On one leg or two legs?

That's very interesting. So the amp with the more severe hum/buzz also has lower gain. It could be due to a lot of things, but I suppose the most likely culprit would be a tube. Perhaps before taking it back to him it would be worthwhile switching tubes between the two amps.

Regards,
-- Al

Maril,

Wow, I just read through all of this, at Jea's suggestion. Let me make sure I understand what amp we are talking about. If I followed everything correctly, you are having the problem with your friend's pair of BAT VK-150SE's, not the VAC amps that are listed in your system profile. And you previously had (or still have) a BAT VK-75SE (similar to the 150SE but in stereo configuration), and it exhibited similar symptoms.

Do your VAC's have these symptoms as well?

I notice that the BAT amplifiers have a bandwidth spec of 200kHz, which is extremely wide, and approaches rf frequencies. I'm wondering if there may be rf interference coming from somewhere that is radiating directly into internal circuit point(s) within the amp, and somehow then intermodulating or being AM-detected somewhere in the amp circuitry, resulting in the sound you are hearing.

Can you tell if the sound is really 60Hz, together with its 120Hz and possibly 180Hz harmonics (corresponding to the power line frequency and its lower harmonics), or could it be some other frequency or frequencies, which would be suggestive of rfi pickup?

The computer that you mentioned is in the room would certainly be one such possible source. If you haven't already, try turning it off AND unplugging it -- as you probably realize some circuitry on the computer's motherboard, and parts of its switching power supply as well, are always powered up as long as the ac is connected. For that matter, unplug all other computers in the house, and if you have nearby neighbors, consider asking them to do the same.

Perhaps you could bring an AM portable radio into the room, or better yet if possible a portable that can receive the long-wave band (below 550kHz) and see if it picks up any interference. If it does, you could track down the source of the interference by walking around with the radio and seeing where it is loudest.

Also, just to be sure, did the XLR shorting plugs you put on the inputs have all three pins electrically connected together (such as by having pin 2 wired to pin 1 and pin 3 wired to pin 1)?

Regards,
-- Al

Maril,

Yes, based on the facts indicated in your last post, I feel pretty confident that is what is happening -- the BAT's, with their 200kHz bandwidth, are picking up interference through the air.

The source of the interference is probably something closer to the left channel amplifier than the right, since the symptom is worse on that side.

Besides computers, other common sources of rfi in the home are:

-- Dimmer switches for lighting (either incandescent or fluorescent). If there is a dimmer switch nearby, make that your number one suspect after the computer!

-- Fluorescent lighting.

-- UPS's (uninterruptible power supplies).

-- Computer network cables.

-- VCR's, DVD players, etc. Like computers, they can generate significant noise when plugged in, even if they are turned off.

-- Possibly wireless devices like cordless phones, wifi, etc. Although they operate at far higher frequencies, they may have switching power supplies or other circuits which emit spectral components below 200kHz.

I have a portable AM radio that can receive the long-wave band (as well as short-wave and the medium wave regular AM broadcast band). You'll be interested to know that it's dial starts at 150kHz (!), well within the frequency range of what your amplifier can amplify!

Good luck!

-- Al

Maril -- Do try what I suggested with a battery-powered portable AM radio, checking to see how much interference or buzz it picks up in the room, and if it is significant, walking around with the radio to try to track down its source.

How close are your nearest neighbors -- maybe it's coming from them.

Aside from interference being picked up through the air, the one other possibility that occurs to me, given that the amps worked fine before being moved to your house, is that perhaps the mechanical jostling that occurred during the move affected something within the amps. Such as, perhaps, a screw loosening slightly, causing the contact between a metal transformer housing and the chassis to lose its integrity.

-- Al

Jim -- He appears to have answered that, at the end of his first post dated today.

-- Al