Little help for beginner?

[hooman]

Hi all
When I switch off the ampli the woofer of the speaker oscillate badly for few seconds.this happen only on one channel. .

when we remove the couplage cap between driver and output stage in fact we remove the feed back .
this problem happen only in one channel .....so how e can find solution in circuit changing ?

 

[hooman]

but best chang in circuit is self bias to fixed bias .if kt88 has problem can solve by capacitor change in negative voltage .when we tern off switch main voltage go down but negative voltage on g1 can send kt88 to cut off zone .no vibration speaker .

 

[ciccio64]

hi all
Just to thank all that try to help
This afternoon I check all component and they were all good
Reducing the feedback did solve the problem
Remain still strange to me why only on one channel
Anyway thanks again

 

[DF96]

As I said, component tolerance. If you measured it, you would probably find a subsonic peak in both channels but higher in the one which oscillates.

 

[hooman]

hi all
Just to thank all that try to help
This afternoon I check all component and they were all good
Reducing the feedback did solve the problem
Remain still strange to me why only on one channel
Anyway thanks again

sorry just carefully i looked at schematic !!!!! the 100 mfd in feed back !!!!

 

[hooman]

i never seen 100mfd in feedback.... can oscillate in output .maybi another quit channel has problem.

 

[waltube]

He said the the cap is not installed


Walter

 

[ciccio64]

There is no capito in the nfb network.
And May i should have one. Probably with a correct site capacitor There this oscillation would not occour
Problem is to find the right value require a lot of test and i have no time right now as going to work. Project for winter time
Thank you

 

[waltube]

No cap!!!
In that position you need only the resistor and , in case, a little cap for compensation but you must have a generator and scope. ( 150-330 pF, p.e.)
In this way you can linearize the frequency answer if there are resonance at high frequency

Walter