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Motorola Hi-Fi power amplifier

This is a very simple, low cost, Hi-Fi quality power amplifier. You can build it 5 ways, like it’s shown in the table (from 20 W to 80 W RMS).

Some comments:
- The first thing that you must do, is to measure the end transistors (T3 and T4) amplifying coefficient, the hfe or в. 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. I was able to decrease it to 10 uA.


I am very contented with this amplifier. It gives a very good sound quality.
Have fun with it!

Jacint Chapo
6. 5. 2005.


Jackson Teneng-von D 2019-12-22 15:27:55
Thanks so much for the schematic and explanation. I have a question:
1- The data sheet states a maximum supply voltage of +- 18V. Why do make an option for voltages higher than +- 18V? e.g. in case of 80W/125W you state a supply voltage of +-22V
2- What would be the value for R13, before tuning to get a suitable value that would correspond to the said low uA in the output?

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jorge loarte 2015-02-07 00:51:14
I find very interesting the diagram I will also try to build sound quality if trubieran a diagram tone control to apply to this amplifier thank you very much your friend peru jorge loarte

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