Light Dimmers

[JPickman]

Hello,

I had the understanding that dimmer switches on lights could mess with the electrical outlets in the studio and created hum and ground loops.

My question is--If you have lights on a completely different circuit in the circuit box than the outlets for the audio equipment....is this still a problem or can dimmers be used?



Thanks for the help.....Pick Man

[knightfly]

Having dimmers on a separate circuit will help some, but there's a good chance you'll still get noise. Here's why:

SCR dimmers work by delaying the turn-on time of the SCR's in the dimmer circuit by a certain amount of EACH CYCLE of the AC power. What this does for dimming, is to decrease the total power available to the lights, so they get dimmer.

What it does to the power itself is another thing entirely - since each half-cycle of the AC power is partly switched off, by the time the SCR switches the part of each half-cycle that is going to be used, the voltage across the SCR is no longer zero, so switching takes place during a part of the cycle where more voltage is available. SCR's switch so fast, that when they DO turn on, there is a very high frequency "spike", or overshoot, in the voltage. Since the switching happens so fast, this spike carries a lot of high frequency energy, clear up into the Radio bandwidth.

Since the interference is now in Radio Frequency bandwidth, it can actually "broadcast" to the wires and circuit boards in audio gear. Since the bursts of RF caused by the dimmer are still at 120 hZ rates (one for each half of the 60 hZ line frequency) you hear a 120 hZ "buzz" in your gear, and finding out which (or how many) units are guilty is a major pain in the ass.

That was an over-simplified explanation, so any of you junior techs out there, please save the flames - I've been an Electronics Tech since 1965, I've worked on Pro Video and audio gear for several years, and currently (pun recognized, but not originally intended) work in Industrial Automation systems, IT systems, and hold two electrical licences.

If you absolutely MUST have dimmers in your studio to be happy, I would get just ONE, higher quality dimmer that fits in the switch box, turn off the breaker to that box, TEST to make SURE it's off, and replace that particular switch with the dimmer.

To try it, set the dimmer about half-way up, take a guitar with single coil pickups, like a Strat or tele, plug in, turn on the amp, and wave the guitar around overhead (near the dimmed lights) with the volume turned up. If the dimmer is bogus, you'll know IMMEDIATELY, in which case you could move it to the dining room for mood lighting, or whatever. If it causes no problems you can hear with ALL your gear on, do the other lights and rejoice...

I hate bullshit so much, that I didn't even try dimmers in my studio. Instead, I installed two different lighting systems on multiple switches. One system uses incandescent track lights with theater gels for different colors/brightnesses, with 4 different circuits so I can turn on just one, any two or three, or all 4 depending on what mood I want. The other system is for working (maintenance, wiring, etc), and consists of flourescents mounted behind all racks, etc - these are ALWAYS turned off when recording, as flourescents and their ballasts tend to also produce hum.

Hope that helped... Steve

[DeadPoet]

interesting subject... would having a power cond between box and dimmer help ?

So like: 1 circuit for audio, 1 circuit for lights, each with their own power conditioners.


Herwig

[knightfly]

Good question, Herwig - thing is with RF interference (RFI for short) - a lot of times, you almost have to TRY it in your own set of conditions. For one thing, there are conditioners, and then there are CONDITIONERS, know what I mean? OK, what I mean is that, just like Behringer Vs. Soundcraft or Neve, etc, there are all levels of quality/design. One might accomplish almost nothing, the other do the job just fine.

There are several kinds of ways to cut noise in the higher frequencies, some being MOV's, capacitors, inductors, ferrite beads, heavy shielded chassis, phase cancellation devices, etc, in any combination.

The only problem is that once the RFI spikes get into ANY wiring, they now have an ANTENNA from which to broadcast - at that time, it's up to each and every piece of gear in the studio to accept or reject this interference on its own.

Some of the better quality dimmers (read "more expensive") have better snubbing components built into the circuit, and that's why my advice above. I can't possibly test every make and model of dimmer personally to find which works and which doesn't, which leads us back to my previous response.

The good news is, it only costs about $40-50 US to find out if you can get away with it - do NOT buy a dimmer based on best price, or I can almost GUARANTEE you won't like it. The better noise control is MOST of what you're paying for with the higher priced units, and that's what will make or break the deal... Steve

[JPickman]

Cool...Thanks for the info Knightfly.....

I don't think it is worth the hassle to make pretty dimming lights....I would rather give good audio than good mood setting to my clients....thx a million...

Pick Man

[DeadPoet]

Thanks Steve. Off course YGWYPF (you get what you pay for... my idea of launching an abbreviation)

I was once in this studio where they wanted some dimmers and they got a leftover from another studio. The thing looked like a anti-tank mine from W.W.II !! huge (circle with 1' diameter), old and rusty, and VERY heavy...
the guy claimed they just got a 500$ gift...

When constructing and shopping for dimmers/conditioners I'll ask the manufacturers..

Herwig

[c7sus]

Hey Knightfly. Does the same theory apply to low-voltage dimmers? Like on 24-48v DC track lighting? I guess the difference would be if the dimming was pre or post rectification.

[knightfly]

Herwig - yeah, those older dimmers were completely different and would NOT cause a problem. They were what is called an Autotransformer - turning the knob actually changes the turns ratio of the transformer, so no RFI is generated.

C7 - yeah, if the dimming was accomplished by just lowering the DC voltage applied to the lights, cool. I've not checked out any of those dimmers as yet, (Probably should) but that could certainly be accomplished with no RFI whatever. Just take a typical DC power supply and replace the voltage reference resistors with a pot. No RF, no prob... Steve

[Dan Merrill]

No hands on with these but, I've read variacs, like these below, are the solution if you've got the dough. Can you comment on variacs Steve?


http://www.iseincstore.com/productgroup.asp?33=12

[Light]

The only way to completely eliminate the hum caused by dimmers is to have them, not just on a different circuit, but on a separate service. This is what we do in (when we can) in live situations. As a lighting guy and a sound guy, I am never as happy as when there are two generators, one for sound, and one for lights. Not only does this insure that there is no hum, but also I can insure the voltage and current are right on, something you never get from the power company.

The scale of what we are doing is different, and you are unlikely to want to use generators, but the concept is the same. If you want to guaranty you will not get hum, you must have a separate service. This being rather expensive, power conditioners will help a lot, particularly a balanced power conditioner.


Light

"Cowards can never be moral."
M.K. Gandhi

[knightfly]

Light brings up a good point, that of separate sources. He's also right about the cost, which makes it not very practical from a home studio standpoint. I'm seriously considering balanced power from EquiTech in my next facility, hopefully to be started in about two years, but even that cannot control airborne RF noise if your individual pieces of gear are not designed well enough to reject RFI. And, unfortunately, once those SCR spikes get into the wiring, you have a "radio station" on your hands. From then on, it's mostly a function of how good is the design quality of your various pieces of gear?

The best way to get rid of noise (any kind) is DON'T MAKE ANY. To this end, the better the quality of the dimmer the less RFI it will generate, as I said earlier. I've DESIGNED things like this in past years, and my simple but effective way of dealing with it is to DO WITHOUT. Again, see my earlier post regarding my lighting systems. I know this doesn't make for a very exciting light show in the studio, but my recordings don't have as many warts as they could have either.

Dan - the term Variac is a trade name for an Autotransformer. Autotransformer is the generic name, Variac is one particular brand of autotransformer - See my earlier post. Heavy, clunky, efficient, but QUIET... Also, not very easy to automate.

Anybody who really WANTS to be able to dim studio lights can do a feasibility study for about the price of a decent dinner for two - just buy ONE of the more expensive dimmers, replace your light switch with it, and do the test as I described it earlier. If it passes, you got lucky. If not, put the thing in your dining room light circuit and tell your significant other you thought it would be romantic. Win/win... Steve

[Ethan Winer]

Steve,

> just buy ONE of the more expensive dimmers, replace your light switch with it, and do the test as I described it earlier. <

I tried that and it didn't work. The $80 solid state dimmer was just as noisy as the $10 ones. Fortunately the electrical supply house was willing to take it back and give me a refund.

The only viable solution is variable transformers. I wrote about this for Recording magazine, and it's listed first on my Articles page:

www.ethanwiner.com/articles.html

--Ethan

[knightfly]

Ethan, that doesn't surprise me - SCR's are one of the nastiest RFI producing devices in electronics with that super-fast switching time. I wouldn't have even suggested that if not for the inconvenience of the only thing you and I know of that actually WORKS; and, since I haven't tested any of the recently offered dimmers I was cautiously optimistic that they might have come up with a snubbing circuit that worked. Guess not...

As I said, I've not looked into the DC low voltage scenario, but if anybody makes a variable one of those it would be worth checking out. After tax deadline I'll see what I can find out on those; if they work, they're BOUND to be cheaper and more convenient to remote than those humungous Variacs... Steve

[Light]

A little pricey, but this would work.

ETC Unison

Actually, a lot expensive. But ETC makes the best dimmers (theatrical or architectural) any where.


Light

"Cowards can never be moral."
M.K. Gandhi