dimmer noise |
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Abe The Babe
Registered User Joined: 16 September 2011 Location: Ireland Status: Offline Points: 336 |
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Posted: 27 March 2016 at 2:21pm |
I did a gig recently in a theatre where the house dimmers caused a lot of buzz on 50%. This wasn't an issue for the shows but it was for rehearsals. I've seen it in several hotels and don't want this to ever be a problem again so I'd like to get to the bottom of it.
As far as I know it's due to sharing an earth with crappy dimmers. This is something I don't get to choose unfortunately. What I usually do which makes the buzz on the output of the desk not as bad is take the desks power from the same socket as the amp rack. transformer isolating the output of the desk from the input of the rack should improve this further. On the stage end I was using BSS DI's which didn't give any buzz presumably due to the transformer balancing. Where this idea runs short is that I needed to use condenser mics on the stage. these buzzed away. There is no cost effective way I see of transformer balancing every channel of my multi that doesn't make providing phantom an inconvenience. Is there something I'm missing? One of my thoughts was if I isolated the mains with a 220v -220v isolation transformer?
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There's a time and place for acoustical reflection.
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shagnasty
Old Croc Joined: 30 July 2007 Location: Guildford, UK Status: Offline Points: 7685 |
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[1]The truth is the
"buzz" is not gonna go away with transformer isolating any signal, it
is coming into your kit from the neutral of your mains supply.
When a tungsten
dimmer is @50% you get the worst effect, they dim by delaying the switch on the
lamp in the mains wave (you can't turn a triac off so you can only wait to turn
it on, they use "snubber" networks in the live (a coil that tries to
flatten out the effect you what is a massive surge as the output flys from 0 to
240V, much harder than the natural progressive it would have made @ full power
as it switches on in the low part of the sine wave) the issue is if the supply
is "soft" the is surge yanks the neutral and you get an artefact in
the mains supply which plays havoc with audio gear, back in the day when you
could tail-in I have bolted 70mm2 cables straight to the busbars in venue to
get a stiffer supply than they offer by default, (ballet job, no buzz was going
to tolerated) but nowdays you can't do that, you only real option is look a
daft gain structure run +14dBv if you can and back your amps off 10dB may help,
but that is not for the faint hearted, it has worked for me as I moved my noise
to signal ratio 10dB in the right direction or if that doesn't work/isn't
practical then you need a Stabline device, which as you say is a 1:1 mains
transformer with a very tight Q that only really lets 50Hz pass, these weigh a
tonne, cost a mint and also isolate you from the mains making your RCDs useless
(and to a point redundant) in reals terms finding an un-mutilated supply may be
your better option if the build has more than one mains board...
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Abe The Babe
Registered User Joined: 16 September 2011 Location: Ireland Status: Offline Points: 336 |
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I run as good a gain structure as possible. The only issue with that is when I get dimmer buzz on the inputs of the mixing desk. One idea which is a bodge was to run a few inputs opposite polarity with the hope of cancelling the noise. Doesn't really work though as the signals get eq'd before they are combined.
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There's a time and place for acoustical reflection.
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bob4
Old Croc Joined: 29 February 2004 Location: Finland/Germany Status: Offline Points: 1842 |
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You might find this interesting.....
http://www.jensen-transformers.com/wp-content/uploads/2015/02/AES-Ground-Loops-Rest-of-Story-Whitlock-Fox-Generic-Version.pdf |
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_djk_
Old Croc Joined: 23 November 2004 Location: United States Status: Offline Points: 6002 |
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Newer dimmers use FET switches to do the reverse of triac dimmers, and are thus much quieter.
No amount of fooling around with ground-lifts, transformers, etc. will kill the EMI/RFI generated by triac dimmers. Properly designed filters inserted in each dimmer will do the trick. http://s56.photobucket.com/user/dkleitsch/media/DampedRFFilter.gif.html?sort=3&o=161 The lamp goes where the transformer is drawn. A poorly designed filter will increase the noise (when lightly loaded). This filter design is for 100W~1KW. Edited by _djk_ - 30 March 2016 at 1:08am |
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djk
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shagnasty
Old Croc Joined: 30 July 2007 Location: Guildford, UK Status: Offline Points: 7685 |
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The good news is the days of 72W 4KW dimmer should be dying off as LED stuff get better, but in real terms most dimmers you will meet on a gig are Avolites Stylee using Power Cubes or Triacs to switch the mains, the issues has been teh same for 30 years and not a lot will get around it..
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Abe The Babe
Registered User Joined: 16 September 2011 Location: Ireland Status: Offline Points: 336 |
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what ever about when high power dimmers are brought in but the dimmers on house lights shoudn't be an issue
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There's a time and place for acoustical reflection.
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_djk_
Old Croc Joined: 23 November 2004 Location: United States Status: Offline Points: 6002 |
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"but the dimmers on house lights shoudn't be an issue"
Not a certainty, unless they are auto-transformer types. Any triac/SCR based dimmer will generate EMI/RFI unless it has properly designed filters. |
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djk
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shagnasty
Old Croc Joined: 30 July 2007 Location: Guildford, UK Status: Offline Points: 7685 |
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Couldn't agree more, and some house dimms are real crappers (chokes the size of Polo Mints) the sheer square metrage of house dimmer wiring is a massive consideration...
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