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#48928 - 02/23/05 01:37 PM Dimming panel & sound board noise  
Jps1006  Offline
Member
Joined: Jan 2004
Posts: 615
Northern IL
Just stopped by the church today. They are thinking of adding some stage lighting and possibly changing out some of the house lighting. House lights are quantity of 5 MH 4-lamp pendant type. These cause interference in the sound system sometimes when kicked on. Also the existing stage lights are PAR 38 floods on dimmers. These dimmers are 10-20 feet away from sound board. They also cause inherence to the sound. Lighting and audio is fed from the same 3 phase 200 amp panel that is located 5 feet from the sound board.

Consultants came through and recommend a dimmer panel for control and for the future expansion of stage lighting. They say it needs a separate 200 amp feed. We would move the entire lighting load to this dimming panel that will be 25 feet away from the board. I am going to call them shortly to see what they have to say. But the questions that initially jumped through my mind are this:

Does interference travel through the wire or through the air (RF)?

If through the wire, how does a different feed correct this? Wouldn’t continuity through the buss at the service render this solution useless?

If through the air, shouldn’t we be considering moving the dimming panel further away?

My role is to figure what it is going to take to get 200 amps to this panel. I just want to make sure our solution is heading in the right direction. If our load is not exceeded, can I come off the existing 200 amp panel (that is assuming the new 200 amp feed is overkill, which I am guessing. We don’t want this place to look like a ballpark) or is that just asking for noise problems? I’ll post anything helpful the consultant has to say.


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#48929 - 02/23/05 02:22 PM Re: Dimming panel & sound board noise  
hbiss  Offline
Member
Joined: Dec 2003
Posts: 886
Hawthorne, NY USA
This is a complex situation which there is no one easy answer to, other than just not using dimmers. The sound system installation and wiring itself can be just as much to blame as the dimmers, lighting and how they are supplied.

Assuming your consultant knows what he is talking about, the additional 200A sub panel needs to be fed from the service not the existing panel. This is to minimize neutral noise that could be getting into the sound system via the shared neutral.

Does interference travel through the wire or through the air (RF)?

Primarily through the wiring and grounds.

-Hal


#48930 - 02/23/05 02:55 PM Re: Dimming panel & sound board noise  
SolarPowered  Offline
Member
Joined: Jul 2004
Posts: 625
Palo Alto, CA, USA
A high-quality dimmer pack will have chokes to reduced both acoustical and electrical noise. This is done by the choke increasing the rise time (dv/dt) of phase-control edges. That should help your problem considerably.

As hbiss said, these problems can sometimes be quite involved. It's important that grounding and shielding of the audio system be done correctly.

Something that comes to mind is whether you have professional or amateur audio equipment. A lot of amateur equipment uses unbalanced connections between equipment, which is quite susceptible to noise, especially ground and neutral noise from the power system. All the connections between components, and from the microphones, should be balanced--that is, with a plus, minus, and shield connection. These are usually carried on either XLR or Tip-Ring-Sleeve type phone plugs. If you have any RCA connectors, or Tip-Sleeve type phone plugs, you may have problems. (I hope I'm not insulting you by explaining this. Also, the vast majority of church systems are pro-grade balanced systems, so this probably isn't the problem. But... it never hurts to check out the obvious.)

The dimmers in my church are fed from a 200A feed to two ETC dimmer packs, and an additional 100A feed to a third dimmer pack. So I'd say the 200A they're looking for is within reason.

[This message has been edited by SolarPowered (edited 02-23-2005).]


#48931 - 02/23/05 05:59 PM Re: Dimming panel & sound board noise  
mhulbert  Offline
Member
Joined: Oct 2004
Posts: 132
Anaheim, CA USA
Ina ddition to what others have said, a LOT of times, the audio side wiring is at fault. Without getting too far in to it, you should make sure that your shields on all of your MIC wiring are connected only to the shield in the cable. The shell of the connectors should not be attatched to Pin 1 (Shield). A metal jack in a metal, grounded box can produce the dreaded "ground loops". You have to check the mic cables and the mics themselves too! Also, on line level runs from the mixer outputs to amps and equipment, only one side of the cable shield gets connected, snip the other end (I like to lift it at the amps).

There is a lot more to this (a good book is "audio system design" by Phillip Giddings, a good read for any electrician), but in my experience, almost all these problems can be attributed to audio cable problems. If you exhaust all other options, put all the sound gear on an isolating transformer, 120/240, it doesn't have that much load and it does not need 3 phase.

Also, I'm sure you checked this, but a N-->G bond at a subpanel or dimmer rack will raise absolute hell on everything. Maybe your problem is as simple as that.

If you need more info about this just ask, i did sound systems and touring audio for 3 years, and we ran into all sorts of things

Cheers,
Mike


#48932 - 02/26/05 10:45 AM Re: Dimming panel & sound board noise  
Jps1006  Offline
Member
Joined: Jan 2004
Posts: 615
Northern IL
thanks guys. Very helpful. Solar, my intelligence is rarely insulted, I don't know if it knows enough to be.

Mike, I will check the N-->G connection. That's an easy one for me and I'm still in my element. Great suggestions already, I'm sure I 'll be back for when we get there. I still haven't talked to the consultant yet. He'll be back Monday.

After talking to the AV guy, noise isn't a persistant problem. He says occationally when the Metal Halide lights are fired up, you get that intial arcing, and sometimes a little pop. But sometimes it's worse than others.

Is that something that can be avoided with an arc like that? Isn't that a case of interference travelling though the air?


#48933 - 02/26/05 02:16 PM Re: Dimming panel & sound board noise  
hbiss  Offline
Member
Joined: Dec 2003
Posts: 886
Hawthorne, NY USA
No, its getting into the sound system through the wiring. How are the lights supplied?

-Hal


#48934 - 02/27/05 02:06 AM Re: Dimming panel & sound board noise  
mhulbert  Offline
Member
Joined: Oct 2004
Posts: 132
Anaheim, CA USA
Is the noise from those metal halides coming in on the mic lines (Mic's on stage--->mixer) or on the lines that go from the mixer to the amps? This will split your troubleshooting area in half. Try this: Mute the sound board, making sure all your outputs are turned all the way off, auxes, monitors, everything. Turn on all the amps, and then start up those MH's...do you hear the interference? If yes, it's on the lines from the mixer to the amps, check the shields at both ends of these cables...one side should not be connected to anything, the other should be connected to pin 1 of the XLR connector (3 pin, mic style connector, the pins are labeled). The shield should never be grounded to conduits, boxes, etc...think of it as a neutral, you only tie it to ground (pin 1)in one spot, and it stays completely isolated therafter.

This is not the case on mic cables, they need the shield to be connected all the way through...but it still should be isolated from chassis grounds, etc. Treat it just like a signal wire.

a board like this one that has a lot of info on live audio is http://srforums.prosoundweb.com/
check out some of the past posts, there are many others in your situation!

Oh yeah, make sure all involved equipment has its ground pin intact and that you have good grounding to all outlets. Also, something I used to do a lot is check the voltage at outlets from Neutral to ground with a digital meter, shouldn't be more than a volt or 2, if you have weird things going on, this may indicate problems with wiring on your side of things.

Let us know what you find out!



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