Hum on one Y side not in the other (Using 2 com mixers)

[Gizmo1977]

The other day I was asked to use two Sonifex mixers for three guests. As there were not enough ins and outs in the snake, the PGM feed for both mixers was splitted with an Y. The main guest had a backup mic and his return was also Y-splitted to a Fischer stick in case the power went down. (Both mixers were connected to the same outlet)
When checking lines I noticed the PGM signal on mixer 2 had noise (none in mixer 1's), the same happenend with main guest's return: it was clean on the mixer 1 side of the Y split cable but there was some hum on the Fischer. At least the Fischer wouldn't have noise in case the mixer lost power.
When I told A1 about that, he said it had happenend many times when power isn't as clean as desired and it's something they usuallt can't fight with at some locations.
I swapped mixers (and also Ys) in case one was worse isolated or ground issues but they seemed to sound similar.

How come that hum noise happen on one side of Ys but not the others?
Could this be deaden by linking both mixer chassises ?(for example with a guitar string).

Thanks in advance!

[Hugh Robjohns]

I'm a bit confused from your description as to what was being split, at what level and how the signals were shared to what...

But it really doesn't matter because the possibilities are countless and remote diagnosis after the event almost impossible.

Noises suggest ground-loop problems, but it could also be RF or even oscillation issues. If the Y-split outputs were balanced and feeding balanced inputs grounding shouldn't have been an issue... but the reason one desk was affected more than another could be down to different internal grounding arrangements or different input circuit revisions.

The devil is very much in the detail....

[Gizmo1977]

Sorry if the text was confusing. Mind my English.

Mixer A had
Mic 1
Mic 2
Ret 1
Ret 2
PGM

Mixer B had
Mic 3
Ret 3
PGM

There was an analogue line leve feed with PGM which was splitted with an Y cable to both mixers.

Ret 1 line level signal was splitted with another Y cable to mixer A and a FischerAmps stick.

Noise happene on mixer B PGM (not on mixer As PGM) and Fischer RET 1 not Mixer A Ret1.

Both mixers were plugged to the same powerstrip so I thought there should be no ground loop issues.

Thanks, Hugh!

[Hugh Robjohns]

Powering from the same strip minimises the size and thus the strength of any ground-loop, but doesn't remove it.

If you suspect ground-loops, the first thing to try is a line isolation transformer in the signal path. If the noise goes away, you have a ground-loop... if it doesn't there's something else going on that will need further investigation.

In your case, I presume the outputs of the two mixers were also connected to something, so there's another ground loop possibility in that path too.

It is unusual that one mixer hummed from the Y-split and not the other... but as I said, you'd need to investigate how the mixer's differ internally to figure that one out.