Potentiometer body grounding odd behaviour

[rabbitz]

I've an equaliser that uses 8 B100K pots and the bodies are grounded back to SG even though it's fine without. The circuit has a small buzz but what is odd is that when I place my finger on the wire linking the pots, the buzz disappears. Is there any way to replicate my big finger with caps, resistors or diodes back to ground?

At times when I poke around with my finger in some circuits I get a hum but I've never had one where it shut up any noise. Most circuits I've built never needs the pot body grounded but when required I use the SG on the pot PCB. BTW, it's in a plastic case so no chassis grounding used.

[duncan2]

Sounds like induced RF interference due to no screening, your body acts like a resistance value to earth so removes the hum.

[PRR]

Not clear: this EQ is a *plastic* box. When your stand near it, it catches the buzz on your body (from wall-wires). If your body is tied to the same ground as the circuit (touch the bus, or RCA plug or power-amp shell), buzz is far-far less.

[anatech]

Hi rabbitz,
You should always ground pot bodies. They are designed to be grounded and they also ground the metal shafts. Otherwise you will induce a hum (or worse) when touching the shaft or body of the control.

If you don't ground these, all bets are off. Same for using a plastic box without any screening or shielding. Sometimes you can get away with this poor practice, but eventually it will get you.

They make spray shielding or you can use foil, or copper sheet in order to shield the box and ground control bodies. I would highly recommend that you do something to shield your circuits and ground control bodies from now on.

-Chris

[rabbitz]

Thanks for the information.

Most of my projects are in metal cases especially the ones in use but for ones that I build out of curiosity I find plastic quick and easy and usually have no problems. The last few commercial items I've bought are in plastic cases.... BD player, streamer, pre/streamer.

[JensH]

Originally Posted by anatech

You should always ground pot bodies. They are designed to be grounded and they also ground the metal shafts. Otherwise you will induce a hum (or worse) when touching the shaft or body of the control.

I agree. Just look at the Topping L30 thread over at ASR. A number of people fried their L30 due to ESD hitting the shaft. From the shaft the ESD pulse went straight to the input op-amp, which was detroyed. And even worse, some people fried their headphones as well
The L30 is an excellent headphone amplifier and the problem has been solved on the latest version.

[Mark Tillotson]

Originally Posted by duncan2

Sounds like induced RF interference due to no screening, your body acts like a resistance value to earth so removes the hum.

I think its more likely that touching the wire grounds your hand so that it no longer injects hum into the nearby circuitry. In typical environment with mains wiring around you it can induce dozens of volts at mains frequency into you, since mains cables are not screened - the wiring around you in the building acts as a faraday cage running at mains potential (or about 1/2 to 1/3rd of mains potential as neutral and earth wires dilute the field).

So the whole environment you are in is at a fraction of mains voltage except for the things that are grounded conductors.

Ironicaly an industrial environment with 3-phase mains would have less problem with mains hum, if you think about it...

[duncan2]

"Grounds your hand " ?-- isn't that the wrong way round .


Your body has a finite electrical resistance starting with skin surface resistance then the resistance of your bodies contact with a ground earth.


Standing on a "duck board" or in more modern parlance some thick rubber insulator stops the current flow of your bodies "circuit " .


I got first hand experience of this in a factory where the maintenance electrician using a metal screwdriver while standing on a duck board touched the live contact on an electrical heater he had---no shock.


I have had many 100,s if not 1000,s of shocks over my life time mains /high DC etc only my thick/high resistance skin which is always dry stopped me from departing this earth.


Yes your right I know about Faraday Shields goes back to WW2 use and earlier and I agree about 3-phase.

[nigelwright7557]

If the hum in your body is out of phase with the hum in the equipment you can get cancellation when touching bare metal.

High value pots can cause problems where there is no hum at either end but with the pot in the middle it hums badly.

Used to have an electric guitar where if I touched strings any hum in speaker would disappear.

[RJM1]

Nigel, the trick to fix that guitar is to put a 1M ohm resistor between the guitar bridge and ground.
It is large enough to not cause a shock hazard but is enough to ground the strings to prevent them from picking up power line noise.
I briefly worked at Gold Tone when they were developing their electric banjo which had this problem and implemented this fix.
Gold Tone Musical Instruments


Gold Tone EBM5 Electric Banjo - YouTube