Film-Tech Forum ARCHIVE: Dimmer buzz on audio GDC > Doremi

This is topic Dimmer buzz on audio GDC > Doremi > Dolby CP65 in forum Digital Cinema Forum at Film-Tech Forum ARCHIVE.

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Posted by Eric Robinson (Member # 2966) on 08-23-2013, 12:27 PM:

I searched around and did not find an existing topic with exactly this configuration.

Problem: Buzzing from the dimmer is heard on the audio channels.

AC mains feeds the dimmer, audio server, and sound rack.

The sound originates from a GDC 2001SX 25 PIN AES output to a Doremi D2A DA converter whos analog audio outputs are then fed to a Dolby CP65 6 channel analog audio input.

I'm guessing that the path from the GDC to the Doremi DA is not suspect since this is a digital signal; however, might the ground connection on this path be providing the buzz?

I suspect the dimmer is polluting the AC mains and the non-filtered audio system is picking it up.

Any ideas on troubleshooting scenarios?

Thanks,

Eric

Posted by Tony Bandiera Jr (Member # 2365) on 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.

Posted by Carsten Kurz (Member # 5396) on 08-23-2013, 02:23 PM:

Try to connect a headphone or speaker, if necessary with a BATTERY POWERED amp in between, directly to one output of the AUDD2A and check if the buzz is still there, and if on all channels. If not, connect the headphone to the HI output of the CP65 and check wether it's there.

Will the buzzing get louder or softer with different levels set on the CP65?

The CP65 input is unbalanced. A set of transformers could solve the issue, but depends. Electronic dimmers are usually worse than ground loop hum.

How did this turn up? New dimmer, new digital installation, any other changes?

- Carsten

Posted by Eric Robinson (Member # 2966) on 08-23-2013, 04:32 PM:

Thanks Tony and Carsten.

Divide and conquer is usually my method as well.

I do like the portable audio monitor idea.

I was planning on taking a scope to see if I could see the buzz as well.

This was a new digital install. It is happening in 3 auditoriums out of 4. The auditorium not having the problem is using an Ultra Stereo ECI60 instead of a Doremi D2A. All auditoriums have the same crappy dimmers. I would guess the problem happened since day 1 of the install, but I wasn't there for the install.

Cheers,

ERIC

Posted by Gordon McLeod (Member # 33) on 08-23-2013, 05:44 PM:

What make of dimmer?
Most proffesional dimmers have a min of 300msec rise time filtering by a choke and a snubbing cap
If the dimmer is the source it should change in pitch as it dims the worst is at 50% output when the largest steep rise time is occuring
Often a extra filter on the dimmer helps also we try to wire the sound on a seperate phase than the dimmers

Posted by Harold Hallikainen (Member # 5405) on 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

Posted by Joe Elliott (Member # 4006) on 08-27-2013, 12:38 AM:

Also check that the negative and sheild on the amps are correct. I had one install that once we pulled the 35, and put in digital, we were getting all sorts of hum, some from the dimmer. We tried everything, and someone after tracing wiring, found that it was the negative and shield swapped. How on earth it did not cause a hum with the 35mm I will never know, but wiring correctly fixed it in both houses we were having an issue with.

Posted by Mark Gulbrandsen (Member # 72) on 08-27-2013, 12:12 PM:

This is an easy one that has happened to me a few times. The dimmer is on the same phase as the sound rack. Just move the dimmer to an ac phase that is not associated with feeding the sound system. Worked for me every time...

Posted by Louis Bornwasser (Member # 3063) on 08-27-2013, 06:13 PM:

Make sure chassis ground is connected on the CP65.

How far away is the dimmer physically from the sound rack? I have had trouble at less than 5 ft.

Different phase is also a good step.