bizarre board/amp issue.

[bniesz]

So, the studio I work in has re-racked most of the outboard gear. It's hooked up in mostly the same fashion, with a few changes (and new racks obviously).

The problem arises when we plug the amps back into the control room outs of the board. Suddenly there's a WICKED buzz going on. the obvious cause is grounding... but the headphones on the board aren't buzzing, only the amps.

Strange.

Now stranger, the buzzing stops when the amps are turned to lower volumes. Alas, not a grounding issue. We discover that the board seems as though is throwing off signal at a much much higher level. So we test. The TRS end of the cntrl out is throwing off around 1.2 V, which seems right. I wanted to test the actual XLR out of the console, however - they are configured as shield, left, right. I figure I gotta test each side across the shield, but that didn't give me the result I wanted (it was way low). What's the correct way to test voltage across a stereo unbalanced XLR connection?

Moreover, presuming that the voltage coming off the board is correct (as tested on the TRS end), what in god's name else could be causing this? We've tested with a different feed - from a Presonus Control Station - which works as expected. But when the board is connected the signal is so hot, the amps buzz unless turned way down. And when turned down, they pass signal much louder than expected.

WHAT'S UP eletro voodoo?

[RodC]

What board, and be 100% sure that the connections on the board you are using are balanced.

I once helped out at a live gig. They ran a 200' snake and used the TRS connections for the runs to the poweramps on stage. They had a wicked buzz and swore to me up and down that they ran all balanced cable ect. The board was a rented board, looked new. Of course they didnt have the manual. Anyway, the XLR connectors on it was balanced, but the main outs TRS were NOT! Blew my mind. I looked in the connection and sure enough it was only a TS connection. We connected to the XLR outs, bam buzz gone.

Do you get a buz if you leave the cables connected to the amps but not the board?

[bniesz]

What board, and be 100% sure that the connections on the board you are using are balanced.

They specifically aren't balanced. It is a single stereo XLR connection, so there is no hot/cold/ground - but left/right/ground.

Do you get a buz if you leave the cables connected to the amps but not the board?

No. But I don't think it's a grounding issue, because the buzz goes away when the amps are turned below 50%. And I don't mean that is gets quieter until it becomes inaudible. It goes from being very loud, to gone, very quickly.

My only guess is that the cables we were using were somehow wired incorrectly. We connected a XLR stereo-Y to TRS cable to the output. I think the TRS is actually configured unbalanced TR - rather than TS. Why this would suddenly become a problem now, I have no idea.[/i]

[audioboffin]

Perhaps you need to look at your grounding philosophy ?

[bniesz]

Problem solved.
The shield on the TRS cables was in fact wrong.
The cable was T=hot, R=shield, S=null.
The cable is now T=hot, R+S=shield.