Film-Tech Forum ARCHIVE: Distortion/crackling/hiss -- cannot identify cause

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Posted by Ken Lackner (Member # 1002) on 06-14-2006, 12:38 PM:

Equipment
CP500
QSC Model 1400 amplifiers
USL XTA-1 "octal module" crossover
USL CM-35 monitor
JBL 4675C-LX 2-way stage speakers

Problem
Theater will commplain there is hiss in the left channel. They say it is bad enough to cause customer complaints, so they shut off the left amp. Usually a few days later they can turn on the amp and the problem is gone. When running pink noise to check the speakers, the left channel sounds "crackly" and distorted. When the pink noise is turned off, I can hear the hiss.

What I have tried so far

Swapped left and right amps. Problem stays in left channel.

Swapped crossover modules.

Measured lo and hi speaker drivers. Each 4 ohms.

Swapped each CP500 card, one at a time, with another house. Each time the problem remained in the original house instead of following the card. Even swapped the motherboard.

When running pink noise yesterday, I could hear the crackling in the left channel. Cycling power the the processor cured the problem, so I assumed it was a procssor issue. I swapped the entire processor with an adjacent house. Problem remained in original house.

Turned off and completely disconnected monitor to see if it was causing noise. Problem remained.

Disconnected digital reader, CP500 bypass power supply, optical, non-sync, and hearing impared. Problem still exists.

Swapped Left and Center amp outputs only to so that the left signal would go to the right speaker and vice-versa. Problem followed to the Center speaker.

Ran new wire from Left processor output to Left amp input. Problem still exists.

Because the problem did at one point move to the the center speaker and because the distortion can be heard in the monitor. I have not gone up to look at the speaker yet. However, I have tried everything else I can think of, so I am going to look at the speaker. In the meantime, any other suggestions?

Posted by Marin Zorica (Member # 1716) on 06-14-2006, 02:18 PM:

Did You try change L and R speakers? Just change cables on amplifiers.

Posted by Louis Bornwasser (Member # 3063) on 06-14-2006, 02:43 PM:

Would guess that left H.F. driver is marginal. For diagnosis purposesd, swap entire driver. Louis

Posted by Steve Scott (Member # 630) on 06-14-2006, 02:46 PM:

At my former theater, the CP-500's would occasionally give off some high-frequency distorted hiss. We often had the issue of the noise embedded within the center channel's signal path. Dolby field bulletin FB200 describes it as electromagnetic interference causing clock waveform distortion. Dolby provides a kit for replacing five resistors on the cat 681 card with inductors to eliminate the problem.

We had soundracks identical to yours Ken, so if this isn't the solution, I'd be interested to know what really works.

Posted by Monte L Fullmer (Member # 2797) on 06-14-2006, 07:28 PM:

..wonder if the rear socket plane on the case is suffering some sort of opens? Had to replace two cases due to case momboard problems...

have you done a complete card swapout from another CP-500 to see if the problem travelled to another house .. if the rear plane might have gone bad?

Posted by Josh Jones (Member # 447) on 06-14-2006, 10:51 PM:

Hmm... any AM radio stations near by?

JJ

Posted by Bernie Anderson Jr (Member # 410) on 06-17-2006, 12:25 PM:

Did you do a complete B chain on the theatre? It sounds to me that Left is too hot. Check SPL. They should be 85db across the stage. LS & RS at 82db to give you a 85db together.

Is it present with both Dolby Digital and Optical?

Bernie

Posted by Jarrad Salmon (Member # 3818) on 06-18-2006, 06:43 AM:

Crackle normally means a bad connection. Either cabling or even a loose card.

Hiss normally means processing dramas/mismatched impedances/wrong input levels/wrong output levels (too high will cause a distortion)

If the problem moved from left to centre, you can eliminate blown speakers.

You mentioned replacing the processor out to amp in cable. And because the sound has moved from left to centre, we know the signal from the amplifier to the speakers is fine.

I know that he CP500's have had motherboard problems popping up from time to time. I'm leaning towards a bad connection inside the processor. Where the cards lock in.

That said it is more common for a cable to fault or a input/output level to change.

Posted by Mark Gulbrandsen (Member # 72) on 06-18-2006, 07:09 AM:

quote: Jarrad Salmon
Crackle normally means a bad connection.

Could be a connection... but as a long time bench tech I can definately say that it can also a failing electronic component.

Mark

Posted by Jarrad Salmon (Member # 3818) on 06-19-2006, 10:41 PM:

quote: Mark Gulbrandsen
Could be a connection... but as a long time bench tech I can definately say that it can also a failing electronic component.

Mark

Can't argue with that... unless it is a loose connection within the electric component!!! [Razz]

(joke)

Hows it coming along Ken?

Posted by Frank Dubrois (Member # 3042) on 06-24-2006, 10:49 PM:

Had the same problem, only it was intermittent. All I did was pull the cards and reseat them. Problem hasnt come back. Probably luck, but worked.