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[Bill Joyner]

I need to install some dimmers in our church sanctuary.  They will carry loads between 500 & 600 watts.  My concern is the interference with the sound system.  Someone installed basic off-the-shelf dimmers from Lowes last year and had to remove them because of interference.  Can anyone recommend some good commercial dimmers that have good rf filters?

Thank you for any suggestions and assistance.

Bill

[David Buckley]

If you are getting dimmer buzz in the sound system, it's down to the sound system not being configured correctly, rather than any noise coming from the dimmers.  All dimmers generate interference, and a well configured sound system will reject it.

You can reduce the magnitude of the buzz by increasing current risetime, and the way most dimmers do that is with a choke, so (all things being equal) a heavy dimmer will create less interference than a lightweight dimmer.  Lutron sell an after market choke called a Lamp Debuzzing Coil you may wish to try.

Footnote A - actually some dimmers don't generate interference, but they are quite unusual, and are either old like rheostatic or variac based dimmers, or expensive, like sine wave dimmers.

Footnote B - some instruments pick up dimmer buzz badly, mainly single coil pickup electric guitars, so strats and teles.

[E. Lee Dickinson]

David Buckley wrote on Sun, 09 December 2007 22:28

If you are getting dimmer buzz in the sound system, it's down to the sound system not being configured correctly, rather than any noise coming from the dimmers.  All dimmers generate interference, and a well configured sound system will reject it.

Can you elaborate on this some, in terms of specific steps to take? We're pretty good about putting all of our sound on the same power, with common grounds, and using all balanced interconnects. We still pick up dimmer buzz sometimes.

Typically it's fixed by finding some piece of gear somewhere (a laptop with a cheap power supply, the direct out on some muso gear, etc) and inserting either a hum eliminator or a DI with a signal ground lift.

Aside from good grounds and running all balanced, what systematic policies should be in place when sharing power with dimmers?

[John W. Boneske]

Don't share power with any lighting stuff that is on your sound system!!! That may cause some of it, but you should not have a noticable hum with lighting...Although I use all DMX stuff...

[E. Lee Dickinson]

John W. Boneske wrote on Thu, 13 December 2007 14:16

Don't share power with any lighting stuff that is on your sound system!!! That may cause some of it, but you should not have a noticable hum with lighting...Although I use all DMX stuff...

Unfortunately, that's not always possible. Plenty of venues only have a single three-phase disconnect.

I guess what I really need to understand is what is the root cause of lighting-system induced hum and buzz? Are cheap dimmers backfeeding harmonics onto the power lines which the sound system is rendering as humm? Are they dragging the ground or neutral potential up a few volts?

In the past someone recomended that all the lights go on one phase, all the sound on another, all the video on another. I just can't see unbalancing the transformer and neutral by 100kw or so.

I will add that, in some instances, it's prudent to bring along a generator or something to move the lights to their own power. Unfortunately, that causes even bigger problems for us, since our sound, lighting, and video are all typically controlled from a single computer.

[David Buckley]

Issue number one you've already got; improper grounding.  The entire audio system should have a single ground point.  Traveling shows manage this well by using a distro, and running power to the mix position by a long cable alongside the multicore.  Installs often want to use sockets on the wall, and then the electricians get involved, and thats the end of the single point ground.

Issue two is high impedance and/or unbalanced signals.  If you have an unbalanced source and it needs to travel some distance, balance it with a DI box.  Everything on stage should be a balanced source to the mixer, either naturally balanced (eg SM58) or go through a DI.

Issue three is a variation of issue two, equipment built into plastic boxes.

Laptops are a nightmare and seem to be getting worse; use a passive transformer isolated DI box, the lowest cost one I know of is the EWI  FDB101, which I use myself and is a great hum lifter.  It may not be the highest quality DI box, but I'll trade losing the racket that laptops make every time.  With computers in general, an external USB or firewire connected audio interface is better than an internal audio card, and much better than using the speaker output...

A low impedance balanced audio cable like goes from a mic to a mixer is almost impossible to get buzz into.  The multicore returns to the amplifiers are even harder, as the signal level is so much higher.

Things that don't make an appreciable difference - using separate power, using different phases.  If this does make buzz go away, it's just telling you there is still a fault to find.

Finally, persevere - you can make your audio system bombproof.

[Duane Massey]

It's not very common to get a "hum" from a cheap dimmer, but it is common to get a "buzz", especially into hi-Z inputs (guitars and basses most notably). A better-quality dimmer will help a great deal, and locating the dimmer away from the stgae also can make a difference.

If you paid $6.95 for the dimmer, it's very suspect. It's been my experience that staying away from the 600w wall dimmers, and going for the 1500w or 2000w dimmers will give you a better shot at not having the problem. Most better-quality entertainment dimmers have chokes built into them, and RFI is greatly reduced.

And sometimes you just have to get a better guitar....