What would cause noise (ground noise?) when touching the volume control and tone controls?

tubeguy76

I have a new preamp that I recently hooked up and I'm getting some interference or what may be ground noise but I only hear it when I'm physically touching the metal volume knob and tone controls.

That sounds defective. I'd return it to the dealer for an exchange.

Sounds like a ground loop. Try connecting to ground separately or a grounding box. Does this happen only on phono or also higher voltage inputs?

@cleeds Ah... it's a "new to me" preamp, but it's a used unit.  Fairly new though.

@antigrunge2 I don't have a phono setup, just a streamer connected to a DAC and the DAC connected to the preamp. 

I can reach out to the preamp maker, just wanted to see if I could solve it myself before doing so.  :)

I can reach out to the preamp maker, just wanted to see if I could solve it myself before doing so.  :)

@tubeguy76 This sounds like the controls are not properly grounded and the case may not be either. I would talk to the manufacturer first, since opening it up to solve it yourself will likely void the warranty.

@atmasphere Yeah that's what it appears to be.  I'll reach out to him and maybe he can help me solve it without sending it back for some TLC.  

Have you tried plugging all your equipment into 1 power outlet strip?

Could be a ground loop.

Joe Nies

Back in the day, I had the same issue with an amp.  It had a two prong A/C plug, and turning the plug over, i.e., reversing the  prongs, solved the problem.

I guess since only one prong was supposed to be driven and the other was supposed to be a ground source, they must have wired the ground source to the chassis ground.  So if that prong was plugged into the driven side of the socket, the chassis wouldn't be properly grounded.  Turning the A/C plug over was the fix.

Your new unit probably came with a three prong plug that can't be turned over.  Maybe the internal wiring of the A/C connection needs to be switched.

If your preamp is a tube unit, it could be that you have a microphonic tube in the circuit, resulting in what you describe. If ss...not applicable.

@daveyf  I really should put more detail when I post. Yes, it is a tube preamp. I did tap on them gently to see if I could detect anything. 

@tubeguy76 Might want to experiment with swapping tubes around. See if that does away with the issue. I had this exact problem with my tube preamp, and with some tube swapping was able to zero in on the offending tube. Sometimes tapping on the tube will show an issue, but not always.

When I touch the metal top of the case in addition to the knobs, the noise will subside.

This sounds like the controls are not properly grounded and the case may not be either.

atmasphere +1

Probably the volume and tone control potentiometers bodies are not properly grounded.

If you have a multimeter, check continuity of the power cord ground pin to preamp case, also check volume and tone knobs to chassis continuity.

As a first step I would put a cheater plug on the lower cable to lift the ground and see if it goes away. 

This sounds like an open earth ground circuit to me with your description. Check your house wiring and see if it checks out proper. You can get a plugin device for this if you don't have one. Which leads to the second possibility, the new unit is not properly grounded internally. This could either be a design flaw or a bad/missing earth ground connection.

@audio-union Good idea, I can check with my little plug checker. That outlet may have an issue. 

@audio-union  I just checked my outlet ground with my checker and it doesn't show any ground faults with the wiring. 

@imhififan  I did a continuity check with my Fluke and it does beep when touching the center ground pin of the power cord and then touching ground points on the preamp chassis.  It beeps when I touch the grub screws holding the knobs on the pots as well.  I opened the preamp and all the grounding/shielding is in place and looks to be well done.  Otherwise the chassis is wood with metal top, front and back plates.

It could just be the nature of the beast.  Tube quality is unknown so I'll try some new ones before going further.

So I tried a few more things but I'm still getting the buzz/hum.  When I touch the ground stud on the back of the preamp (for the phono stage), it goes silent.  Maybe I can run an external ground wire?

Does this happen when no sources are connected to the preamp? If not, try an optical cable between your streamer and dac. I had a similar problem years ago that just couldn't be resolved until I did that. Very strange but it worked. 

So I tried a few more things but I'm still getting the buzz/hum. 

@tubeguy76 

If you did so without talking to the manufacturer then you were wasting your time. This is a problem that has a very specific cause and I suspect that manufacturer, if a good one, would like to know about it.

I've encountered this problem before- as I mentioned in my prior post, its most likely a grounding problem, such as the controls themselves are insulated from the chassis since the chassis is finished with either paint or anodizing (both of which insulate). If the control bodies are not grounded they can pick up noise; when you touch them that noise pickup will be more obvious.

If the chassis is not properly grounded it can have the same effect. The chassis is supposed to be a safe harbor in which the audio circuit resides. If its not grounded, it can instead inject noise. Its worth getting this fixed.

Call the manufacturer (or dealer, if you can't contact the manufacturer)!

@tubeguy76   Since you confirmed that your preamp is a tube preamp, it would be instructive to know what make and model and also the tube compliment that it uses. While a grounding problem is likely, my suggestion about a tube issue is also possible. 

IMHO,It is a little crazy in this day and age that manufacturer's still have problems with grounding issues! 

@daveyf I'm going to not mention the manufacturer right now until I get a chance to speak with them about it.  I don't want to cause any issues for them in case the issue is really mine and not theirs.

I did try the cheater plug to remove the ground and the noise was still there so I'm thinking a tube or maybe a bad transformer?  We'll see, I'll update the thread as soon as I can.  :)

I fixed it.  The left channel output ground wire had fallen out of its terminal block.  It's quiet now.  :)

Use a power conditioner plugged into one outlet.

And then buy a new dog