the pot in this schematic crackles only when it sees a signal, any ideas?

[iandy4]

Hi Guys,
I made a circuit based on R.G.'s splitter/blender.  I thought I had it working perfectly accept just now when I installed the pot of the circuit into a foot pedal I noticed that the pot slightly crackles but ONLY when the guitar signal is present.  The pot is completely quiet when there is no signal from the guitar (while rocking the foot pedal back and forth).  Any idea what is causing this?  I figured it has something to do with d.c. voltage across the pot.

[petemoore]

d.c. voltage across the pot.
  Looks like DC staging caps aren't included/[needed?], also that some DC might indeed going through the pot.
   Whether that's it or not, since it's verified layout and all..should work.
  Scritchy pot action''ll do that too though.

[R.G.]

Hmmm. Don't know off hand. Just thinking out loud.

You changed to a balanced bipolar power supply. The cap on the wiper should not be necessary.

IS there any DC across the pot from any to any pin?

The + input on IC1B should have a large-value resistor pulling it to ground. Maybe this is putting DC on the pot when you put in signal.

C2 is probably not needed with a bipolar supply. Check with a meter whether the outputs of IC1A and IC1B are at nearly 0V. If they are, you can remove C2 and substitute a short to ground. The other caps should remain there for "circuit hygene" reasons.

Is your rocker pot old? Old pots in rockers are often scratchy, which actually means they make and lose contact between the wiper and the resistive element as you move them. It takes a voltage difference to make this crackle. In  the absence of a DC voltage, some of them will crackle with signal voltage.

But I don't see any obviously smoking gun.

[PRR]

[iandy4]

wow... Ok, I spent about about two hours taking measurements of before and after the modifications you guys suggested.  The last thing I tried was another pot.  It was the pot! I had figured that the crackling sound of a bad pot would always be present no matter what but it was definitely the pot.  Thanks guys.  Sorry, that should have been the first thing a checked!

[MJG]

I agree with PRR.  You need a high-value resistor (1M or so) going from between the cap and the "+" input to ground.  The opamp input bias current, while miniscule, will gradually build up a bias voltage that will be very mysterious when it appears.

Just my tuppence.

[iandy4]

Ok, I will definitely add that resistor.  Thanks for the heads up!  By the way, contact spray cleared the crackle right up.