Hi all,

I'm running into a problem. I made the following schematic. When I touch the DIRT potentiometer I hear a continuous humming sound (GND problem?). I can't figure out what's causing it. I have already replaced the DIRT potentiometer, C2 and Q1.  No effect. Everything else works properly and it sounds like it should be.



Does anyone have an idea what could be going on?

Sophia

By "touching" you mean wiper rotation or pots housing (outer casing)..??

Out of curiosity, what's the purpose of R1..??
(other than alter pot's track..)

Both, wiper and pots housing.

When I get close to the potentiometer with my fingers I already hear a hum coming up. By touching it it becomes a lot more intense. I also notice the same hum (when approaching) around C2 and Q1 but the worst is the potentiometer. Does this explanation help to clarify my problem?

The problem was present on a pcb. I then copied the schematic on a breadboard and there I experience exactly the same problem. Could it be a wrong component choice or is there a mistake in the schematic?

R1 is a pulldown resistor?

Sophia 

Is it in a box yet? Maybe it just needs the pot housing to be grounded.

Pull down resistor goes the other side of C1.

The order that C1, R1 are drawn is irrelevant: both drawings would be equivalent.  In any case, R1 is superfluous, since the pot and R2 provide a pull-down path to ground?  (I think R1 was put there out of habit.)

Quote from: idy on May 06, 2025, 09:48:43 PMthe other side of C1.

If you're referring to R1, it's in parallel with C1. There is no "side".
PS: what bluebunny said.

Quote from: Sophia2001 on May 06, 2025, 05:20:42 PMon a pcb [...] on a breadboard

The pot body definitely needs to be grounded to prevent this. Otherwise you're injecting all the noise radiated by your body (a big salty water antenna) into the input of a high-ish gain circuit. When it's bolted to a conductive box it will usually be grounded through the jacks' sleeve contacts, but on a floating PCB or breadboard it is not.