Noisy volume pot

[DIY Bass]

I may be overthinking this. I have built a compressor from a kit. I was using a sine wave and an oscilloscope to fault find. Think I got the issue but I noticed another one. I don't have a schematic but the output goes through a volume pot right at the end before the bypass output. I can see a nice clean wave when the pot is at either extreme, but there is quite a lot of noise in the signal in between. Having said that I can't hear any differences in the tone between the bypass and non-bypass. I don't claim to have great ears though. I just want to do any sorting out before it goes into the box. Is this a problem or pretty normal? If it is a problem is it likely to be bad pot or something else?

[Rob Strand]

I can see a nice clean wave when the pot is at either extreme, but there is quite a lot of noise in the signal in between. Having said that I can't hear any differences in the tone between the bypass and non-bypass. I don't claim to have great ears though. I just want to do any sorting out before it goes into the box. Is this a problem or pretty normal? If it is a problem is it likely to be bad pot or something else?

It's pretty common.  The most common scenario is the volume pot used has a high resistance (as in it's nominal resistance value).    The output impedance of a volume control is maximum when it is centered (for a linear pot).  That does two things:  The center position becomes the most susceptible to noise and also it forms the lowest frequency low-pass filter with the cable + amp/effect capacitance.  (It's also worst case for  thermal noise.)

[thermionix]

You're hearing this noise with your ears, or seeing it on the scope?  Does that count as noise?  Is this like a "tree falls in the forest" thing?

Having said that I can't hear any differences in the tone between the bypass and non-bypass.

There shouldn't really be much of a tonal difference, but if you hit the strings hard you should hear the volume clamp down.

[DIY Bass]

You're hearing this noise with your ears, or seeing it on the scope?  Does that count as noise?  Is this like a "tree falls in the forest" thing?

Having said that I can't hear any differences in the tone between the bypass and non-bypass.

There shouldn't really be much of a tonal difference, but if you hit the strings hard you should hear the volume clamp down.

Yes exactly :-)  I am seeing it on the scope, but not really hearing it.  I will try and get some photos to show you what the scope is seeing.  Even better might be to work out how to post a short video showing the rotation from end to end.  And yes, I was trying to say that I am hearing what I expect to hear, which is no real difference between the bypass and non-bypass sound (especially as I tend to like pretty mild compression settings)

[Rob Strand]

It could be wide-band noise.  If you have a band-limit/low-pass filter on your CRO try flipping it.  Otherwise you can put a simple 20kHz to 50kHz RC low-pass filter at the output and probe that; you need an R smaller than the probe impedance but larger than the circuit impedance.  If it's wide-band noise then the filtering will reduce the noise you see on the display.