Film-Tech Forum ARCHIVE: Buzzing Sound Caused By House Lights

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Posted by Jason Setzer (Member # 3949) on 10-12-2006, 12:39 PM:

Ok in one of our theatres we have a constant 60hz buzzing sound. I know it is coming from the house lights cause when I turn off the breakers or switch the buzz goes away. I moved the breakers to a different spot in the box but that only worked for a day. The only lighting equipment I see up in the booth is 2 Sebco transformers(which I believe is for the aisle lights ). Can anyone give me a couple pointers on where to go from here without calling a tech out. Also the buzz is in all channels.
Just for reference the amps are QSC and the processor is a cp500. This theatre is a Regal theatre we took over that was built in 1999.

NM The problem was that whoever ran the electrical in this building ran the dimmers and house lights on different phases, plus shared nuetrals!(update)

[ 10-15-2006, 06:47 PM: Message edited by: Jason Setzer ]

Posted by Louis Bornwasser (Member # 3063) on 10-12-2006, 02:13 PM:

The physical distance from dimmers to sound is part of the equasion.

A recent war story like yours: the ground was not bonded to the sound rack, just to the outlet strip. This green wire was properly connected and ALL HUM departed. Good luck. Louis

Posted by Dave Macaulay (Member # 813) on 10-13-2006, 01:41 PM:

Check the grounding on the sound rack.
There's no reason to have all lighting on one phase, to keep phase loads balanced they should be spread across phases.
You cannot run a 120V device from a three phase system (or even a single phase system) without a neutral. You could, theoretically, (mis)use ground as the neutral but that would be illegal, unsafe, and fuching insane.

Posted by David Buckley (Member # 2600) on 10-13-2006, 02:11 PM:

Dimmer buzz being heard in the sound is almost always due to a problem in the sound system. I say "almost" 'cos one day I may come across a system where it isn't the audio system's fault, but it ain't happened yet.

Generally, dimmer racket its due to having different parts of the audio system (and in particular, two ends of a signal carrying cable, and even more particularly, an unbalanced audio cable) connected to different ground points. For a quiet sound system, you need to have a single audio ground point.

Posted by Mark Gulbrandsen (Member # 72) on 10-13-2006, 07:25 PM:

Actually it is preferable to put multiple channels of simple... up, mid, down dimmer on the same power phase, but not on the same breaker. Some dimmers will flicker if not connected to a common phase. Many simple multiple dimmers cannot always control when they fire the SSR or triac... large permanent stage dimmers have this firing control built into the computer that runs the rack allowing them to be evenly loaded across three phase power.

I believe CFS dimmers are one brand that have this problem, Lutron dimmers also generate aot of noise and garbage. Some dimmers generate ALOT of garbage or hash... meaning alot of residual signal that can also be rf and that can get back on the AC line. If your sound system is on the same phase as the dimmer then some of that garbage the dimmer is generating could get back into the sound gear. Good grounding and seperation are also factors that have to be considered but do not guarantee no noise going back. Some dimmers can be mounted close to sound gear and others cannot. Your tech should be familiar enough with many of them to advise you properly.
Note that the large coil often a toroid is on the dimmer output to smooth the AC so you don't hear the filament sing in indandescent lamps. Some models of Professional stage dimmers are available with different size coils depending on the size load attached.

Mark

Posted by Gordon McLeod (Member # 33) on 10-14-2006, 01:13 PM:

The purepose of the torroid coil or filter choke is to reduce the rise time of the turn on of the SCR or thyristor.
A small snuber of caps is recomended to reduce hf noise back on the power lines
CFS/Raven dimmers must be on the same phase as the control box generates the sync to the ac line for fireing
In most complexes I always do the panels so that the loads are on the same phase for each similar device

Because loads are always changing as things switch on and off balancing the phases is not realy aw important as is noise supression

Posted by Demetris Thoupis (Member # 833) on 10-14-2006, 02:09 PM:

Mark,
Actually the Lutron have introduced the Adaptive modules for their Homeworks series range and we have included them in some residential installations and are almost garbage free. In a theater through where we installed the old Homeworks with the U dimmers we had no problems with sound humming or buzzing at all. It was a brand new installation in all with Kinoton projectors and CP650's with QSC amps in all screens.
D

Sorry for the edit. Usually is the transformers used in the halogen lamps that create the huge problem. If you have a lot of halogens driven by transformers. try replacing them with 240 halogens and see what happens.

Posted by Jason Setzer (Member # 3949) on 10-15-2006, 08:16 AM:

We found the issue to be caused from the following.

1) The internal dimmers are on a different phase than the house lights.

2) Whoever did the electrical for this building forgot to run some nuetrals?@@F! [Mad]

Once we wired the dimmers on the same phase as the house lights ths buzz went away.

Posted by Gordon McLeod (Member # 33) on 10-15-2006, 12:27 PM:

Since the dimmer feeds the house lights they have to be on the same phase so I don't understand the issue
If it was the dimmer control was on a different phase to the power module in the likes of a Raven or CFS dimmer all that should happen is flickering
No where in your post do you say what make of dimmer is involved
Shared neutrals are common problem as it is legal and it needs to be specified in the tenders that continous seperate neutrals be run to all loads

Posted by Jason Setzer (Member # 3949) on 10-15-2006, 06:39 PM:

Gordon,

The dimmers are internal dimmers and I do not know the name of them ( I will find out though). As far as the phase issue I meant the automation for the dimmers is suppose to be on the same phase as the dimmers . I do know that there are shared nuetrals that were suppose to be fixed but never were.

[ 10-16-2006, 09:38 PM: Message edited by: Jason Setzer ]