Explanation of the circuit:
Class A amplifier to amplify the voltage
OP amp to amplify the current
Sallen-Key filter to cutoff certain frequency
Inverting amplifier to change the negative voltage to positive
Optocoupler to vary the brightness depending on how loud the sound is
FET to drive the large amount of LED's I have.
I designed this at school but had limited time so getting it working was the most important goal.
The signal was a near perfect replication of the line-level audio with a gain of 10, until the lowpass filter.
The lowpass filter seemed to really mess up the signal.
Using a scope, the output with no signal was almost the applied voltage. With a low frequency, the negative peaks would dip down to about zero, with no positive alternation. We couldn't figure out why this was happening, so we just fixed the problem and put this signal into an inverting amplifier.
Also, I have a large capacitor across the bus. When I disconnect power, short the busses, and reconnect power, it still will not work. I have to disconnect power and wait 1-2 minutes to reconnect power for it to work again.
I am using a CMOS LM324 OP amp, so the idea that a voltage on one of the OP amp's input sounds like the problem, as I don't know of any capacitor on the circuit that would be slowly charging without discharging, except maybe one of the coupling capacitors? Anyway, thanks for your help and I will appreciate any more help you are willing to give.
Also, for doing projects like this and maybe some radio broadcasting circuits, how important is a oscilliscope? I would really like to have one, but can't really jusitfy spending $300 for a hobby.