Film-Tech Forum ARCHIVE: Bad crossover = Distortion

This is topic Bad crossover = Distortion in forum Film Handlers' Forum at Film-Tech Forum ARCHIVE.

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Posted by Paul Konen (Member # 41) on 12-23-2006, 02:28 AM:

Equipment:
Dolby CP-500
QSC Model 1400 AMP
JBL Speakers

Got a complaint that one of my channels was blown. Checked behind screen and speaker box and horn produce output when running pink noise through it.

Listened to channel through booth monitor. Sounds clear when listening to Processor output, but Amp output sounds distorted. Replaced amp and still the same thing.

Could the crossover be causing the issue? Can you directly replace it with another one or does it affect the EQ of that channel?

Thanks

Posted by Kenneth Wuepper (Member # 1174) on 12-23-2006, 07:10 AM:

Paul,

Your black box technique for servicing this system has not gone far enough. What you have not done is to exchange the speakers with another known to be good working channel's speaker. That will tell you immediately if the speaker is bad. Poor wiring and loose connections to the speaker can also cause bad sound and distortion.

When you say that the speaker produces pink noise are you looking at an RTA? It is almost impossible to hear distorted pink noise by ear unless the speaker is very badly damaged or not working at all.

Replacing the crossover or any other speaker component should be followed by an RTA study of the sound produced by that speaker system. It will certainly "work" but not be "optimum".

KEN

Posted by Paul Konen (Member # 41) on 12-23-2006, 12:32 PM:

Thanks Ken. I did forget to mention that I did replace the driver on the horn and made sure that both the box and horn were connected well.

I will probably have to get my engineer in to have a look.

Posted by Kenneth Wuepper (Member # 1174) on 12-23-2006, 02:55 PM:

Paul,

Is there any DC voltage present across the speaker terminals with the amp connected but no audio signal present?

Output Transformer Less (OTL) amplifiers may be capable of placing DC on the speakers if the amp fails.

KEN

Posted by Gordon McLeod (Member # 33) on 12-23-2006, 04:20 PM:

What make of crossover is being used
since you swappped the amp you can rule that out

Posted by Louis Bornwasser (Member # 3063) on 12-23-2006, 07:56 PM:

I would have started with swapping the input wire on the amp with another (center & left for example). You could then make a determination if the amp/speaker combo or the processor output is the problem; then do the individual swapping. Louis

Posted by Kenneth Wuepper (Member # 1174) on 12-23-2006, 09:52 PM:

The Black Box system is very effective in servicing complicated systems. The process consists of exchanging units (black boxes) along a signal path. A known to be good piece is exchanged for a doubtful one. The last piece to be exchanged and make the system work is the defective one. It is however necessary to follow a system in making these exchanges. As Louis pointed out, he would start at the processor and work to the speaker. This metod of following the signal through the system is most effective. Having monitoring points throughout a system is a faster way to localize a problem. If you can monitor the signal as it leaves the processor, then as it leaves the amplifier output the localization is faster than switching out cables. It is also helpful if the system has been installed with cables and connectors to facilitate this substitution process. Many of today's digital products are serviced by technicians who only change out circuit boards in this black box routine for repairing complex systems. Component level service is becoming too time consuming and requires specialized tools for use in the field.

KEN

Posted by Gordon McLeod (Member # 33) on 12-23-2006, 10:29 PM:

If the booth monitor shows a clean signal out of the processor and a distorted signal out of the amp the problem has been to a large amount narrowed down
Since I assume he is using the octal plugin crossovers try swapping one