02-06-2021, 07:13 AM
Ahh, I see. Trying to analyse this circuit has had me scunnered, especially trying to figure out where ground is sometimes. so you see low Z and think it's effectively ground? That makes sense.
That 0.22u cap has been bothering me,but this is the problem here,trying not to use too many coupling caps to block DC. Moving the 0.22u to the IP CF is a good idea. I encountered the same problem yesterday trying to move the vol pot from the IP to after the tone stack/before gain stage. I'll come to that later, but first off I wondered if we could loose the 2nd OP CF? Maybe not,we need low Z to "drive" the tone stack. I tried loosing the IP CF, that loose us a bit of gain. Without the IP CF I measured a drop of about 200mV. Just thinking out loud.
As mentioned I moved the vol pot yesterday and a tone cut SW, to SW out the tone stack, the amp oscillated, but switching the tone stack back in oscillation stopped! : ) Oh the Gods do like a laugh. This reinforces my suspicion that the power supply is resonant at very low frequencies, I'm going to go back and do a few sums and see if I can solve that.
Andy.
That 0.22u cap has been bothering me,but this is the problem here,trying not to use too many coupling caps to block DC. Moving the 0.22u to the IP CF is a good idea. I encountered the same problem yesterday trying to move the vol pot from the IP to after the tone stack/before gain stage. I'll come to that later, but first off I wondered if we could loose the 2nd OP CF? Maybe not,we need low Z to "drive" the tone stack. I tried loosing the IP CF, that loose us a bit of gain. Without the IP CF I measured a drop of about 200mV. Just thinking out loud.
As mentioned I moved the vol pot yesterday and a tone cut SW, to SW out the tone stack, the amp oscillated, but switching the tone stack back in oscillation stopped! : ) Oh the Gods do like a laugh. This reinforces my suspicion that the power supply is resonant at very low frequencies, I'm going to go back and do a few sums and see if I can solve that.
Andy.
