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Author
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Topic: Dimmer buzz on audio GDC > Doremi > Dolby CP65
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Tony Bandiera Jr
Film God
![](/ubbmembers/2365e.jpg)
Posts: 3067
From: Moreland Idaho
Registered: Apr 2004
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posted 08-23-2013 12:39 PM
Yep, use the old divide and conquer method: (When the buzz stops, the step you just did is most likely where the problem exists.)
1: Disconnect the GDC from the Doremi... 2: Disconnect the Doremi from the CP65... 3: Try other format buttons on the CP65... 4: Disconnect the CP 65 from the amplifiers/monitor 5: Turn off booth monitor (CE MS100's are especially noisy and suspect) 6: Shut down amplifiers one at a time
(If the buzz does NOT go away at a given step, reconnect/restore/turn on the equipment in that step.)
So, if the buzz goes away in step 1, the GDC is the problem , step 2, the Doremi , step 3, whatever is connected to formats that have the buzz, step 4 the CP65 itself, step 5 the monitor, step 6 the amplifier(s).
If this has come on suddenly it is most likely a fault in the CP 65, either power supply module or oddly enough, the CAT 150 card. (Just helped a local room where the CAT 150 failed so badly it injected noise in ALL formats except non-sync.)
If none of the above clears it up, you need to enlist an electrician to check the theatre's electrical system for a loose or failed ground and/or neutral...that can be in the booth panel, the main panel, meter socket or the power company's service drop.
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Harold Hallikainen
Jedi Master Film Handler
Posts: 906
From: Denver, CO, USA
Registered: Aug 2009
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posted 08-24-2013 12:31 PM
I think the responses are very good, though I suspect the risetime is 300us instead of 300ms.
This is possibly ground loop noise. When did the noise start? Is this an existing installation where the noise just started, or is it a new installation? As others have suggested, try unplugging audio cables to try to determine where the noise is coming in. It MAY be a ground loop between the server and the D/A converter. Current through the shield of the AES cable (if there is one) raises the voltage on the D/A chassis above that on the CP-65 (with its unbalanced inputs). This would put hum in the system, but dimmer noise is even more audible due to the higher frequency harmonics. So, if you can unplug the AES cable between the server and the D/A and have the noise go away, if you're not already using an unshielded cable, give one a try. I'm using standard CAT 5 more and more for AES/EBU. Since the AES/EBU source and destination both have transformers, they break the ground loop.
Good luck!
Harold
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