On one leg or two legs?

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

Jea48,
I will check the power cords tonight.

Al,
Now, that you said it, I do have wireless router next to my desktop computer in the basement, transmitting to my wife's laptop, she's using in the kitchen upstairs. Transmitter is closer to the L channel amp. Also, Comcast cable goes along the same left wall to the desktop in the back of my listening room.
Can't wait to get home tonight and disconnect all that crap, to see what happens.
Will keep you guys posted.

Jea48,
I have been working alot in this weather. hevac ya know.
Houstonreef can put any load on the other side that he wants. I would put another rooms outlets on it myself or light. Make sure there are no dimmers or florecent lights. Do not use kitchen outlets & motor loads are bad. I do not think the projector would be good he will connect the ground and common through the cables. It does not matter the amps if the breaker only has a 120Vac going throu it may not trip correctly.

Houstonreef: Where are your amps to your other equipment?
Do you have tube equipment, phono stage, have you tried to change out some cables. move equipment around a little can stop hum in some instances. Your equipment mite not like each other. Are your power cords 90 deg to your interconnects or 6 inches away as this can cause RF interferance. All a/c create RF. You could just have a bad shield or bad cable.