Hi-Fi quality power amplifier. You can build it 5 ways, Please follow the table (from 20 W to 80 W RMS).
Description:
The first thing that you must do, is to measure the end transistors (T3 and T4) amplifying coefficient, the hfe or ?. If their disagreement is bigger than 30 %, the amplifier would not give a clear sound. I used MJ3001 and MJ2501 transistors, and this disagreement was around 5%.
Before the first ?turning on? you must short circuit the inputs of the amp, and put a mA-meter on the output, than turn the amplifier on, and tune the R13 pot, to decrease the DC current on the output, to some uA-s, or in a lucky situation to zero.
It gives a very good sound quality.
Have fun with it!