really bad hum or possible ground loop

[michaelnorman]

hi everyone,
I'm having problems when we are dimming our lights. When we dim them we get a really loud buzz through most of a foldbacks and IEM's. The buzzing is only there when we start dimming them and with some lights its really loud and others dont make a noise.
What would be a solution to this?
As far as i know, our dimmer pack is on a separate circuit as it has a 3 phase outlet. Sound system I believe is on 1 circuit.
I spoke to electrician who attended's the church, and the only solution that both of us came up with that would be heap and easy to fix is to move the sound system on to a different phase as the one we are using could be getting interference from the dimmer.
Thanks in advanced

[waynehoskins]

If the dimmers are on all three phases, I doubt that moving audio to either of the other two phases would solve anything, since there are dimmers there as well.

I think the problem is likely poor filtering (or lack of it) in the dimmers. This tends to embed noise on the power wiring, which will couple through any common power path between the dimmers and the audio, including quite often neutral and ground wires. As filter components or improved methods of dimming cost money, cheap dimmers often omit filtering entirely.

What dimmers do you have? Are they centralized or distributed? Does any of the power wiring run for any considerable distance parallel to and nearby the affected audio signal wiring?

Juggling power phases like you're thinking is only a band-aid (and I doubt it would be effective in your case); the root problem is still there. At my church, as with many, lighting is on all three phases, audio is distributed across all three phases, video is on one or more phases as necessary, and there are no dimmer noise problems.

[michaelnorman]

im not sure what type of dimmer we have but its a old analogue dimmer that we got second hand, will check what type it is when im there tomorrow night.

When you say "Are they centralized or distributed", im guessing you mean are the dimmers all in the same place or spread out, if that is the case, the dimmers are located all together in the same room where we store our amps.

i dont believe that we have any power and speaker cords running parallel to each other as power we have going through the roof and speaker and mic cables are going through the floor

[tdangelo]

Just adding on to what Wayne said ...

In a three phase electrical system it is important that loads (your audio power draw, video power draw, lighting power draw, utility power draw, environmental draw) be balanced across the three phases. Pulling substancially more draw on one phase (aka "leg"), than another can cause hum/noise problems as well.

In a single phase system the noise is easier to diagnose.

Yo may want to ask your electrician friend to put an amp probe on the mains to see how much is really being drawn on each leg.

[wogster]

Originally Posted by michaelnorman

im not sure what type of dimmer we have but its a old analogue dimmer that we got second hand, will check what type it is when im there tomorrow night.

When you say "Are they centralized or distributed", im guessing you mean are the dimmers all in the same place or spread out, if that is the case, the dimmers are located all together in the same room where we store our amps.

i dont believe that we have any power and speaker cords running parallel to each other as power we have going through the roof and speaker and mic cables are going through the floor

Any kind of shielding between the dimmers and the amps, dimmers, especially older ones, can put out a lot of RFI, can put out quite a large magnetic field, and can produce a lot of heat. None of these are things your amplifiers are particularly fond of.