With a 1k load the darlingtons have such a low current that they do not even get warm.
But with an 8 ohm load the darlingtons are passing a high current which causes them to heat which causes then to conduct more current from the positive supply to ground which causes them to heat more - more current - more heat - more current - more heat ..... blow up.

Why do you drive the darlingtons with three emitter-follower transistors in series? Emitter-followers don't have any voltage gain.

 

Congratulations on getting it to work, Carib, but where are you getting your voltag gain? You said you were going from 50mV to 50 volts, but I see no voltage gain at all. Like Audioguru said, you have 3 emitter followers in series. That means attenuation, not voltage gain. Plus 3 followers in a row make no sense. Care to explain?

 

One last thing. Your hum is coming from your supply. Since it is unregulated, it suffers from ripple voltage. This appears as a 120 Hz sawtooth wave riding your supply voltage. The more current you draw from the transformer the worse this condition gets.

Most amplifiers use unregulated supply voltage only for the output amp since it has unity gain at most. This is because as you turn up the volume the music or whatever drowns out the hum. But at lower levels the hum is irritating. For this reason you need to decouple your voltage amplifiers from the raw voltage supply. One way is to use a resistor and a capacitor as a voltage divider, another is to use zeners with your voltage amps and another is to use regulators.

 

HI guys, the push pull circuit is working perfectly and even i maneged to reduce the hum, the last problem is that i have put the circuit on a veroboard, and dont know why the hell all stages are clipping at the positive cycle, especially when i connect the stages together . for example the first stage amplifies correctly but when i connect the second stagem it clips at the positive cylce at the output of the first stage! it worked correctly on the breadboard, dont know why this happened and i rellay could nt find whats going on

 

Was each stage connected to the next stage on your breadboard? Or did you just now connect them all together for the first time?

 

Hi guys, I located the problem , it just was a shorted coupling capacitor, BY ANY CHANCE WHAT CAN I DO IN ORDER TO AVOID CLIPPING WHEN I CONNECT THE POWER STAGE TO THE FINAL VA STAGE PLEASE?

tHE VOLTAGE GAIN STAGES ARE WORKING PERFECTLY BUT WHEN I CONNECT THE PUSH PULL STAGE THE CLIP AT 15V PEAK TO PEAK FROM 60V PEAK TO PEAK

 

Try increasing the current in you bias circuit to about 50mA. You have 5mA and this may not be sufficient. For 50mA use 1k resistors.