Dimmers & Sound Systems

Status
Not open for further replies.
D

David Klipper

Member
Location
Houston
My church asked me to look at a buzzing noise coming from the sound system in the youth ministry building. The buzzing is only distinguishable at the lowest dimmer setting. The sound board is plugged into a receptacle (via a UPS) immediately downstream from a (2) 1000w dimmer switches which control (6) 100w incandescent floods each. The floods are in a separate conduit run. The building is only 2 years old, but the dimmers are cheap (residential grade), and the contractor was sloppy (I've spent the last 2 years correcting many errors, malfunctions and failures). I have 20 years experience in residential/commercial/industrial/service and the last 15 years as an industrial electrical designer. I have never come across this problem, so never had to think about it. Will changing out the dimmer help (better quality dimmer or electronic type? any suggestions)? Running a new line off the other phase doesn't seem logical (harmonic noise may still occur). I hinted they may have to keep the dimmers off when using the sound system if new dimmers don't work, but this seems smug. An isolation xfmr is too expensive. Any thoughts or suggestions would be appreciated!
 
I would check for neutral/ground current first, simple enough to test.

play with the dimmers second, and try a different type,

Most likely the UPS you describe is not a true UPS in that it supplies power from utility when not on battery.

If this is a plug in sound board surely they can spring for a ISO trans, just to test the system


On second thought.....

Unplug the UPS with the soundboard operating, play with the dimmers see if this clears up the noise.
 
What is the purpose of the UPS; is this a digital desk or is there a PC nearby? Test your system without the UPS if you haven't already. My suggestion though, ditch the UPS, a sound board is not a critical load. Also, is there a hum component or just a buzz.
 
Just a buzz. Yes simple UPS (local cord connected) is for PC. Never thought to disconnect soundboard and plug directly in wall outlet. Thanks for reply (to both y'all)!
 
a sound board is not a critical load
Click to expand...
YOU EVER BEEN IN ONE OF THOSE MASSIVE CHURCH'S, tell that to the Reverend. IMO a critical load is in the users mind. But I would bet the UPS was installed to minimize the effects of PQ issues which it can address minimally depending on the manufacture and model.

If you power the system from the UPS while on battery you can rule out some of the issues associated with improperly grounded neutral.
 
Yes cheap dimmers are notorious for "noise" especially at the dimmest settings because thats when the SCR or Traics are shutting off most of the cycle and turning on when voltage is highest (also generating more heat) so replacing them with higher quality might help. You should probably run a few tests first and try to determine if its electrical or RF noise or both and see if it can be filtered or solved by an isolated ground, etc. Unfortunately sound boards are notorious for picking up both electrical and rf noise as well.

I never thought of testing with the UPS unplugged... w/power and ground isolated if there's still noise it would tend to be an RF problem. Unfortunately if its powering anything more than a mixer it probably will die real fast... amps take power, and unfortunately most UPS don't make a clean sinewave and will probably make much more buzz in operation. Another thing we do for testing is lift the ground for all audio equipment (NOTE this is unsafe and is for testing only!!!) If buzz lessens try testing a differnet circuit preferably not in the same MWBC so seperate neutral and ground. Whenever we're wiring auditoriums and classrooms we always run completely seperate feeds for lighting and sound systems... but trust me the buzz still tends to be there even with the best equipment.

And yes I've been involved with massive churchs for 30 years doing sound, lighting, and video.
 
Last edited:
David,
If removing the UPS from the system has no effect, can you tell us how close the dimmers are to the sound station?
The reason I ask is that even though the wiring is in metal conduit, the face of the dimmer is a good radiator of RF energy, and the sound equipment if close enough can pick up the noise from the dimmer(s). I've solved some of these type of problems by using a steel cover plate on the offending dimmer.
Of course this is not an option if the dimmer has a finned heatsink on the face that prevents this. Aluminum has almost no low frequency RF mitigation so the heatsink is useless in this regard. Steel is a very good barrier to low frequency RF energy. The Steel conduit association has a lot of info on this. http://www.steelconduit.org/
 
Last edited:
99.99% of the time dimmer buzz in sound systems is a sound system problem in need of fixing. And many of those times its down to the absolute basics, like there isn't a single common ground point for the audio system.

And the other 0.01% of the time - I've no idea, still waiting to see one of those...
 
I agree with dbuckley...make sure there's not a sound board, grounding or mic cable wiring problem first.

Pull all of the input cables off the sound board and turn down the input pots (volume controls). See if the hum is present with the dimmers on. If it's not, start adding input cables one at a time and set that input pot back to it's normal position. Keep trying until the hum comes back...you might be able to isolate this to a single mic circuit.

Also, keep unused mic channels turned off at the sound board; unterminated mic cables can sometimes suck in noise or RF.
 
Rant Of An Electrician

Rant Of An Electrician

While I am never surprised by what may or may not cause electrical problems, I can tell you in the last week I have corrected 4 hum issues at a HUGH Church, a major Cable channel, an apartment building and a data center.

Everyone of them had the exact problem in various forms, a grounded neutral downstream from the main service or separately derived service, each and every error was a byproduct of sloppy electrical work. IN each and every case the electrician was given the opportunity to find the problem and in each case they said they had expended all possible avenues and the issue was with the equipment. I try not to be blinded when I investigate these issues but based upon my past experiences I always look for ground current first, when a building has separately derived distribution systems it simplifies locating the problem over a building with 208/120 distribution.

Accidentally grounded neutrals such as 6/32 cut into the wire, wire nut fell off, neutral intentionally grounded downstream for what ever reason (OH THEY LOVE THAT GREEN SCREW), defective busway neutral shorted to ground, neutral from one transformer tied into circuit of another transformer. Primary Neutral pulled into a delta wye transformer and connected to the secondary XO/Neutral/grounded conductor, 120 branch circuit neutral connected to a 277 vac circuit, neutral bonded on the load side of the Window Ct of a GFP system, EGC tied into neutral bus downstream of main service and of course a EGC utilized as a neutral because they forgot to include a neutral in the circuit, (I have seen this on 20 amp branch circuits and on a 400 amp feeder).

Then there was the electrician at a code seminar I was at 3 years ago that asked why manufactures packaged a 10/32 X 1" green screw with every panel, BIG MOUTH me butted in assuming He did commercial work and I said not to worry throw them away, WHOOPS he does track homes and had a large box of these screws. I stuck to my guns and told him to throw the evidence away.

So if sometimes I seem cynical, short or just upset, consider that I just cannot believe the lack of care lack of education and just plain sloppy work I see EVERYDAY and it really bothers me. I go out of my way to assist educate help and it seems to me MOST do not care WHICH REALLY BUGS ME.

*Hum- not always in the audio but also in video seen as hum bars-scrolling lines...Used here as an all encompassing term.
 
Last edited:
iwire said:
An IG?
I think that would be an entire waste of time and money.
Click to expand...

I didn't say it was my first choice... closer to my last, but sometimes it does work. I don't like that the mixer is downstream or anywhere near the dimmers (we always run them seperated)
I'd try lifting audio equipment ground first (cheap/easy test)
If mixer & amp are seperate try isolation boxes/lift grounds on their feeds. Only use low impedance inputs/ mics.
I'd try high quality dimmers... (I have free access to good used ones). Then depending on results could try putting all the audio equipment on the same phase as lighting, then opposite phase. (just run cord to mixer from where the amps are)
I spend more time installing electronics than electrical, including studios, auditoriums, churches, restaurants, clubs, etc.
Part of the problem with Triac dimmers is their use of zener diodes to trigger the SCRs. Its kind of like putting an arc welder next to an AM radio.
 
Forget the IG.........Install it RIGHT from the beginning, megger all neutrals prior to final terminations and you'd be surprised how humless the system will work....
 
Donny So let me see someone explains something you do not understand, agree with or care to hear so you want to shut them up. HMMMMM you're one GOOD? engineer? Lets not learn something.
 
Last edited:
DaveTap said:
I didn't say it was my first choice... closer to my last, but sometimes it does work.
Click to expand...

If there is an electrical interference / noise on the EGC it will also end up on the IG as and IG is really not isolated from the EGC.

If there is a situation with current flowing on the EGC perhaps an IG can help with that, but I don't see that as the problem with the dimmers.

Besides, even if you install an IG it will very likely become compromised by incidental contact with other grounded objects.
 
back in the tube days it was different, but it boggles my mind why, in this age of cheap electronics, that all sound systems aren't built with iso transformers and dc power to completely wipe out all unwanted harmonics.
 
Initial thinking says look into the class 2/3 side of the issue. Proximity of conductor and cables, and grounding of the equipment on the audio and chassis side of things. Not that I would suggest cutting the grounds off of cords, but for troubleshooting purposes it may help to find the 'pick-up' for this noise. It is obviously finding its way to the ampifier somehow.....

And now for a wacky AV guy story.... Several years ago I was installing some transormers in a hall to supply a big AV presentation it was day time and all of the skylites were open so the whole hall was day lite with bright sun - and I see this guy crawling around on the floor with a hand-held flouresent shop light starring at the cables on the floor. Apparently this guy was trying to find a break in the cable by listening to the hiss created by the light on the PA system.
 
e57 said:
I see this guy crawling around on the floor with a hand-held flouresent shop light starring at the cables on the floor. Apparently this guy was trying to find a break in the cable by listening to the hiss created by the light on the PA system.
Click to expand...

I was watching rodies set up a concert and the audio snake that runs from the front of house to the stage had broken conductors in it.

The sound guy cranked up the system to '11' and dragged the end of a live extension cord along the audio snake until the he found the spot with the breaks by the very loud AC hum that came over the system. He then cut into the snake and made repairs, he repeated this a number of times and fixed a few breaks, I thought it was pretty clever for someone that looked pretty wasted.
 
Status
Not open for further replies.
Top