Y-Not noisy in input A

[crossadam77]

Hi, just wondering if anyone has had a similar problem.

I have a rack with 2 preamps (ADA Mp1, and Bogner Fish) running into a TC Electronics G Major, then into a Peavey 50/50 power amp (running stereo) into 2 Quad boxes.
I basically have both pre-amps running at the same time (1 left and 1 right) - pretty much as independent rigs (but in 1 rack)

I bought a Y-not to control them via midi and it functions perrfectly... (however)

For simplicity lets say;
Bogner Left side, and MP1 Right side. (both amps on at the same time)
Clean channels sound great (hum free), however when I change (both amps to lead channels at the same time) there is a loud hum coming from the Left speaker (even when guitar is turned fully down). I've worked out that whatever preamp is plugged into the "A" input to the Y-Not, it's that side/speaker that hums loudly. i.e.. if I swap preamps around it's always the "A" side that hums.

I've tested all the cables, bypassed the effects, swapped power amps, changed speakers everything etc… I'm pretty sure it's something happening at the Y-not stage (ie. If I disconnect the Y-not all together and hook up the preamps individually with a power amp they both work fine (noiseless) when stand alone mode.

Would appreciate any suggestions… (pic attached).
Adam,
Melbourne Australia.

 

[crossadam77]

Re: Y-Not noisy in input A (CABLING PICTURE ADDED)

Diagram added. Thanks Adam

 

[rjmmusic]

Did you try using the Iso B switch? (definitely turn on the buffer if you're using it)

Also, check how you're powering the Y-Not - if it doesn't have an isolated output,. it could be causing a ground loop.

Another thing to try is make a cable with the ground disconnected on one end. Use this to connect from the A output to the preamp. This will probably help. You can also use an isolation transformer in place of the ground-lifted cable.