It should work now, jasnc.

 

ok i opened it

 

You sure started a popular thread here, jasnc!

 

Thanks PRS for your help!!...i've got a strong hissing now on the speaker..how can i eliminate that??...also if one of the transistors of the push-pull gets burnt is it supposed that the other one gets burnt too?

 

No, if one is hotter than the other you probably have a biasing problem. Your voltage between the 4 diodes and the two 10 k resistors should be 12 volts. This is by inspection and due to circuit symmetry. Also, the voltage at the junction of your emitter resisters should be 12 volts. The voltage of the top xistor base should be 12+.6+.6=13.2 volts and the lower xistor should have a base voltage of -13.2 volts. If these voltages are not right, you need to do some more investigation to make sure every component is healthy and doing what it's supposed to do.

As for the hissing, it's probably due to ripple voltage on your supply rails. This voltage arises due to the high current being drawn from your supply. It's 120 hertz and has a sawtooth shape. Adjust your oscilloscope to sensitive setting and look at your supply rails with the scope ac coupled.

What happens is that your voltage amp picks up this signal and amplifies it, sending it to the output. The way to deal with it is to decouple your voltage amps from the main supply. I think I need to draw this. The idea is to make a voltage divider from a capacitor and a resistor. I shall draw it.....

 

The decoupling is via voltage divider action:

 

the hissing arises when i plug in the signal...

 

Sounds like an oscillation problem then. It will be at a higher frequency, but the fix is still to decouple your voltage amps.

 

the problem persisted..

 

the problem seems to come from the power stage...cause when i quit the power stage and the take output from last pre-amp...the hiss disappears..