Here is another problem with my circuits, that I cannot solve.
I am working on analog active tone control – Bass, Volume and Treble with TL081/82 chips.
The reason I put volume control on the output of first TL chip is to reduce hum in the tone control ciruit by seting volume to zero while not playing music.
At first I thought that the hum comming from potentiometers was because of testing it on vero board and not on pcb so I designed a PCB.
Putting the circuit on a PCB was a little better, because the hum was reduced by GND polygon all around potentiometers and circuits. However, by touching the potentiometers with my hand I get a lot of hum and buzz on the output. When I was get close with my hand to the potentiometers, the buzz and hum starts to get stronger, moving the hand away, the hum and buzz reduces to little value and so I believe that the problem is with the potentiometers.
Then I decided to ground potentiometer by soldering the shield to ground and getting plastic covers on potentiometers; the hum and buzz was reduce to a minimal level even when touching it.
The idea was good for a little time, but when I put potentiometers in metal housing, then the all housing will be at ground potential.
So I unsoldered the potentiometers from ground and attached them in metal housing "see picture above" and the problem come back. When I touch the metal housing (not the potentiometers) buzz and hum start going even louder.
How can I solve this problem with potentiometers or hum in circuit? I've also tested and resoldered a lot of identical new potentiometers, but the buzz and hum does not go away.
Using potentiometers in plastic shield is not solution, because a lot of audio amplifiers on the market use the metal shield potentiometer and work perfectly.
Please help, I already spent a tons of hours solving that problem.
UPDATE
Hi everyone, I just resolder some components and test @Andy aka proposal:
The hum does not go away with these solution :/, the most noticeable are potenciometers for bass and treble. When I toch the potencimeters hum start go very loud, but when i tuch myself to the ground with other hand the hum go away.
For faster understanding, I uploaded hum on vocaroo, here is the link:
https://vocaroo.com/i/s1oMDxBu4z3h
Best Answer
This is not a sensible approach for a buffer amplifier: -
The pot's wiper directly connects to the virtual earth point of the inverting amplifier and this is asking for noise problems. At the very least you need to insert a resistor in line with the wiper of the pot.
A better method is to have the wiper feeding the non-inverting terminal of the op-amp and having a feedback resistor and grounding resistor like this: -