A persistent hummmm...

I have a Zesto Bia 120 all tube, Class A amplifier. I am currently using it with a pair of Volti Razz speakers - pretty efficient horn speakers. My problem is a hum that is audible from the listening chair. Most music masks the sound, but in the quieter passages, there it is. It hums the same way when nothing else is connected to it - just amp to speakers, or when the preamp is hooked up. I have had an electrician out to the house to check the ground. It was good, but he put two more 8 ft. copper bars in the earth - no difference. I had sent it out to George Counnas, the designer/owner of Zesto. He checked it out (and upgraded it while it was there). He couldn't make it hum. 

I have tried using an extension cord to other power outlets in the house, and the hum was no different. I have changed speaker wire in case my regular wire (no shielding or conventional insulation) was acting like an antenna. I have used a iFi DC filter. Obviously, I have changed over the tubes (KT88's). I use a PS Audio Power Regenerator, and it hums less when the amp is plugged into that device than when it is plugged directly into the wall.


When I use a high powered Class D amp, I do not get hum. When I use an old Scott 299A all-tube integrated, I do. That makes me think something is making the transformers hum, and the Class D doesn't have them in the same way.  


I remember reading that Michael Fremer had a persistent hum with his gear, and finally changed out a lot of his house electrical set up. My two electricians can't see a problem, but clearly, there is one. Anyone have any suggestions for other things I could try?


I live in the Boston area of Massachusetts - does anyone know electricians or audio experts who specializes in these kinds of problems? 






Hi OP!

I think the place to start it to understand if the hum is inherent to the amp, or perhaps a ground loop issue.

Disconnect all inputs and listen.  If you still hear a humm its an amp issue, and as I think you suspect, may only be noticeable because of your high efficiency speakers.  OTOH, if the hum vanishes with no inputs, it's a ground loop or upstream problem.

I had a hum problem once, although mine did not involve tube amplification nor high efficiency speakers.  I found isolation units from Jensen Transformers were effective when all else failed.  Of course, I had to get comfortable with the idea of inserting an isolation unit in the signal path, but I couldn't hear any sound quality degradation and the hum was eliminated.


So, on my (so far) futile diagnostic journey, I disconnected all input to the amp. and then unplugged them. The amp, with nothing plugged into it, or even sharing the same wall plug, continues to hum. I have to say the builder, George Counnas of Zesto, has been very accessible and involved with my process. Besides taking the amp back and checking it out, he has called me more than once to check on my progress. I think he has made an honest effort to help. 

I"m not a technical person, so trusting my gut is not really a substitute for process, but my suspicion is that the culprit is somewhere in my house. 


I really appreciate that a number of you have tried to think with me.



Interesting. Leaving the speakers attached, with the inputs disconnected I’d put a multimeter on the outputs, set it to AC and see how many AC volts you are getting. It may be that your few millivolts of AC (humm) are "normal" but also too much for your high efficiency speakers.

 I know very little about this, but Vandersteen suggested I try touching a wire (any old hookup wire) to the various chassis of the system and to the 3rd ground wire of a wallplug (just take the stripped wire and shove it into the round (not spade) hole.....one component was the culprit...gone now.