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par cans + dimmer = ?


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#1 giap vu

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Posted 17 January 2009 - 11:41 PM

Hi,
Wondering if anyone can explain why when par cans are dimmed there is humming/buzzing in the audio? The reason for ask is because I'm working on a feature and Sound was complaining about this problem.

Thx
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#2 Phil Rhodes

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Posted 18 January 2009 - 05:41 AM

Probably because you were using triac dimmers. You can also get audible filament buzz out of big tungsten lamps on dimmers, but that's an audible sound as opposed to RFI.

The science:

Triacs work by abruptly cutting chunks out of the mains waveform, which creates a hard electrical edge with a very fast rise and fall time. Theoretically, an infinitely small rise time has infinite harmonics, that is, it emits radio energy at all frequencies simultaneously. While your dimmers wouldn't have created a perfect theoretical zero rise time, it would still have been short enough to emit radio energy over a wide range of frequencies. More modern dimmers get better and better at this, but it's difficult to guard against it entirely.

Solutions include moving the dimmers, lighting and audio equipment far apart, using triac dimmers with better suppression, or using non-triac dimmers such as a variac (a variable transformer).

Again, all that assumes you weren't just hearing filament buzz from the lights themselves, although variac dimming should solve that.

P
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#3 freddie bonfanti

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Posted 18 January 2009 - 06:34 AM

jesus Phil, why dont you become a spark then? i mean youve got the knowledge and the grumpiness, youd be perfect...want me to put a good word?
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#4 Phil Rhodes

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Posted 18 January 2009 - 05:28 PM

To be honest, I suspect that the glue on a postal envelope has the knowledge to become a spark, but that's not really the point, is it?

Phil
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#5 freddie bonfanti

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Posted 18 January 2009 - 07:21 PM

you've never met the nice sparks, Phil...the ones who never fell to the dark side...we love our job! we dont grumble all the time, we help out other departments!!!

come and join us you grumpy bastard
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#6 giap vu

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Posted 19 January 2009 - 11:08 PM

Thank you PHIshizzle! Our key grip figured a variac could solve the problem but the best boy elec begged to differ. another problem we were having was lamps blowing. Both our 250 inbetweenies blew out. We were running power off a 400 amp generator and again no one could figure out why this was happening. The gaffer suggested it was the end of the bulbs life but it seemed too much of a coincidence. Any theories?
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#7 Phil Rhodes

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Posted 20 January 2009 - 05:39 AM

Two at once doesn't seem like too much of a coincidence.

I assume your generator output was properly calibrated? Were you moving them around while they were hot?

And:

you've never met the nice sparks, Phil


No. No, I haven't.

P
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#8 freddie bonfanti

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Posted 20 January 2009 - 03:17 PM

Two at once doesn't seem like too much of a coincidence.

I assume your generator output was properly calibrated? Were you moving them around while they were hot?

And:



No. No, I haven't.

P


oh i forgot, youre from London...never met nice London sparks either...or grips, focus pullers, loaders...whoops ;-)
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Visual Products

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rebotnix Technologies

Ritter Battery

Tai Audio

Aerial Filmworks

Glidecam

Willys Widgets