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Author Topic: qsc plx amps clipping to beat with no signal  (Read 11293 times)

Jim Roach

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Re: qsc plx amps clipping to beat with no signal
« Reply #10 on: January 28, 2013, 09:30:39 PM »

I'd unplug the cables from the amp outputs and see if the problem is still there.

I've seen the exact same response from a monitor amp at a festival. An overly helpful stagehand plugged the mix 2 cable into the output of the wedges on mix 1. Mix 1 wedges sound funky and mix 2 amp was metering clip lights with the beat.
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Jeffhtg (Jeff Kenney)

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Re: qsc plx amps clipping to beat with no signal
« Reply #11 on: January 29, 2013, 01:20:56 AM »

I'd unplug the cables from the amp outputs and see if the problem is still there.

I've seen the exact same response from a monitor amp at a festival. An overly helpful stagehand plugged the mix 2 cable into the output of the wedges on mix 1. Mix 1 wedges sound funky and mix 2 amp was metering clip lights with the beat.

in this particular case i traced the cables (very simple single run to a pair of subs).. all cables were as supposed to be..

at the end of the night i turned on the amps and ran some pink thru them and all subs played just fine. (after they had been off for 45 min)
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john lutz

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Re: qsc plx amps clipping to beat with no signal
« Reply #12 on: January 29, 2013, 01:23:55 PM »

The back EMF theory is what I thought of at first, although I don't think I've ever actually seen it.

perhaps back EMF weirdness is more common than realised?  Crown XTi will also act up when connected channels are not being used.
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paul bell

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Re: qsc plx amps clipping to beat with no signal
« Reply #13 on: January 30, 2013, 08:51:20 AM »

I would agree it's from the CEMF coming back from the speakers. The clip light on these amplifiers compares the input to the output-if there's a difference, it illuminates.

Adding air on the backside of a QSC amplifier is a good idea. It matches their airflow direction. I like amps that cool like this, it keeps the rack cool. Amps that run the other way cause heat to build up within the rack.
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Jeffhtg (Jeff Kenney)

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Re: qsc plx amps clipping to beat with no signal
« Reply #14 on: January 30, 2013, 10:11:24 PM »

Well I have gotten no where in my research to try and narrow down the problem. 2 brand new plx2502's arrive tomorrow to replace these older units. I will then take the older units to a local amp genius to have him diagnose them and test everything.

1. I feel it COULD BE the thermal switch/fan on the amp

2. it COULD be the amps just getting old and failing

3. it COULD BE the fact that im forcing air from new fans into the back of the rack

This rack has run configured like this a hundred plus times with nay ever a clip or barely orange light.. so SOMETHING caused these amps to do what they did. I will post results later. They are oooold but in good shape.
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Tim McCulloch

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Re: qsc plx amps clipping to beat with no signal
« Reply #15 on: January 30, 2013, 11:35:37 PM »

Well I have gotten no where in my research to try and narrow down the problem. 2 brand new plx2502's arrive tomorrow to replace these older units. I will then take the older units to a local amp genius to have him diagnose them and test everything.

1. I feel it COULD BE the thermal switch/fan on the amp

2. it COULD be the amps just getting old and failing

3. it COULD BE the fact that im forcing air from new fans into the back of the rack

This rack has run configured like this a hundred plus times with nay ever a clip or barely orange light.. so SOMETHING caused these amps to do what they did. I will post results later. They are oooold but in good shape.

I'm with the low voltage crowd.  Big iron Crown Macrotechs would go into ODEP sooner with soft power.
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Dave Gunnell

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Re: qsc plx amps clipping to beat with no signal
« Reply #16 on: February 02, 2013, 12:38:16 AM »

Another thought--while I'm not familiar with the design of the PLX amps, is it possible that the clip light will flash if you overdrive the front-end circuitry?  I know of at least one mixing console whose channel 'clip' light actually monitors several points in the signal path for clipping, and will light if you clip anywhere in the path.  Not sure about power amps though.  I believe that in many (most) amp designs there is front-end input circuitry before any gain attenuators.

My .02
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John Roberts {JR}

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Re: qsc plx amps clipping to beat with no signal
« Reply #17 on: February 02, 2013, 10:25:03 AM »

Another thought--while I'm not familiar with the design of the PLX amps, is it possible that the clip light will flash if you overdrive the front-end circuitry?  I know of at least one mixing console whose channel 'clip' light actually monitors several points in the signal path for clipping, and will light if you clip anywhere in the path.  Not sure about power amps though.  I believe that in many (most) amp designs there is front-end input circuitry before any gain attenuators.

My .02
I've actually designed consoles that way... 8) Useful IMO to see clipping anywhere in the audio path, even if faders are pulled down.

Amplifier clip limiters generally detect when the primary gain path loses feedback or the output can not accurately follow the input signal. This will report clipping from too much input signal and/or not enough rail voltage. It will also report on current limiting if the amp is loaded down with too low impedance. 

I vote for flaky power... or shorted speaker wires.

JR
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duane massey

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Re: qsc plx amps clipping to beat with no signal
« Reply #18 on: February 02, 2013, 05:50:45 PM »

Long shot, but.... QSC's are known for having volume pots that get really flaky when they are dirty. I have seen (and serviced) amps that would "clip" when you tapped on the volume knob even with the knob turned completely "off" and no signal connected. Maybe the vibration from the sound system was causing this? Doubtful, but should be easy to duplicate.
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Duane Massey
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Jeffhtg (Jeff Kenney)

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Re: qsc plx amps clipping to beat with no signal
« Reply #19 on: February 04, 2013, 01:31:38 AM »

I've actually designed consoles that way... 8) Useful IMO to see clipping anywhere in the audio path, even if faders are pulled down.

Amplifier clip limiters generally detect when the primary gain path loses feedback or the output can not accurately follow the input signal. This will report clipping from too much input signal and/or not enough rail voltage. It will also report on current limiting if the amp is loaded down with too low impedance. 

I vote for flaky power... or shorted speaker wires.

JR


the offending amps are now in for bench tests..  on my bench the fans work, the amps work, and we tested all cables twice and checked all connectors. Everything on our end is good.

If it was voltage why did it happen after amps were hot. I vote for something in the powersupply, old amps, or cooling perhaps due to airflow restriction or the addition of rack cooling fans.

will update when i get a real answer
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Re: qsc plx amps clipping to beat with no signal
« Reply #19 on: February 04, 2013, 01:31:38 AM »


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