Film-Tech Forum ARCHIVE: intermittent electrical discharge/interference sound in speakers

This is topic intermittent electrical discharge/interference sound in speakers in forum Film Handlers' Forum at Film-Tech Forum ARCHIVE.

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Posted by James Larden (Member # 5594) on 02-09-2011, 07:59 PM:

Hi All,

I was wondering if I could get some help with this issue.

Two days ago in our main theater on campus we had a overwhelming noise come from the stage ch. speakers. I can only describe it as akin to hearing lamp strike in a poorly grounded system, but it was sustained for about 30 seconds and much louder. I immediately turned off all the amps, but the sound persisted. I couldn't pinpoint it because it was so loud and aggressive, and about 5-10 seconds after I entered the cinema it cut out. Yesterday morning it happened again, lasting about 10-15 seconds before the morning film studies class began. Just now it happened again this time it was only on the surround speakers.

I thought that it might be inductance from some nearby lighting dimmers which have been causing us some trouble, but the phenomenon appears at random and not when the dimmers are turned off and on. I called our service tech and he said our CP-55 volume control can short out, and the pot can go to max with all lose of control. But the system was not passing audio at a loud volume, just the noise and the amps were off. I thought that we might have a faulty amp that is passing DC, but I am not sure.

here is the gear:

Oppo BDP-93 BR deck (coaxial s/pdif out)
Cambridge Audio P2000 pre-amp (6 ch. analog out)
CP-55

L,C,R amps Bryston 3B
Surround and Sub amps QSC USA 900

screen ch. speakers EV 5045/DH16?
sub JBL 4647
sur. JBL 8330

Posted by Louis Bornwasser (Member # 3063) on 02-09-2011, 08:29 PM:

I don't think the EV's can take much loud hash.

With the amps OFF, I cannot see how you have any sound. Louis

Posted by Tony Bandiera Jr (Member # 2365) on 02-10-2011, 12:28 PM:

I agree with Louis..if the amps were off there is no way the speakers were creating the noise.

What I suspect is that you need to get an electrician in there IMMEDIATELY to look for signs of an arc fault in either some equipment, light fixtures, dimmer rack, junction boxes or conduits. Do it NOW before the fire.

DO NOT let them take this casually...it may take a lot of time and effort but I guarantee they WILL find something damaged by an electrical arc fault!

Posted by Gordon McLeod (Member # 33) on 02-10-2011, 02:31 PM:

Bryston power switchs have been known to fail and the amps still powered up when the switch is in the off position

Posted by Sam D. Chavez (Member # 1841) on 02-10-2011, 05:43 PM:

Maybe cell phone interference.

Posted by Dave Macaulay (Member # 813) on 02-12-2011, 06:29 PM:

An amp may store energy in capacitors and work for some time after powerdown, but even Bryeton amps can't make "loud" sound from a loudspeaker for more than a couple of seconds with no AC power. The load power with high signal will suck the caps down immediately.
So what's up? Is the sound actually from the speakers? Do the power switches not work? Maybe someone is hammer drilling some holes to mount new equipmeny nearby? From too much experience listening to seat guys hammer drill a few thousand holes while I'm trying to install projectors, the sound goes a long way.