I built up a 100 watt bi polar amplifier.
I fired it up and got loads of noise on the output.
So I had a look around and found the dc servo op amp was making the noise.
So I looked up the spec on it and I had accidently put in a 5 volt supply op amp on +/-12v. So I removed the op amp.
Powered up again and found I was getting a 1Hz square wave on the output.
I thought I had blown something up with the wrong op amp so checked all components on the pcb and they were fine.
I really couldn't make any sense of it.
Adding audio put the audio on top of the square wave so it looked like amp was working ok.
I checked the CCS's and they looked fine.
All dc volts looked right.
I then decided to pull an op amp from a previously working pcb.
It then worked fine.
I had a look over the circuit and realised that without the dc op amp in there was a positive feedback path from output through dc servo 1 meg resistor through integrating cap and back into the input of the amp.
This caused the 1Hz oscillation.
I fired it up and got loads of noise on the output.
So I had a look around and found the dc servo op amp was making the noise.
So I looked up the spec on it and I had accidently put in a 5 volt supply op amp on +/-12v. So I removed the op amp.
Powered up again and found I was getting a 1Hz square wave on the output.
I thought I had blown something up with the wrong op amp so checked all components on the pcb and they were fine.
I really couldn't make any sense of it.
Adding audio put the audio on top of the square wave so it looked like amp was working ok.
I checked the CCS's and they looked fine.
All dc volts looked right.
I then decided to pull an op amp from a previously working pcb.
It then worked fine.
I had a look over the circuit and realised that without the dc op amp in there was a positive feedback path from output through dc servo 1 meg resistor through integrating cap and back into the input of the amp.
This caused the 1Hz oscillation.