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Try 100pF here. This capacitor is called a "Miller capacitor" and serves to make the amplifier stable. If the amplifier is still unstable try increasing the value, but over 220pF and something is wrong with your PCB layout. You may also be able to make it smaller and have the amplifier stable but 100pF is a safe value.
 
circuit singing

i try 100pf ,then this circuit can sing with no noise.
but ,when th input is open ,i touch the input with my finger , the bias still jump to high current , why this happened ? maybe my body became antenna , so seduce some RF to this circuit , make the circuit unstable . i dont know why , how to solve it. is it must that heatsink connected earth? thx
 
hi,john-china:
you can do same thing to your another amp,this will happen too,try it!
I think human body has a very low votage(static ),so when your finger touch the input end,it seems to input end accept the signal,
and you will heard the"hooter " from you lound speaker...

regards
WINCO
;)
 
C5 could be in the range of 47-100 pF . Bass performance may be improved by adjusting the C4 value. Try 470 uF 25-35 V there.

john-china said:
i try 100pf ,then this circuit can sing with no noise.
but ,when th input is open ,i touch the input with my finger , the bias still jump to high current , why this happened ? maybe my body became antenna , so seduce some RF to this circuit , make the circuit unstable . i dont know why , how to solve it. is it must that heatsink connected earth? thx

It would be useful to put a passive RC filter on the input to cut the HF at around 50-70 kHz. Try 4K7 and 470 pF.

Cheers

Alex

(x-pro)