Output Cap Question

[Greek Acrobat]

Just beginning to understand the necessity for coupling caps. A practical problem with an output cap though...

I'm making a pedal with lots of gain and the voltage from the end of the ouput cap to ground is about 4.5 - 5.5 V. I understand this is supposed to be zero (?). How do I remove this voltage leakage? Increase the value of the output cap?

I've tried 220uF to no avail, I think I don't have any higher than that to hand.

[petemoore]

My outcaps range generally between .01uf and 2.2uf, 4.7uf being a rare exception.
 Right...no DC should be leaking past the DC coupling caps [input and output caps].
 This problem could be cause by something else?

[Arno van der Heijden]

There shouldn't be any *DC* voltage after the output cap.  You may have a faulty cap if that's the case.

[Greek Acrobat]

If it's caused by something else then what could it be? Somehow DC is getting from in front of the cap to the back of the cap, so I guess it must be a capacitor thing. Unless I'm measuring those voltages from the wrong places (?).

I tried different caps and none solved the problem so the faulty caps idea is probably unlikely but still the most likely.  :wink:  I'll go mess with more caps, see if it can't be fixed.

Oh, and capacitance adds up with caps in parallel right? Standard grade physics was a while ago.

[R.G.]

All caps leak a little. Tie a 1M resistor from the outboard side of the output cap to ground. If that does not reduce the voltage from the outside of the output cap to ground to zero, then you have either a bad cap or a short of some kind allowing DC to leak through.

This is also the "why" for input and output pulldown resistors preventing clicks.

[Greek Acrobat]

The tie down resistor didn't work fully - reduced the voltage to about 0.3 V. I tried using a non=electrolytic cap and problem solved.

Which is interesting. Do electrolytics go bad easily? What kind of conditions can they survive? They have been sitting in my garage all winter so they could have been exposed to sub-zero temps if that makes a difference?

[nirvanas silence]

My only idea is that maybe the polarized cap was in backwards?