The gremlin of studio EMI

[eschertron]

Gentle readers,

I suspected my (20+ year old) Carver PM-300 was radiating some buzz/hum into the other components in the rack. It does hum audibly (acoustically) but it's not too bad next to the fan noise from the computer. On my way to proving the buzz does get picked up by the mic preamp, I also discovered that an A/D unit in the rack also radiates enough to be picked up by the mic pre. So I have these data:

1. it's only channel 2 of the mic pre (ART tube preamp system - original model) that picks up noise. In addition to noise picked up when an offending unit is brought into proximity with the mic pre, I can induce noise by moving a 1/4" patch cable (signal out FROM the mic pre channel 2 to the A/D input) near the noise source. Note that channel 1 does not exhibit either of these behaviors. I have swapped patch cables and inputs on downstream gear - the problem is in the mic pre.
2. putting one of my wife's cooking sheets between the mic pre and the other unit will add shielding and reduce the buzz.
3. all this was done with nothing plugged into the mic pre, and with gains turned up to clearly hear the quality of the noise. Channel 1 can be turned up to the point that hiss is apparent, and a 'little' buzz is in under the hiss. Channel 2 has large amounts of the buzz, and hiss comes up about the same as channel 1. So channel 2's buzz is WAY above the noise floor.
4. With the PM-300 turned off (and unplugged for good measure) there was still buzz. That's how I discovered the A/D unit was also being picked up.

Am I on track to assume there's an issue in the mic pre that needs to be addressed?
What would cause one channel and not the other to become sensitive to radiated noise?
The fact that a cord plugged into the output will pick up and transmit the buzz must be a clue; something to do with the output impedance of the channel?

I have a schematic for this unit on a USB stick, not with me right now. will post when I can find it. Thanks for any comments.

[eschertron]

ART tube preamp schem

art Tube Pre M3 sch.pdf

Not this exact unit, from what I can tell, but the only one I've found around the net.

Looking at this schem, I see unbalanced outputs only use half the U2 chip. I'll see what difference I get using balanced outputs.
Also the phase switch is between the final buffer and the outputs. I'll massage that as well.

Still thinking that it's an output impedance issue. Am I right in thinking that I can load down the outputs to detect if one channel is failing to deliver sufficient current?

edit:
I'll see what difference I get using balanced outputs. No difference.
I'll massage the phase switch as well. No difference. Sigh.

[The Dude]

Since yours is a 2 channel mic pre, does it use 2 tubes, or the channels each use a triode of one tube? If 2 tubes, try swapping them around and see if the noise follows the tube or the channel.

[eschertron]

Since yours is a 2 channel mic pre, does it use 2 tubes, or the channels each use a triode of one tube? If 2 tubes, try swapping them around and see if the noise follows the tube or the channel.

Thanks for the post, Dude! It uses 1 double triode, and I did swap it out for a different tube while the unit was open. No change. I'm mystified as to what can make one channel such an antenna for local magnetic fields.