Years ago (1970's),
Just a fun post its a shame someone doesn't make these modules..
Classic quasi complementary output stage
Note in the simulation additional resistors in the emitters of the power transistors. (0.22 ohms) stability improvement.
I used TIP41a in the 70's YMMV. Input cap is 2.2uF.
Note the output capacitor between the speaker and amp module. It has been suggested to increase the value of the capacitor as shown on the back of the box for the module.
A company called Bi-pak made modules (pic)
As an early audio (phool) I bought a couple of these and connected them to some speakers and a record deck.
It was a pretty good experience you just bolted them to a heat sink even the chassis was good enough.
So after trawling the net and with lots of help the circuit was identified from an Ebay sale (pic)
Here is the circuit for any intrepid builders.
Mooly did a simulation as well, the THD was a shock this is what I was listening to at age 15.
I thought it might be interesting for guys here, I have posted the same info in other places.
Special thanks to Biggles and Mooly.