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Old November 21st, 2011, 04:32 PM   #1
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Why am I getting this interference in audio from dimmer?

I recently made a change in my studio and now I get interference in the audio whenever I dim one of my lights lower than about 75%. You can hear it in the first half of the attached 5 second clip and then it goes away when I adjust the dimmer to 100%.

I tracked it down to adding a 10' XLR cable to the existing 20' XLR cable. When I use either the 10' or the 20' cables alone, I don't get the interference... only when the two are combined. Changing the camera to battery power doesn't affect it and the XLR cables aren't closer than 4 feet from the power cables.

The audio chain is Rode NTG-1, Shure FP33, EX1R. Any ideas why this might be? TIA
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File Type: mov dimmer_clicking.mov (1.09 MB, 59 views)
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Old November 21st, 2011, 08:11 PM   #2
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Re: Why am I getting this interference in audio from dimmer?

Sounds like a triac dimmer. These work by chopping off the normal sine wave household AC power into little squared-off waves. Reducing the size of the square waves reduces the power, dimming an incandescent lamp. The squared-off stuff is noisy and some mics can pick up the "splatter" from a few feet away from a circuit with a dimmer on it.

I had the same problem last weekend -- the dimmer-equipped circuit covered the two rooms where we were shooting and couldn't be disabled. I had to move the boom around to find a relatively quiet spot.

Triac dimmers.. hate 'em !

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Old November 21st, 2011, 09:38 PM   #3
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Re: Why am I getting this interference in audio from dimmer?

I have three of these but only one causes the interference and it's only when the two XLR cables are used. Router Speed Control

Does that make sense with the Triax theory?
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Old November 21st, 2011, 11:27 PM   #4
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Re: Why am I getting this interference in audio from dimmer?

Triac dimmers are ALWAYS a problem. It is amazing that you have been so lucky that they don't ALL buzz.

You could try connecting the XLR shell to pin-1 (ground) on the female ends of your cables, That would extend the shield to the junction between the cables.

Note that serious studios just avoid those cheap/simple dimmers altogether precisely because of the problems you are complaining about.
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Old November 21st, 2011, 11:36 PM   #5
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Re: Why am I getting this interference in audio from dimmer?

It's worse when the DP/gaffer uses them to dim tungsten fixtures. They can't hear the buzz and it ain't their problem.

Real dimmers are something like this: Mole-Richardson Variac Large, clunky, and silent ( electronically speaking ).
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Old November 22nd, 2011, 06:21 AM   #6
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Re: Why am I getting this interference in audio from dimmer?

RIchard, Stephen... thanks. that makes sense now.Connecting the XLR shell to ground makes perfect sense. I like to understand the physics of things so I can tweak things better. It helps me be "lucky". :-)
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Old November 22nd, 2011, 04:33 PM   #7
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Re: Why am I getting this interference in audio from dimmer?

Right on Les :) When I started out as a teen, I was given a set of tools which included a screwdriver with a neon handle which lit up if I poked it across the AC mains. And I did too a couple of times.

In our first built studios in 1980, I specd the expensive proper dimmers all through, the idea being to dim the lights during sessions after sundown. They certainly worked as a marketing tool for the first year then most established clients never used them.

Same with coloured globes for the rock clients, although that idea lasted for the duration and were easy to change back.

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