emf is a little devil

This is not CAR audio, but it is AUDIO related, and I know there's knowledgeable people here that may not be in the general section. Here's the situation. I have an intelligent lighting fixture used in concerts that contains an electronic ballast system. This ballast is inherently going to give off an Electromagnetic Field, and that is presently the problem. When any of the guitar players get within about 36 inches of this unit, a very piercing 4KHz signal is produced. Because of how sensitive the pick ups on the guitar are (This is a hard rock group and they put all their knobs at 11, and turning it down isn't much of an option, because it's part of their style) it is taking the the harmonic noise created in the EMF and sending it as a tone. It is not being intercepted by the belt pack on the guitar. I verified this by running the guitar to the racks via hard line. The simple options are to either put a fence around the lights and not allow the band to travel near (not do-able) or get rid of the lights completely (absolute last resort) Now, our Front of House Engineer posed a noteworthy issue, our stage is presently not grounded. They believe that grounding the stage may give some of the EMF somewhere to deaden itself. I can't decide how much weight should be put into that. The idea of a Faraday screen was my first thought, but our Stage Manager says he has tried this before on a different gig and to no avail, but I have no knowledge if it was applied correctly. I'm open to all range of suggestions, but professional opinions only please. This is a problem who's ultimate solution allows the guitarists close proximity access to the lights, and keep the lights in their present positions. I thank you all in advance for allowing me to pick your brains on some possible solutions. This isn't car audio, but it's audio in the long run. That and there's some uber knowledgeable people here. Thanks guys p.s. No, I can't say who the artist is, my job could be put on the line by putting out something of this nature with their name in tow. But they would appreciate any intelligent solutions we could conjure up.Greed is for amateurs.

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There are many people here more knowledgeable than I however I will give you what came to mind for me. Can the ballast be relocated? I know you said it was built in but it may be a separate unit inside the case and therefore may be able to be relocated off stage. The other obvious answer is to get a different light that has an outboard ballast but I do know how expensive lighting can be. I don't think that grounding the stage will help but it may. The problem I see is grounding it reliably at every location then, even after you did that, I don't see it solving the problem as there would still be something emitting an EMF very close to the instruments. It may be worth a shot though. Hopefully someone with a little more electronic theory knowledge gets on this and helps out more than I can. Richard2008 Scion xB

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"a noteworthy issue, our stage is presently not grounded." I have no problems lifting signal ground but power ground, Never. This is a huge liability. FOH and the stage should be bonded together with the same ground and power source. What brand of Mover are you having the problem with? In 18 years I haven't heard of this problem. EMI and RFI are very difficult to deal with. The good news is you have narrowed down the culprit. Is the lighting Rig grounded or sharing the same power feed? Have you tried swapping out the fixture?

audiocableguy wrote: What brand of Mover are you having the problem with? In 18 years I haven't heard of this problem. EMI and RFI are very difficult to deal with. The good news is you have narrowed down the culprit. Is the lighting Rig grounded or sharing the same power feed? Have you tried swapping out the fixture?

This is my first time coming across the problem myself. They are Martin MAC 700 fixtures, and it's not just one, it's everyone that is on the floor. I proved to our LD that it was our lights by bringing out a Coemar 1200 Wash and a Vari*Lite 3500 Wash, and the same problem occurs. Yes, the lighting rig is grounded, but sound and lighting power are on 2 different transformers. Stage was grounded, and to no avail. I used a sheet of lead this morning and laid it over the fixture and it stopped the sound. Unfortunately this isn't a practical fix for the issue. I'm still debating on building a Faraday's Cage around a fixture to see what it does for me.Greed is for amateurs.

Disorder,chaos,anarchy now THAT is fun!!

jazzcustom131 wrote: Yes, the lighting rig is grounded, but sound and lighting power are on 2 different transformers.

Try eliminating the ground loop.  Build a ground-to-ground jumper and see if it helps. (Third prong to third prong, hot and neutral not wired)

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The Mac 700 is a short arc MH bulb that uses a mechanical douser. No dimmer. Electronic ballast. Once struck, the bulb is "on". Control is a modified RS-485 (DMX). The moving head sits on the ballast while the "brains" are in the arms. A guess would be there is an opto splitter isolating the lights from the board. If DMX gets funky grounding the light wigs out. My Guy at Martin has left so I don't know any of the new staff. I will call my buddy at Bandit and see if he might have some answers. It's too late back East to get anything tonight. You might want to try a post on prosoundweb.com and see if you might get some answers from other tour guys. I do some more research and see what I can come up with.

The problem might just be with the pickups gained so high. If they are singe mode magnetic pickups, they will puck up about everything. Dual Humbucking magnet work much better by canceling out interference. The other option would be piezoelectric pickups. I've had bands who want to play to a crowd of 2500 with stage volume. I remind them I have a Full rig to do that for them. Is it possible to lower the gains on the pickups and add wedge volume to gain the players perceived loudness on stage? Just throwing out ideas at this point...

audiocableguy I guess I forgot to mention, I'm a board level repair tech for these fixtures, so looking for grounding issues was one of my first thoughts. At this point, I'm at the point of believing that in order to solve any problems, we will have to build Faraday Cages around the fixtures. It may look odd, but the audience wouldn't think they were anything more than Pyro pots. Unfortunately the Backline techs are dead set that it is my problem. Though I may be the cause, there isn't anything more I can do besides experiment with Faraday Cages and I'm willing to bet the lights will simply be struck from the set before I'm allowed to do that. As far as the pick ups go, they are EMG Active pickups, I don't actually know anything more about them, other than what they are doing and looking for. And being cranked up so high on the gains is apparently part of the band's sound, and reducing it would change the tone from the instruments... Or so I'm told. I have a hard time believing that personally, but until I have to send the output from a band directly to a car audio system, I'll never personally know. Thanks for all the help everyone, I'm going to hold my ground that the guitar techs need to properly shield their pickups, and that the only thing I have left to do is a Faraday Cage, which won't be cheap... I'd need about 10 square feet per fixture, and there's in excess of 20 of the units.Greed is for amateurs.

Disorder,chaos,anarchy now THAT is fun!!