Why is my Fuzz causing my vol. knob to make noise when turned?

[GuitarMatt]

I'm prototyping a Fuzz Face-type circuit on my breadboard, but when I turn the volume knob down, it crackles and hisses. I've tried modifying some component values but no luck yet. And no, it's not just a noisy volume knob independent of the effect; I only notice this noise when going through this effect. Any suggestions?

Schematic for reference:

[patrick398]

Sounds like there's DC present, do you have an input cap, is it the right way round?

[GuitarMatt]

 I do have an input cap, but it's a non-polarized ceramic capacitor of about 0.2 micro. (I know it differs from the circuit I posted, It's used in the Vox ToneBebender which is the same as the Fuzz Face, but with slightly modified values.)

This might be more accurate to what I've built:

[ElectricDruid]

I agree with Patrick - it sounds like there's DC present. If it's the volume knob that's crackling, then it's the output cap that's more important. That's the one that blocks DC from getting onto that 50K pot. Have you got that 0.0047u/4n7 cap in there? Is it correctly soldered or could there be a short across it somewhere? Or across the tracks that connect to it? Or even a bad cap?

If it's in place and there's no obvious DC across the pot, then honestly I don't know what's going on!

Tom

[GuitarMatt]

I should clarify: It's when I turn my guitar's volume knob up or down when I hear the crackling, not the pedal's output knob. I have checked both the input and output caps and they are properly connected and of the correct value...

Edit: So I just discovered that when I take out the 1M pulldown resistor before the input cap (in blue on the Vox TB schematic) the noise goes away and my guitar's vol knob acts as normal. But it's not a fault in the resistor, because other resistors do this too. Why? 

[ElectricDruid]

Oh, ok, I see. I misunderstood.

Humm, that *is* odd then. Is there any way the 9V power could get to the guitar's pot? Obviously one end is connected via the ground (positive ground effect, so to +9V in this pedal) but the other end should be blocked by the input cap. If that cap were bad or shorted, the guitar pot would become directly connected to the circuit and would have 9V across the section you were using.

[idy]

It is a positive ground circuit, and that "pull down resistor" is instead pulling the input up to 9v. Plug a cable into the input and see if you can measure some volts on the other end of the cable, tip to sleeve.

wait a minute, does this mean you can't use pull down R on positive ground? I never thought about it...

Normally you would check if the capacitor is leaking. But that would be worse without the R. Yours gets better so...

[willienillie]

wait a minute, does this mean you can't use pull down R on positive ground?

Not at all.  It goes to ground just like in a neg ground circuit, no problem.

Plug a cable into the input and see if you can measure some volts on the other end of the cable, tip to sleeve.

Good plan.  I think something is just not wired correctly.  Pics might help.

[FiveseveN]

wait a minute, does this mean you can't use pull down R on positive ground?

It only means "positive" and "negative ground" are misleading terms. Is zero positive or negative?