help with build

[Marc.yo]

ok, i've got a problem. when i adjust the volume control on my pedal, there is a LOUD crackling noise. i am using LM741 with the whole diodes in the feedback loop and such so i am wondering (hoping) that its the LM741. any ideas?

[GibsonGM]

Crackling is mainly caused by a bad pot, or DC riding along with the audio signal.   Check across the pot for DC, could be a cap missing before it?  I would suspect that before the 741.  Swap it out if you can just to eliminate that as a problem.   Also measure the output for DC (just measure across a patch cord plugged into the output).   No DC should be at the output!

It would be helpful to know what pedal it is, and have a link to a schematic!  Is it a Dist +?

[Marc.yo]

i don't have a schematic right now as i can't get at a designer here's the functional parts tho, if it helps


its an op-amp based fuzz that goes like this


jfet booster - .1uf cap - inverting op amp with diodes in the negative feedback and 470k resistor and 1Meg pot -> high cut -> 100k volume control

voltage divider using 100k resistors, feeding as vref into the + input of the op-amp, and a 100uf cap running parallel with the vref to ground

and i tried using a TL071, its still there but its reduced


thats all i can give for now

[petemoore]

  It's a matter of biasing the opamp then providing the details involved with DC blocking and creating a signal path.

[Marc.yo]

ok, i assume that i'll be using a cap for dc blocking? what size and where and in series or parallel to ground.

[GibsonGM]

Try a 1uF in series right before the 100K volume pot.  See if it changes the tone too much...go up or down from there....but it should cut the crackle right off, and you'll have your answer...

[Marc.yo]

k. thanks man. i'll try that tonight