Grounding Issue? Volume Pot is noisy.

I recently just finished a headphone amp. It is a clone of the El Estudiante from wtfamps.com.

I have tracked down a noise issue that is preventing me from calling this amplifier done.

When I touch the volume pot (ALPS 100KAX2 100K Ohm Audio Taper Stereo from PartsExpress), there is a lot of noise or hum on the output.

At first, I suspected the circuit was not earthed to the chassis properly, because the noise got louder when I touched the metal of the chassis. I spent a lot of time ensuring that the chassis connection is not the problem. After much trouble shooting, I know for sure the culprit is the volume pot.

If I remove the volume pot from the front panel, the chassis is no longer reactive to my touch. However, all I need to do is touch the metal shaft or front of the pot, and the hum get loud.

I checked for continuity between the ground pins on the pot and the shaft and there is none. If I run a jumper from the metal on the pot to the star ground in my amp, the noise goes away and the circuit is dead quiet.


How can I remedy this? There's no easy way to ground the metal on the pot, solder won't stick. Are these pots supposed to be internally grounded?

An externally hosted image should be here but it was not working when we last tested it.
 
The volume control's metal case acts as a shield, so the control should be mounted directly
to the metal chassis for grounding. Mounting the control on a nonconducting panel won't work,
and will inject hum into the circuit, especially when the (metal) knob is touched.
 
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The volume control's metal case acts as a shield, so the control should be mounted directly
to the metal chassis for grounding. Mounting the control on a nonconducting panel won't work,
and will inject hum into the circuit, especially when the (metal) knob is touched.

Wouldn't this be bad for my grounding of the circuit, as I already have one point in the chassis that is connected to ground?

The chassis is powder coated aluminum, and is not connected to the pot's shielding.
 
A pot shield doesn't cause ground loops, since no audio current flows in it. You can find a lug that fits
the pot shaft, and run a ground wire to the power supply ground. Something like this (metric is best):
Abbatron | HH Smith | GH Plugs

An aluminum plate (at least an inch square) with a soldered wire and a punched hole for the pot
would also work, if you can't get a suitable lug.
 
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The coward's way out is to use a plastic knob, but even then you may hear noise when your hand
is near the pot shaft, since the pot's shield is not grounded. With a 100k pot, there could be noise
even if your hand is not near the pot from stray pickup, so the pot's case should really be grounded.
 
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The coward's way out is to use a plastic knob, but even then you may hear noise when your hand
is near the pot shaft, since the pot's shield is not grounded. With a 100k pot, there could be noise
even if your hand is not near the pot from stray pickup, so the pot's case should really be grounded.

The knob I have is aluminum over plastic. Just my hand touching the chassis caused an issue.

My solution was simple, I found a ring terminal, at the local big box store (since there is no such thing as an electronics store locally :( ), that fit the front tab. I made sure it fit snug, and sandwiched it between the chassis and pot. Then I ran a ground wire to the chassis ground. Quieted the amp right up.


Now I have a different issue with RFI.
There is no issue at my home, but at work, I pick up one of the local FM stations. The RFI doesn't increase with the volume knob. It is constant.
Another fun note, my desk neighbor gets the same interference on his noise cancelling bluetooth Bose headphones.a

Can this be getting in on the power cord?
I have 33pF caps accross the input jack. And I have 47pF caps from the post volume pot signal to ground (as a filter).

My signal wire is all shielded.
I troubleshot different RCA interconnects. (No change)
I troubleshot a different source (no change).

If it matters, I am using Qobuz on pc => FXAudio DAC-X6 => this amp; Removing the DAC-X6 didn't remove the interfernce.

Taking the amp home removes the RFI :)


ANy other troubleshooting I should try?
I'm considering adding some aluminum tape to the inside sides of the chassis where there is only wood.
 
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Are you sure it is the FM? An AM signal is a lot easier to demodulate.

May affect which tower you need to cut down. Or how many pFd to try before the axe.

I can distinctly hear the dj say 93.7 WBLK. However, i don't know for sure on what Frequency it is entering the circuit. How closely regulated is the RF band? I wonder if there is someone nearby that is rebroadcasting the signal?
 
Probably it's coming in through the output and feedback loop. Try coiling up the headphone cord
into a small bundle and see if the interference is affected.

I will try this on Monday. Interesting idea. I have 2 different pairs of headphones, and one seems to play the interference louder. Although I have no measurement of that. But that one has a split wire to each ear, while the quieter one has a single wire.
 
Now I have a different issue with RFI.
There is no issue at my home, but at work, I pick up one of the local FM stations. The RFI doesn't increase with the volume knob. It is constant.
Another fun note, my desk neighbor gets the same interference on his noise cancelling bluetooth Bose headphones.a

Can this be getting in on the power cord?
I have 33pF caps accross the input jack. And I have 47pF caps from the post volume pot signal to ground (as a filter).

My signal wire is all shielded.
I troubleshot different RCA interconnects. (No change)
I troubleshot a different source (no change).

If it matters, I am using Qobuz on pc => FXAudio DAC-X6 => this amp; Removing the DAC-X6 didn't remove the interfernce.

Taking the amp home removes the RFI :)
.

An RF filter is a series RC network.
Simply putting a cap across the input isnt really good enough.