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Dimmer Hum


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Hi everyone


Variation on a common problem! I'd appreciate any thoughts or suggestions on this one. Here's the sequence on events.


First full tech rehearsal. Sound team arrive and setup in our church for our production of Godspell. We use a company that specialises in sound for theatre (both professional and amateur). I arrive, turn on the dimmers. No problems. Rehearsal runs fine.


Last Tech rehearsal (the next day). I arrive first and turn on the dimmers. Sound team arrives, powers up their gear. AWFUL hum. We track this back to the dimmer. We try everything to fix it, with no success. In fit of despair, I suggest turning off the dimmer and restarting it.


Problem solved - no more hum!


Now, nothing had changed between the two days. No power supplies changed. Being a church, everything runs from a single distribution board. The dimmer has it's own 32A 3-phase circuit. The sound gear (desk, amps, speakers etc) all runs off a common 20A circuit. The stage band runs from a separate circuit (I know, not ideal, but we have no choice).


It appears, that

sound then lights = no hum

lights then sound = bees in the speakers


Question - why does the sequence of powering up for the show apparently matter?



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It could be the switch itself. If it didn't make great contact the first time arcing across the contacts could have caused the interference and by switching it off and back on you could have cleared the fault.


As I'm sure you suspect the sequence shouldn't make a difference.

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I've had situations (not specifically involving lamps) where inexplicable hum is suddenly present on parts of a system - ie. mons, when items happen to be connected in a particular order. It would be solved by disconnecting/reconnecting some or all signal links with FOH, or powering down/up one half of the system. I've never understood why or tracked down exactly what could cause this (or found any earth fault), but I mention this as a similar example! Note that this is not the same as having a hum present when a signal lead is connected to a device that does not have power.
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