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Sound system noise from dimmers

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Can anyone give me the technical reasons why sound systems develop noise when operated off the same power supply as dimmers. This might help me sort my problems out.


Current example


2 strand LD90 dimmer racks wired single phase, Red Phase Yellow phase, from main distro, 100A cable only 80A fuses tho :)


Lots of 13A sockets, phase unknown, load capacity unknown, earth could be anywhere, all I know is that the cabling isn't routed along the same run as the dimmer mains but probably eventually is sourced form the same point.


When setting up a band event, unfortunately having to use battered old kit and kit with unbalanced connections, keyboard amplifiers, cheap powered monitors etc. Everthing fine until switch dimmers on and suddenly everything sounds like it's plugged into a fridge.


Am trying to get the electricians to label my 13A outlets and tell me something about the electrical wiring of the building.


Any help much appreciated




PS if you say buy better kit, please forward all replies to the management team at the venue

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I have been working in a new venue where we have exactly the same problem with 2 LD90s however mine are 3 phase, the only way I can see to stop the problem is call in the company that installed it, I have not yet had a chance to speak to the company but should do in the nbext day or 2 and let you know what ours is so you can check yours.

from what I have been told so far the install company cant find any problems... I dont want to mention any names but will say that I will be interested if anyone has any ideas of what this could be as the company I am going through is well known and I doubt very much would say nothing was wrong if it was.


ill let you know if and when I find out what mine is, but I have found it is reduced with higer levels on the dimmers.


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I need to go to bed (!) but here is a brief answer.


Dimmers produce radio frequency interference (worst at 50%). This can be transmitted to audio equipment via two methods:


1. Through the mains. The interference is transmitted back down the supply cabling from the dimmers and gets into the sound equipment through the mains supply. Solutions are (I) run dimmers off separate phases to the PA and (ii) don't feed PA systems from the same power circuits as the dimmers (have separate cabling back to the main electrical intake in the building).


2. Through the air. The interference is transmitted from lighting cables (most commonly between dimmers and lanterns, but also from dimmer supply cabling) to audio equipment or cables (cables are the usual cause). Solutions are (I) keep PA equipment and cables away from LX equipment and cables, (ii) sound cables should cross LX cables at right angles, (iii) if sound cables and LX cables run parallel, try separating them by a foot.


Inadequate or incorrect earthing can contribute to both the above.


Low level audio cables (for mics) are obviously the most sensitive to interference. Speaker cables are extremely unlikely to be a problem. Older or cheaper audio equipment is more susceptible (especially with unbalanced connections as you mentioned, but don't think balancing is a cure-all).


To find out what's causing the noise will take a bit of detective work and logical fault-finding. In your situation it could be many things at once :)

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Common causes of dimmer hum on the sound system are :


Mains borne RFI coming down the line from the dimmers to the sound gear.

Getting the noise and lights on seperate phases really helps this. If that's not possible install some nice chunky roxburgh line filters on the mains feeding the sound system, and maybe the dimmers as well. Don't be tempted by cheap no-name filters, they don't work as well and occasionally fail in entertaining ways.


Radiated RF will be picked up by anything that looks like an ariel, so Don't coil cables. This is _really_ important. Zig-zag both audio and lighting cables like you were taught, and you may well reduce your RF problems significantly. Scattering a few clip ferrites around the signal level audio cabling can also help here.


If those two don't solve, or at least reduce, the hum then you may well have earthing issues. Poor earthing, and earth loops, can contribute to hum big time. Those are probably best left to the people who installed the system.

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It does seem to me that LD90s are most suseptable to this. One venue I worked in had a nice 63A single phase connector located under the dimmers. "Great" said the noise boy. I'll plug in my distro there. Hmm.... maybe not. In that situation it was the feed to the amps which caused the main problem (that was all the distro was being used for), but as has been said already, it could be a combination of factors in your situation.


Good luck




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Running sound on its own phase isn't going to be practical in most theatres - for reasons of balancing dimmer load across the phases.


What you want is a separate 'clean feed' for the sound system, probably from its own dis board, with local RFI filters, and a sensible earthing scheme. You then want to make sure that all sound equipment and only sound equipment is plugged into this system. For a quick fix, RS do 4-gang 13A blocks with mains filters on the plug end, which can help.


Also, you could look at getting some DI boxes, taking everything to balanced connections, and some mic cable with better screening.


Finally, remember there's not a lot you can do about the audible buzz coming out of parcans at 50% except keep you mics away!

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Cheers for all that,

however the biggest problem is that we do not have an installed sound system to use. Subsequently we create a sound system from the best of what we have available at the time. Know how to minimise airbourne noise as was mentioned, but it is coming through the mains supply as well. I would use the blue phase, only I don't know where I've got blue phase! Nothing is labelled in the building and all the wiring is hidden, and RCD's and MCB's are in locked distro's! :)


Going to see if any particular circuits on the dimmers cause the most interference, but from working there for a year and a half I'm pretty sure that it is every lighting circuit.


In answer to all your suggestions


It isn't a professional theatre, and hasn't been designed as one (don't get me started)

All mic cables etc are of high quality, made them myself.

And yes I am a professional sound technician, I do use those DI box thingys.

Wouldn't entertain the rfi filtered 4 ways as they are just rubbish.

Parcans are currently not in use as are electrically dangerous, other generic lighting 7m up.

Only becomes a problem with a full band set-up not just two speakers and a amp (usually)


Think the design of the building acts as an ariel, entire builing is made from very large very interconnected exposed steelwork.

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Owen, You say that the problem mainly occurs when you are set up for a band, is it the same band/backline equipment. Some of the more domestic backline equipment available (including Marshall and most other major manufacturers) have design problems with their grounding and can be very susceptible to inducing hum through a system, Peter
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Only becomes a problem with a full band set-up not just two speakers and a amp (usually)

Ahh, the clue may be in the “full band set-up†bit!


Have you got all the sound equipment connected so that it has a common earth point? If say your desk is referenced to earth at the back of the “stalls†or wherever, and the backline / amps are on stage connected to earth there, you can find a voltage between the earths. This will cause a current to flow in the screens of your audio cables, and this will cause a hum.


These "earth loops" can be the very devil to cure.

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Simple cure! Disconnect the earths!


Seriously, I have actually seen an idiot sound tech unhooking the earths from the plugs of some big marshall stacks to cure the hum.... :)


If the building is fairly new then you'd be within your rights to get the electrical contractor to come in and 'explain themselves'. Although you might find that the ground is up to code, even thought it's not really up to the audio standards. Another thing for Bryson's list....

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Looking for jobs at the moment, so it hopefully won't be a problem much longer!

Have tried every configuration of electrical supply for the FOH desk, but the main problem lies in the monitors, keyboard amps and of course the bass guitar!


When the monitors have a bad earth (unbalanced Connection) it seems to induce back into the FOH PA.

Foh Pa desk runs the monitors.


PS have experience of fender passport PA systems (drum and keyboard monitors) doing this if the wind is blowing the wrong way anyway.


Thank you for all the help.


Please don't over do the replies, have all the info I need.

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