plx qsc headroom?

Is this amp OK?
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nick mineau
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plx qsc headroom?

#1 Post by nick mineau » Fri Jul 12, 2019 7:54 am

i all,
i run qsc plx amps, and i have a question about the -10db lights. can anyone tell me if they are true -10db?
the reason i ask, is some of bills cabs are 11+ohms, and because of this i wonder if i have a bit more then 10db before clipping when those lights are just starting to flicker?
im not sure if the -10db light responds to load on the amp, for example, with a .5 ohm load , i assume it might clip right as the -10db lights flicker, but w no load , it might take 15db over the -10db light to reach clip? wondering if anyone can confirm this for me? I only ask cause i like to know exactly how much (gas) is left in the rig? thanks Nick

Bruce Weldy
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Re: plx qsc headroom?

#2 Post by Bruce Weldy » Fri Jul 12, 2019 8:36 am

nick mineau wrote:
Fri Jul 12, 2019 7:54 am
i all,
i run qsc plx amps, and i have a question about the -10db lights. can anyone tell me if they are true -10db?
the reason i ask, is some of bills cabs are 11+ohms, and because of this i wonder if i have a bit more then 10db before clipping when those lights are just starting to flicker?
im not sure if the -10db light responds to load on the amp, for example, with a .5 ohm load , i assume it might clip right as the -10db lights flicker, but w no load , it might take 15db over the -10db light to reach clip? wondering if anyone can confirm this for me? I only ask cause i like to know exactly how much (gas) is left in the rig? thanks Nick
Those should be input signal meters.....it has nothing to do with what's on the other end of the speaker cable. The gas in the rig is going to be determined by your limiters. Set them correctly and you'll know how much is left in the tank by looking at the output on your board.

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Re: plx qsc headroom?

#3 Post by SethRocksYou » Fri Jul 12, 2019 11:43 am

Hi Nick,

You can verify what Bruce said, and see it firsthand if you disconnect your speakers. The input signal lights wont be any different with the amp making zero watts than they would with them hooked up.

nick mineau
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Re: plx qsc headroom?

#4 Post by nick mineau » Fri Jul 12, 2019 12:31 pm

OK that what i thought. so its not true -10db from clip when the amp is driving a load w varying resistance... its just a ballpark figure.

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Bill Fitzmaurice
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Re: plx qsc headroom?

#5 Post by Bill Fitzmaurice » Fri Jul 12, 2019 1:06 pm

It's -10dB from clipping the input stage, not the output stage.

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Re: plx qsc headroom?

#6 Post by SethRocksYou » Fri Jul 12, 2019 3:04 pm

Stating the obvious; if the input clips, it's a clipped signal on the output too.

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Re: plx qsc headroom?

#7 Post by SethRocksYou » Fri Jul 12, 2019 3:10 pm

I'm not 100% certain, but I don't think the output stages clip. I think if the current demand outpaces the amplifiers rail power supply, like in the case of too low speaker/s resistance, the available rail voltage just drops and the power supply circuitry makes a lot of heat trying to keep up. I think anyway. I'm sure someone will straighten that out.

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Bill Fitzmaurice
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Re: plx qsc headroom?

#8 Post by Bill Fitzmaurice » Fri Jul 12, 2019 3:20 pm

SethRocksYou wrote:
Fri Jul 12, 2019 3:10 pm
I'm not 100% certain, but I don't think the output stages clip.
They do, for the same reason any stage clips, because the signal voltage attempts to exceed the rail voltage.

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Re: plx qsc headroom?

#9 Post by SethRocksYou » Fri Jul 12, 2019 3:28 pm

Bill Fitzmaurice wrote:
Fri Jul 12, 2019 3:20 pm
SethRocksYou wrote:
Fri Jul 12, 2019 3:10 pm
I'm not 100% certain, but I don't think the output stages clip.
They do, for the same reason any stage clips, because the signal voltage attempts to exceed the rail voltage.
Would too low of impedance induce a clip?

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Re: plx qsc headroom?

#10 Post by Bill Fitzmaurice » Fri Jul 12, 2019 3:33 pm

It is a factor, as the amp is more likely to try to deliver what it cannot.

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Re: plx qsc headroom?

#11 Post by SethRocksYou » Fri Jul 12, 2019 3:39 pm

Interesting. Thanks Bill :thumbsup:

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Re: plx qsc headroom?

#12 Post by nick mineau » Fri Jul 12, 2019 3:52 pm

the qsc manual states, the -10db light indicates 10% of full power, the -20db light indicates 1% full power. to me this seems to indicate it is indeed monitoring the output stage? i don't see how this could be? the amp cant possibly predict when it will clip w a reactive load?

so if bill says it monitors input side this is what i assume he meant and qsc did:

lets say it has a max un-clipped voltage output of 60volts @4ohms (900watt) does qsc just assume.

it takes 1.5volts on the input side to reach this voltage because the amp has fixed 40x gain. (40x gain) x (1.5v input) = 60 volts output = 900watts 4ohm
based on this, they then say, in theory .475volts input (times 40x gain) would then create 19 volts output which = 90watts@4ohms
so when we see .475volts on the input of the amp section. light the -10db light, cause output clipping will be about 10db later if a 4ohm load is used.
and if you use any other ohm load, it'll be close enough?

i assume this monitoring would be done after the gain nob rather then before it, which is how they know its a fixed gain.

the thing i don't like, is lets say im on a long extension cord, and saggin to 100vac, im running @2ohms. i might clip the output much sooner then they thought, so my -10db lights are more like -8 lights.
and the oppsite, im running bills subs at 11 ohms, on solid 120power. well know -10 might be more like -12db

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Re: plx qsc headroom?

#13 Post by Bill Fitzmaurice » Fri Jul 12, 2019 4:07 pm

I'm assuming the clip lamp is tied to the input stage as that's a simple circuit, whereas having it tied to the output stage is a very complicated circuit. One thing's for sure, never use a clip lamp as a speaker protection device, because it isn't.

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Re: plx qsc headroom?

#14 Post by Grant Bunter » Fri Jul 12, 2019 4:12 pm

Assuming equal impedance across the band width of the sub isn't helping you here.
Nominal impedance is just a design parameter.
Actual impedance is a product of frequency.

Don't assume a load on your amp across the band width.
Check out the response chart of your driver, see if there is anywhere it's response drops below nominal impedance.
If so, running a pair of cabs parallel per channel is half that value, and you're better off not adding in the acoustic impedance from the cab when calculating this stuff...
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Re: plx qsc headroom?

#15 Post by ACUA » Fri Jul 12, 2019 7:31 pm

the gain dial on amplifiers is a pot on the input side of the amplifier, as far as I know the gain on amplifiers is a fixed amount and the control we have is the input signal strength. depending on the make and manufacture and the amp topology the gain can be a lot of things, for the sake of this, I will say 20/1 gain, now there is a max output so when we put to much signal in and the gain ratio is over the max output, well the signal clips
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