On one leg or two legs?

Well,
I already tried turning off all circuit breakers, no help.
Tomorrow I'll bring the amp to the living room to try with another pair of speakers and another outlet.
I have two other pairs of less efficient speakers, so I'll give it a try as well.
BTW, what's your take on the high chassis voltage, higher with the correct polarity?

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

Almarg,
Yes, you're right, I'm talking about my friend's 150s.
I no longer have 75-SE, but am considering buying it again, that's the whole reason for this post.
VAC didn't have this symptoms, just some ground loop, that I corrected.
The hum is mostly coming from the tweeters, but can also be heard from mid and bass drivers.
Just a little bit of history: what started me on this wild goose chase, is a significant hum coming through the speakers with a phono input selected- I could see woofers moving.
In another lenghty troubleshooting session with Kevin of KAB, we narrowed it down to phonostage amplifying some AC interference. I could also hear phonostage picking-up a radio signal (actual radio broadcast).
All this information combined with BAT's bandwidth, gives your theory an extra credit.
I will experiment with computer and radio tonight.
Shorting plugs- I don't know how exactly they are wired, I'll look into it.
Thanks for your help. I hope you, Jea48 and others will hang with me for a while longer, until this gets resolved, or I commit suicide.

BTW, what's your take on the high chassis voltage, higher with the correct polarity?
02-01-09: Maril555

Well the lower chassis to equipment ground voltage is the correct orientation polarity for the amps. Seems odd to me both amps would have the primary of their power transformers wire incorrectly.

By chance are you using aftermarket or home brew power cords on the amps?
If so it could be the cords are wired wrong.
The wiring on the 3 wire NEMA plug could be right but the hot and neutral on the IEC are reversed. Easy to check with your multimeter. Just pull each cord from the amps and check the continuity from end to end on the pcs.

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