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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.

enter image description here

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.

enter image description here

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.

enter image description here

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: enter image description here

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

mrkefca
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1 Answers1

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This is not a sensible approach for a buffer amplifier: -

enter image description here

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: -

enter image description here

Andy aka
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  • Hi @Andy aka thanks for help. Circuit like this: https://i.stack.imgur.com/6zsns.jpg Will fix the problem? – mrkefca Jun 19 '18 at 13:54
  • I can't say definitively it will fix the problem because the noise/hum may have arisen due to something unmentioned/unforseen in your question. – Andy aka Jun 19 '18 at 15:54
  • Hi @andy aka, please check my update on the post. I test your proposal and do not work best. I also uploaded sound acording of hum. Any suggestions? – mrkefca Jun 19 '18 at 16:01
  • I can only recommend fixes for the bad circuit you had in your original schematic. Like I said if there are other problems elsewhere then fixing a bad circuit isn't going to reduce noise/hum. "Do not work best" - does this mean it is exactly the same as before? If so then your problem lies in other areas that are not revealed by your question. Such as lack of decoupling capacitors on your pre-amp power supply or the need to reduce noise on those power rails due to excessive ripple voltage. – Andy aka Jun 19 '18 at 16:07