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Topic: Subwoofer problem during 4-01-02 evening chat

Will Kutler
Phenomenal Film Handler

Posts: 1506
From: Tucson, AZ, USA
Registered: Feb 2001

posted 05-02-2002 08:11 PM

Last night a large group of us were in the chat room and one person had a subwoofer problem (distortion). Excuse me, but I forgot who it was

Anyhow, the person was running a Century w/RCA soundhead (exciter lamp) and DTS. I gave him some advice, but need to add a few things...so hopefully he'll catch this thread.

On the sound head, there are 8 rubber bushings that hold that cushion the sound components casting to the soundhead. This is to prevent microphonic pick-up. Make sure that those bushings are not cracked and falling apart!

Check all of the sound head wiring and make sure all electrical components are good.

Check the grounds on all sound system components.

Make sure that the sound-head is properly aligned and that the slit lense is not dirty or filling with oil. Having the newer style narrow slit lense will also enhance performance.

Make sure that exciter power supply is in DC.

Make sure speaker wires and other component wires are not crossed.

Check the subwoofer gain for proper adjustment.

Make sure your impedences are correct.

Check your installation against the schematics in the approperiate manufacturer's manuals.

Maybe some other Film-Techers can add to this...especially about the DTS unit..

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Darryl Spicer
Film God

Posts: 3250
From: Lexington, KY, USA
Registered: Dec 2000

posted 05-03-2002 03:41 AM

hmmmmm could be bad cones either they have cracks in them or they are trying to go out. We have had to replace some cones at our theater and bad drivers can cause some pretty bad distortion in the subs.

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Karen Hultgren
Master Film Handler

Posts: 492
From: Agoura Hills, CA, USA
Registered: Mar 2000

posted 05-03-2002 12:25 PM

Whomever is having the DTS problem, send me an email with more information like type of cinema processor, was he using a setup disc to test the system, is it the same in analog, etc.

Karen at DTS
khultgren@dtsonline.com

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John Westlund
Expert Film Handler

Posts: 204
From: Burney, CA, USA
Registered: Jun 2001

posted 05-04-2002 06:22 PM

It is I that am having the problem with the subwoofer. We just replaced one of the 18" JBL drivers and the amplifier for it. The cinema processor is a CP55 with the 441 installed. Some of the traces that had to be jumped were incorrect but the tech corrected them. The problem is that the subwoofer is distorting now when in digital and I believe that it is distorting in analog as well. The subwoofer also doesnt seem to have to power that it used to have. It used to shake the glass out in the box office. The amp is the same that was used before just newer (QSC 1500a running in bridged mode). I did e-mail Karen but I thought I would let the rest of you take a shot at curing this problem. Thanks
John

BTW-Checked all the speaker wire polarity and they are all correct and the connections are good.

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Gordon McLeod
Film God

Posts: 9532
From: Toronto Ontario Canada
Registered: Jun 99

posted 05-04-2002 08:25 PM

If it is also a problem in analogue then the DTS is probably not the culprit.
It is most likely the bass extension card that is at fault or the amp
Also it might be worth measureing the speakers impedance and also check that it is securly mounted in the cabinet

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Darryl Spicer
Film God

Posts: 3250
From: Lexington, KY, USA
Registered: Dec 2000

posted 05-05-2002 01:57 AM

When you say you checked the speaker wireing, did you remove the the new 18" cone and check that the wires were put back on the new cone correctly? Actually if they are switched internaly just switch the two wires around on the back of the cabinet and see what happens.

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Richard Hamilton
Phenomenal Film Handler

Posts: 1341
From: Evansville, Indiana
Registered: Jan 2000

posted 05-05-2002 04:43 AM

Polarity isn't going to cause distortion.

Rick

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Ethan Harper
E-dawggg!!!

Posts: 325
From: Plano, TX, USA
Registered: May 2000

posted 05-05-2002 04:45 AM

Actually if he has got two subwoofers one in phase and one out of phase, he will end up overdriving both of them to get the correct level thus the possibility of distortion.

------------------
"And, hey! You be careful out there too, gosh darn ya!"

~Ethan.

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Dave Macaulay
Film God

Posts: 2321
From: Toronto, Canada
Registered: Apr 2001

posted 05-05-2002 10:40 AM

If he has the dual-18 subwoofer and the drivers are connected with opposite polarity it sure as hell sounds distorted. This applies a high pass filter effect that barely passes the highest in-band frequencies. Sounds very very strange, and gets extremely distorted if one has the power available to try and get the required SPL out of it during a b-chain alignment. If one driver just gets reversed somehow you would have no real sub effect in a film show, and isolating the sub (switching off the other amps) the sound would sound distorted.

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Darryl Spicer
Film God

Posts: 3250
From: Lexington, KY, USA
Registered: Dec 2000

posted 05-05-2002 10:54 AM

If the new driver shares the same cabinet with another driver you will have to remove the new driver and check the wires on the back. If it is a one driver cabinet that is sharing the same amp with another cabinet you will need to switch the wires on the back of the cabinet that the new driver is installed in.

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John Westlund
Expert Film Handler

Posts: 204
From: Burney, CA, USA
Registered: Jun 2001

posted 05-05-2002 06:55 PM

Thanks for all the help. Today we took the speakers out of the box and I checked them all and made sure all the polarity was correct and that the cones were moving the same way. GOT IT FIXED. Just one more question. Which way is better, the cone moving in or out?
Thanks
John

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Jon Bartow
Master Film Handler

Posts: 287
From: Massachusetts
Registered: Nov 1999

posted 05-06-2002 07:19 AM

The definition of a speaker requires that the cone move both in and out. IIRC a big cone will usually appear to move "out". However, if you have a speaker that is just moving out or just in without going back... Then you have a very sick amplifier.

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Karen Hultgren
Master Film Handler

Posts: 492
From: Agoura Hills, CA, USA
Registered: Mar 2000

posted 05-06-2002 12:09 PM

John Westlund, I didn't get your email but if you are having distortion in analog as well as digital, it is not a DTS problem. It is either somewhere in your cinema processor, speaker amp, or speaker.

Karen at DTS

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