Dimmer 73 Audio Hum

[Call911]

Hey All,

Random problem that has been happening recently. We have a 1300 seat high school auditorium, built in 1996. Recently our lighting dimmer 73 hasn't been working, no problem, we have a ton of extra nearby that we can use until that is fixed. The weird issue is whenever dimmer 73 has power applied to it, it creates a nasty hum/hiss in the audio system. I know the two systems are seperated electrical (isolated transformers, etc), so I can't figure out why it would be humming. I've looked at dimmer 73 plug, nothings wrong. I can't imagine the wire between the plug and the dimmers would randomly go bad (we haven't had any construction or anything recently).

My role is TD, and we have in house electricians, but I need to at least point them in the right direction towards the problem. Any ideas? Thanks!

(FYI, don't know how the title got changed to "humconst", should be hum)

[Nelson]

I wonder if there is a bad connection on that circuit that is causing an arc. That may be sufficient to cause a hum in the audio system. Even if the dimmer isn't "ON", there still may be enough leakage current to sustain an arc somewhere.

[BGW]

I'd bet dollars to donuts the problem is in your rack. What system do you have? If it's modular, go ahead and yank the dimmer module and check it (smell it too) for damage. It depends on the manufacturer (system topology), but in some cases when semiconductor devices go bad, they can generate some insanely noisy/dirty power. It sounds like channel 73 is piped adjacent to sound conduit. Hence, when the channel is activated, the lighting wiring is inducing nasty things into the sound cables.

With the module removed, take a multimeter to 73's socket and (1) check for voltage leaking in from another circuit (very unlikely) and (2) check the resistance between the pins. You should have infinate resistance between line/ground and line/neutral, and almost none between neutral and ground.

If the socket wiring checks out, swap another dimming module into 73's slot and give it a test.

I think if the actual wiring were compromised, you'd have a breaker pop every time you'd try to run 73. It sure sounds like a failing/failed rack module to me.

[David Ashton]

I think you will find a neutral/earth wiring cross on that circuit

[WooferHound]

I would try hooking up a light to the dimmers output. The filament would short out the noise that's being generated.

[macwhiz]

I think you will find a neutral/earth wiring cross on that circuit

Neutral and ground/earth are supposed to be connected, as neutral is eventually connected to ground

[Nelson]

Neutral and ground/earth are supposed to be connected, as neutral is eventually connected to ground

Correct, but only in the service entrance panel.

[65535]

Still leaves potentially hundreds of feet of cable for any interference to travel and be picked up by the audio system. Most decent consoles and other processing hardware probably wouldn't have an issue due to their power supplies filtering.