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Topic: Unbalanced loads on QSC amp

Mike Colley
Film Handler

Posts: 25
From: Dacula, GA
Registered: Jan 2000

posted 05-22-2001 06:48 PM

We are adding a second sub to one of our auditoriums and I would like to know: is there anything wrong with this scenario? One QSC Model 1400 amp running: Channel one- One JBL stage speaker (8 ohm, center channel), Channel two- Two JBL 18" subs (8 ohm wired parallel-?four ohms?). Would this seriosly compromise performance of the center channel? Any other possible problems? Thanks in advance for the help.

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Rick Long Jr
Expert Film Handler

Posts: 211
From: Toronto, Canada
Registered: Jul 2000

posted 05-22-2001 07:41 PM

Off the top of my head, I'd say it should be okay, but it would be helpful to know more details about the system, such as do the other screen channels use one amp channel or are they bridged? As far as having different speaker loads on each channel I cant see a problem as long as the amp is configured for stereo operation. In some bi-amp situations, one amp is used for both frequency ranges(i.e. channel 1 feeds the H.F. horn and channel 2 feeds the lows.) Usually the bass cabinets are between 4 and 8 ohms, but some High freq. compression drivers can be 16 ohms.Some amp manufacturers even make active crossovers that do just this. However if you are running bridged amps on the other screen channels, I would not suggest underpowering the centre channel.
Cheers!

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Antonio Marcheselli
Phenomenal Film Handler

Posts: 1260
From: Florence, Italy
Registered: Mar 2000

posted 05-23-2001 05:13 AM

Mike,

Stereo channels in an amplifiers are two differents amplifiers. You should not have any problem with the center channel.
However I wonder if one channel of an QSC 1400 is enough for two JBL 18" Subwoofer...
I didn't found info on QSCaudio.com about your amplifier so I can't help you but that amp seems a bit overloaded to me.

Bye
Antonio

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Gordon McLeod
Film God

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

posted 05-23-2001 08:40 AM

It isn't a problem we do it all the time
The amp will have no problem with 4 ohm loads

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Mark Gulbrandsen
Resident Trollmaster

Posts: 16657
From: Music City
Registered: Jun 99

posted 05-23-2001 12:03 PM

It will work fine but the question again is what is the size of your room, what db level are you shooting for and will the 1400 let you achieve that level?
Mark @ GTS

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John Gordon
Film Handler

Posts: 62
From: Earth
Registered: Jul 2000

posted 05-23-2001 03:30 PM

Hello Mike,
As stated in the comments above, you can run two different loads on the 1400 amp. As Gordon stated, it is done all the time.

There is a caution though, and that is be sure not to underpower your room.

As per your questions, yes wiring two 8 ohm drivers in parallel will yield a 4 ohm load. The power rating of the 1400 at 4 ohms is about 400W. Make sure this will be enough power for your configuration.
Using a target goal of 113dB SPL (+20dB above headroom) for center of auditorium, free field calculation shows no larger than a 40 foot room (length). What is the size of your room?

------------------
John Gordon
Cinema Applications Engineer
QSC Audio Products, Inc.
1675 MacArthur Blvd.
Costa Mesa, CA 92626
(714) 957-7100
john_gordon@qscaudio.com
http://www.qscaudio.com

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Paul G. Thompson
The Weenie Man

Posts: 4718
From: Mount Vernon WA USA
Registered: Nov 2000

posted 05-23-2001 06:22 PM

I don't think you will have a problem. Those QSC's have plenty of OOMPH!!! to do the job.

However, make sure the amplifier is operationg in the STEREO mode, if you are using one channel for your subs and the other channel for center (or anything else.)

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