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? 

 

Thanks,

 

David

dtorc

@dtorc What happens when you short the input of the amp?

Have you tested the input tubes? They are far more likely to cause any amp to hum, if the amp uses an AC filament connection (and most do).

The test that the tubes need to pass in this regard is called the 'leakage test' and it measures resistance between the cathode of the tube and the filaments of the tube. If the measurement is anything less than infinity (no connection at all) then the tube can inject hum or buzz into the amp.

How clean your AC power is can affect how much the tubes can cause this problem if they have leakage issues.

Post removed 

Sorry - in the list of things I said that I tried, the cheater plug should have been first. No luck there.

 

I have changed the inout tubes a few times, and that doesn't seem to affect it. I don't know whether there are quieter versions out there ( I used JJ 12 Au7's). 

 

I think the that the power, which comes from the PS Audio Power Regenerator is pretty clean, at least according to the scope built in to the unit. But something is off, clearly...

 

Thanks for thinking with me...

 

David