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Tube preamp ground loop issue?

A

AndrewCAudiophile

New Member
A friend of mine builds tube preamps. After listening to one of the designs at his house, I had to bring one home it sounded fantastic. Clean, clear and dynamic. No issues...yet.

I bring the preamplifier home. I proceed to remove my solid state preamp, and in place of that I put the new tube preamp. I turn on the preamp, then the power amp...even without a source connected and no volume, I get a terrible ground loop/low hum/hiss noise. It's loud enough to hear through music even at moderate levels. You can see from the video I have posted the noise I am speaking about specifically.

I bring the preamplifier back to him to look over, he said everything looks good inside. We hook up with his system it works flawlessly again. I bring back home, same issues for me again. I am starting to think that my house is cursed and I don't know where to go from here. I hear the potential the preamp has, but the noise is so audible I can't enjoy listening to music with it.

Here's what I have tried at home:
*Swapping cables 2-3 times.
*Plugging everything into one outlet on one surge.
*Plugging into seperate outlets.
*Placement of the preamp relative to other stereo components.
*Changing power amplifiers 3 times.
*Using a dummy style power cable without a grounding pin on the preamp.
*Using an audiophile surge protector.
*Ensuring RCA cables are fully connected.

Any information or possible solutions would be more than appreciated. If someone can help me figure out this problem, they are a savior.

https://youtube.com/shorts/J9iUge5EmJo?feature=share
 
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Try a a 1$, 3 prong adapter and eliminate the ground.
Make sure the output rca jacks are plugged into the amp,
and if using a source, say Aux, plug those rca jacks into a source, CD, Dac ect
 
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Do you mean something like this? Refer to picture. I did already take some pliers and pull the ground pin out of an old power cable and tried that with preamp with no luck.

Not quite sure I follow the second bit: ".
Make sure the output rca jacks are plugged into the amp,
and if using a source, say Aux, plug those rca jacks into a source, CD, Dac ect"

I'll swing by the department store tomorrow and pick one of these adapters up if it's going to resolve this issue for me. Do I use that to plug the preamp into the outlet? Do I use to it on a surge?

Thanks for all of the info.
 

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Well if you already pulled the ground out of the power cord Same thing
If I forget to plug my Rca’s in to my Dac as I have 2 separate tube systems and switch out often.
If the pre-amp is on Aux, it produces a audible hum kinda loud
Depending on the volume setting of the Pre
 
Thanks for getting back. Yes I did pull the ground out already and tried that. If there are any possible solutions to what is going on here please share I am so baffled.

Best wishes!
 
I'd check that Neutral and Phase are not reversed in your outlets. The larger blade is Neutral and the smaller is Phase
Another thing that may be the reason, not all RCA connector ground shells are the same diameter, unfortunately. Make sure your cables have a snug fit to RCA jacks. Tweak them one at a time and see if hum changes.
 
Okay, so you're plugging this preamp into a receiver? What input are you using? Are your interconnects shielded?

What are the blue-glowing tubes? Are those regulator tubes? They may be generating some fierce RF that's being picked up by the receiver or the interconnects. You say you've tried placing the preamp differently--how far away?
 
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I'm connecting a tube preamp to a few different solid state power amps and same issues. Those tubes just act as VU meters for the output. I've tried 3 or 4 sets of interconnects. I have moved the preamp away from everything 5 or more feet or so and same amount of hum/hiss. It sounds like a 60hz hum rather than a 120hz hum. As I said the preamp was used with my friend system and worked flawlessly. When I bring home it doesn't work properly.
 
So Just to be clear, you haven't tried direct plugging it into an outlet? or as I would, different circuits in your house?
 
I have tried plugging directly into the wall..into a surge also. Preamp and also on same outlet..even two different outlets, and no result...into different outlets in the house in different rooms, no result still. I have had to hold myself back from damaging my system in anger/frustration, lol...
 
Is there any other friends or family close by that have separates/tube equip local to you?
even a fellow AK'er that you can just bring your Pre-amp, and see if the Hum follows?
Could it be that your repair shop's less sensitive speakers would not reproduce the Hum?
did the tech put it on a sensitive Scope?
 
Any chance you have a TV or a computer wired in with this? I actually ran into a hum problem last night using not my equipment. As soon as I connected the computer source it hummed. Unhooked the cable line and about half of it went away. I moved the computer to share a power outlet with the rest of the gear and it shut up. The preamp has a ground. The amp does not, but the computer does and the cable line is grounded as well.
 
DC riding on your AC supply?
Try shorting the inputs on the preamp?
Big transformer nearby?
Flourescent lights nearby?
Have your friend come to your house with his oscilloscope and test the output signal on the preamp?
Confirmed no hum through amp(s) only, no preamp connected?
 
Another reason ground loops occur is a crossing of neutral and ground in the outlets and at the box...

It's pretty common in older buildings for neutral not to be properly separated from ground...

I use a tester like this for checking if outlets are wired right...IMG_20211213_143952.jpg
 
Been that way for as long as I'm aware of. Multiple decades at least. I have heard of older homes with 2 wire non-grounded outlets having the neutral looped over to the ground screw though. Luckily my old house with 2 wire outlets had ground wires in the box, so it was no problem to get real grounds.
 
Our local code has neutral and ground separated in the box with a insulated 6 gauge copper bridge wire to join them...

Most of the places I work on are 50 plus year's old or more and I've seen some really bad electrical messes especially if there are additions to the building...
 
Yes, some service panels have separate neutral and ground buss bars with a connection between them, some are combined. End result is that neutral and ground are tied at the service panel. Sub panels are not always required to have neutral and ground tied.
Good to check local codes and practices. Our house had some amateur wiring that reversed hot and neutral, didn't connect grounds, had wire connections outside of junction boxes.
Recently had a bad ground on a GFCI cause hum in one of my amplifiers.
Safety dictates that ground prong is never cut off, but audiophiles are willing to sacrifice a lot for their music.
 
So basically just flip the power cord around and plug in upside down? Or just move around while on and see if the sound goes away? I gave up on it for a few days I will get back into messing around this week. Was about to purchase a ground loop/hum eliminator box not sure if that may work.
 
Nyjblbob said:
Is there any other friends or family close by that have separates/tube equip local to you?
even a fellow AK'er that you can just bring your Pre-amp, and see if the Hum follows?
Could it be that your repair shop's less sensitive speakers would not reproduce the Hum?
did the tech put it on a sensitive Scope?
Click to expand...

I can almost guarantee that it will work in someone else's house. Worked fine in my friend's house who built the preamp. Bring home and it didn't work at all.
 
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