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Author Topic: QSC PLX amp's limiting for subwoofers  (Read 1292 times)

Benjamin Goulart

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QSC PLX amp's limiting for subwoofers
« on: March 12, 2019, 12:40:12 pm »

https://forums.prosoundweb.com/index.php/topic,90762.10.html

https://www.qsc.com/resource-files/productresources/amp/discontinued/plx/q_amp_plx_series_usermanual.pdf

The manual on page 8 seems to be slightly at odds with Bob's comments in this thread, indicating two advantages to having the limiter off: 1) it makes pushing into the limiter less pleasant-sounding, therefore less likely to be exploited, increase RMS, and thereby cause thermal damage to woofers, and 2) slight non-limited clipping apparently adds "punch" to subwoofers.

I am concerned that using only an active crossover before the subwoofer amplifier, though, will create a situation where amplifier clipping could send loud high frequencies way above the crossover's LPF attenuation into the sub drivers, though.  Isn't that a bad thing that could either damage the woofers or at least permanently alter their characteristics and response?
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Taylor Hall

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Re: QSC PLX amp's limiting for subwoofers
« Reply #1 on: March 12, 2019, 01:03:52 pm »

The real takeaway here is Matthew's comment halfway down, in that if you're trying to extract that extra 1-2db of output and running solidly into clipping, you don't have enough rig for the gig.

Sure you *can* drive things a bit harder with them off, but is it worth potentially sacrificing everything downstream?
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Benjamin Goulart

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Re: QSC PLX amp's limiting for subwoofers
« Reply #2 on: March 12, 2019, 01:20:12 pm »

The real takeaway here is Matthew's comment halfway down, in that if you're trying to extract that extra 1-2db of output and running solidly into clipping, you don't have enough rig for the gig.

Sure you *can* drive things a bit harder with them off, but is it worth potentially sacrificing everything downstream?

The manual seems slightly at odds with that, too, because it talks about increasing loudness through increasing RMS, not some magical increase in potential peak power.  Either way, you can still apparently push through the limiter to whatever peak power the supply on these amps can push out if you crank it enough.  I'm worried with the limiter On that the amps will compress and the clip light won't come on as quickly, with a corresponding increase in RMS.

I might be wrong, but the manual seems to be indicating that the limiter is not just tamping down the clipping peaks but is in the chain prior to the clip even occurring when it's activated and prevents the peaks from even occurring.  Bob and the manual did both use the term "audible" each, though, so maybe in both cases they're meaning the clip lights are always on when limiting and/or clipping and pushing into them such that you can hear the clipping is the only time compression is occurring.

I definitely do not want any additional compression of the audio signal I can avoid as most mastered music program content is already compressed to heck nowadays and I've already encountered thermal issues with some idiotic producers and people with defective front-end gear, but I'm worried about those high frequency clip harmonics doing something to the sub drivers if the limiters are off.

If I turn the limiters off for the QSC sub amp(s), isn't the clipping an increased risk to the woofers even if there's reduced risk of the opportunity to increase RMS and thermal issues on them?  If my first indication of too much gain with the limiters off is the sound of a squared waveform "click" high frequency distortion harmonic in the subs generated by the amp that's fed low-passed content from the active crossover, haven't I just messed the woofers up a bit each time I hear it?  I've never heard that with the limiters On.

I guess the root of the question is really: Does having the clip limiter activated on the QSC PLX line reduce the likelihood of the clip light coming on at the same given input gain?
« Last Edit: March 12, 2019, 01:59:07 pm by Benjamin Goulart »
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Taylor Hall

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Re: QSC PLX amp's limiting for subwoofers
« Reply #3 on: March 12, 2019, 02:26:40 pm »

snip

The amp works just like an engine, sure you can rev it higher and maybe get a couple more horsepower out of it, but the chances of floating a valve or spinning a bearing also increase. If your cabs can handle the extra few watts of output (if the manual is to be believed), good on them, but will they be able to handle the accompanying distortion or over-excursion from the highly disgruntled signal? Again, if you're at the "safe" limit of the amp and need more juice, the solution is not bypassing the gain knob, it's getting a bigger amp.

That is correct, the LEDs indicate when the clip/limiting actually occurs not when it is about to occur. It is a preemptive measure in the signal chain to be sure that you're not damaging the amp or connected equipment. As before, are you confident enough in your signal being so predictable that running without the limiters on won't release the blue smoke?

If you're worried about additional signal compression because you're at the absolute limit of your amp's headroom, then you need more headroom with better hardware. Not cranking said amp to 11.

Of course there's an increased risk, why do you think the limiters are there to begin with? That nasty compression you get from active limiting should be your early warning to back off and reassess your game plan, find your weakest link, and replace it. Turning the limiters off is merely a band-aid on a crack in the dike.
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Benjamin Goulart

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Re: QSC PLX amp's limiting for subwoofers
« Reply #4 on: March 12, 2019, 03:32:41 pm »

The amp works just like an engine, sure you can rev it higher and maybe get a couple more horsepower out of it, but the chances of floating a valve or spinning a bearing also increase. If your cabs can handle the extra few watts of output (if the manual is to be believed), good on them, but will they be able to handle the accompanying distortion or over-excursion from the highly disgruntled signal? Again, if you're at the "safe" limit of the amp and need more juice, the solution is not bypassing the gain knob, it's getting a bigger amp.

That is correct, the LEDs indicate when the clip/limiting actually occurs not when it is about to occur. It is a preemptive measure in the signal chain to be sure that you're not damaging the amp or connected equipment. As before, are you confident enough in your signal being so predictable that running without the limiters on won't release the blue smoke?

If you're worried about additional signal compression because you're at the absolute limit of your amp's headroom, then you need more headroom with better hardware. Not cranking said amp to 11.

Of course there's an increased risk, why do you think the limiters are there to begin with? That nasty compression you get from active limiting should be your early warning to back off and reassess your game plan, find your weakest link, and replace it. Turning the limiters off is merely a band-aid on a crack in the dike.

I don't own most of the sound equipment in the particular venue in question, so I'm making due with what they've got mostly.  I'd rather not bring my amps or cabs into the venue to get more headroom.  I'm just trying to figure out the safest way to run the amps that are installed after I've rewired and rearranged the system several times. It's sounding great now, but I don't want compressed music without knowing it's happening if the clip light isn't lighting but limiting is happening before that.  I also don't want to damage drivers with high frequency harmonics accidentally that's a risk if I turn the limiter off on the PLX amp.

There is a soft limiter built into my analog active crossover I installed into the system that I have set just above the max signal of the digital mixer that's running into that, mostly so as to prevent spikes if something is disconnected or the mixer power cycles.  They were literally using a band EQ for a poor man's crossover before, with all the speakers in a clump under the DJ table, and amps' outputs wired directly into amps' inputs.  It was an awful mess and sounded even worse than it looked.  Now I'd stack this system up against anything in the region, at least on sound quality if certainly not max output.

I'm using the PLX 3002 in stereo mode with two 8 ohm subs, one on each amp channel, that are specified as Continuous Pink Noise: 800 watts and Peak Power: 3200 watts each.  These are reputable high-end monster subs taken from the owner's former large nightclub.  So the specs are probably legit.  The amp is specified as 550 Watts RMS into 8ohms in stereo mode at only 0.03% additional THD over the input signal, but I presume is capable of peaks of about 2200 Watts 8ohms per channel.  While that might be below the driver ratings, I am still concerned about having the limiter off and high frequency clip harmonics occasionally making it to the drivers from the amp since the crossover is before the amp and the subs have no passive filter -- good for efficient use of my power available, but potentially unsafe with clipping running direct into the drivers, right?

I also realize that the only way to prevent any possibility of thermal destruction of the woofers would be to have an amp that's one fourth of the driver rating in RMS, which would obviously be a measly 200 watt per channel into 8 ohm amp.  I don't want to downgrade the amp to something else in the rack of that wattage.  I'd rather keep that bigger amp (the largest in the venue that various meth heads over the past years haven't already fried) on the subs.  I'm not always at the venue and can't always babysit it when the system is used.  I try to be there, but there are one or two people I sort of trust to use it and I can't be there on their nights.  I'm trying to maximize output and minimize risk... trade-offs.  For the nights I'm in there using the system for my own purposes, I'd like everything to still be functioning without hiccup or prior driver thermal damage, with drivers that still sound the same.  Do high frequency harmonics into subs permanently alter the subs' characteristics?

The other guys know not to run the sub amp into the red, but we really need more output on the subs for certain situations.  This last week I've just reduced the digital mixer's pad from -12 to -6dB and adjusted the crossover's limiter accordingly (raised it slightly to compensate).  Before it was impossible to get the PLX to ever go into the red -- the digital mixer earlier in the chain would hard limit at double red before that and people know to keep that mixer out of the red.  It says it on the top of it :-)  Now with double the input gain and the PLX knobs at max, it's possible with the digital mixer into the red and you have to back the amp off from max like a notch or two.  I plan on putting a sticker on the PLX to say not to turn that up to max.  There is an infrasonic filter on the crossover that's already activated and I plan to switch the PLX 30hz HPF on, as well, which will give us the equivalent of a 8th order 27.5hz HPF and, from prior experiments with this system before, will probably prevent the red from ever lighting with the PLX limiters On.

So you're saying the clip light is not less likely to come on when the clip limiter is on or are you saying it will light up just as frequently either way?  If the clip LED light on the front never comes on when the limiter is switched into On position on the back of both channels, then limiting and compression is never occurring on the PLX 3002?  If that latter situation is the case, then I will leave the limiters activated on it as I have for the past year.  If that's not the case, then I'm tempted to turn the PLX limiters off, but then am worried about occasional clip harmonics getting to the drivers and possible associated risks from that.  I don't know if that risk is real, though.
« Last Edit: March 12, 2019, 04:13:05 pm by Benjamin Goulart »
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Chris Grimshaw

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Re: QSC PLX amp's limiting for subwoofers
« Reply #5 on: March 13, 2019, 09:35:49 am »

The amp is specified as 550 Watts RMS into 8ohms in stereo mode at only 0.03% additional THD over the input signal, but I presume is capable of peaks of about 2200 Watts 8ohms per channel.

Your assumption here is incorrect. If you said 2200w/ch at 2ohm, there's a chance that a very-short-duration burst would put up that sort of figure. The fact is that these amps switch rails, and the high-voltage rail is very squishy. That's just the nature of the beast.

You have 2x 8ohm sub cabinets, each rated for 800w. Not sure that really counts as a "monster sub", but I digress.
You want an amplifier that can put out somewhere around 1-1.5KW into 8ohm. Maybe a little more if you're feeling brave.

I'd recommend bridging the PLX3002 and putting the cabs in parallel. That'll get you 1500w per cabinet, which will be around 4dB more output than the 550w per cabinet you're getting now.
If you do go for that route, it's possible (albeit unlikely) that the amp will cook your drivers without ever touching the red. You'd need to play some proper dubstep which has long-duration sine waves for any problems to come up, and even then, you're likely to pop a circuit breaker.


You're also getting very hung-up on HF harmonics cooking your sub drivers. The short story is: don't worry about it.
The longer story is that the effect is pretty negligible in two ways:
- The extra power from the HF harmonics is actually very small compared to the increased average level of the intended signals - of the order of a few percent.
- Subwoofer drivers typically have lots of inductance, which means the resistance goes high above a few hundred Hz. Even if the amp is putting out square waves, the actual power dissipated from those HF harmonics would be very small - resistance is high, so current flow is low.

Chris
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Paul G. OBrien

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Re: QSC PLX amp's limiting for subwoofers
« Reply #6 on: March 13, 2019, 03:08:02 pm »

I don't own most of the sound equipment in the particular venue in question, so I'm making due with what they've got mostly.  I'm just trying to figure out the safest way to run the amps that are installed
So you think turning off a layer of protection would somehow increase safety? Do not turn off the clip limiters.

I don't want compressed music without knowing it's happening if the clip light isn't lighting but limiting is happening before that.
You are worrying about the wrong thing, the small amount of compression the clip limiters produce is a better tradeoff to make than the nasty shit that happens to the signal when an amp is overdriven.

I also don't want to damage drivers with high frequency harmonics accidentally that's a risk if I turn the limiter off on the PLX amp.
Ahh no, you can't damage a large subwoofer with high frequencies, that big voice coil acts as a natural low pass filter attenuating the level it sees as frequencies rise. But that isn't an excuse to turn off the clip limiters, a clipping power amp sound really bad.

There is a soft limiter built into my analog active crossover I installed into the system that I have set just above the max signal of the digital mixer that's running into that,
If it is adjustable you need to set it much lower than that. A digital mixer should NEVER be allowed to clip.

I'm using the PLX 3002 in stereo mode with two 8 ohm subs, one on each amp channel, The amp is specified as 550 Watts RMS into 8ohms in stereo mode at only 0.03% additional THD over the input signal, but I presume is capable of peaks of about 2200 Watts 8ohms per channel.
No, it might only be producing 600w or so when distortion rises to the 1% threshold where the clipping light activates.

While that might be below the driver ratings, I am still concerned about having the limiter off and high frequency clip harmonics occasionally making it to the drivers from the amp since the crossover is before the amp and the subs have no passive filter -- good for efficient use of my power available, but potentially unsafe with clipping running direct into the drivers, right?
Forget high frequency harmonics your main concern for an unsupervised system is abuse of the speakers(overpowering) so you need to do everything possible to protect them.

I'm trying to maximize output and minimize risk... trade-offs.
The other guys know not to run the sub amp into the red, but we really need more output on the subs for certain situations.
Then you need more subs and amps or a more powerful amp and much better limiting. The subs you have might take a bit more power... assuming they are as legit as you say, but only just a bit and that won't really produce a lot more SPL. If you need a lot more output you need more boxes it's as simple as that.

I plan on putting a sticker on the PLX to say not to turn that up to max. 
And you're gonna trust people not to turn it up? You know you can't do that.  All amp gain controls must remain at max, clip limiters engaged and your adjustable limiter must be set right at or just below the onset of amp clipping, this will produce maximum output as safely as possible. If that doesn't produce enough output then this venue doesn't have enough rig for the gig.
« Last Edit: March 13, 2019, 03:24:18 pm by Paul G. OBrien »
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Steve Litscher

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Re: QSC PLX amp's limiting for subwoofers
« Reply #7 on: March 13, 2019, 04:52:47 pm »

If my math is correct... the difference in output between an amplifier making 550-watts and one that makes 800-watts is about 1.5dB.

If it were me, I'd do as Chris suggested - bridge the amp and run the subs in parallel and then hope that other DJs don't push things into the red, otherwise you'll have some clicking drivers (or worse).

Real life story... we had some EAW SB180R subs (1000-watts AES rating) and several QSC PLX3602 amps when we first started the sound company. Originally ran the PLX3602s in stereo (725-watts per sub), and would just touch the clip lights. Sounded great, but we wanted more output... so, we ran two subs per amplifier with the amp in bridged mode (3600-watts). This provided the often touted "1.5x amplifier output" rating.

Didn't take long for the subs to start having issues when we pushed them too hard. Eventually had to replace all of the drivers with 18Sound drivers. Not fun, not cheap. Expensive lesson learned.

I'd argue if you need more output, cranking the amp isn't the way to go. I'd add more subs, or swap them out for something with more oomph. I don't think anyone is going to notice a 1.5dB increase in output, especially at the risk of frying drivers.

Benjamin Goulart

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Re: QSC PLX amp's limiting for subwoofers
« Reply #8 on: March 14, 2019, 05:41:21 pm »

https://qscprod.force.com/selfhelpportal/s/question/0D50y00005r2S5G/does-having-the-clip-limiter-activated-on-the-qsc-plx-line-reduce-the-likelihood-of-the-clip-light-coming-on-at-the-same-given-input-gain?s1oid=00DE0000000ZwhP&s1nid=0DB0L000000Kysr&emkind=chatterCommentNotification&s1uid=0050y00000ES1lL&emtm=1552498956903&fromEmail=1&s1ext=0

Bob answered the clip limiter thing pretty clearly.  Sounds like you guys are all of like minds on this subject.  I will leave the clip limiter on.

Last night I noticed the subs only sound slightly louder past 3 o'clock on the amp's volume controls with the digital DJ mixer just peaking into the first red and not actually clipping yet.  Those last two volume notches on the QSC amp caused the subs to mostly just sound worse even when the amp isn't yet in the red yet.  It's hard to push the QSC to clip with one notch remaining on its volumes and quite easy at max on those pots.  I can't entirely tell if it's the room (especially the ceiling) doing most of the additional rattling or the subs.  Running it the whole night at or just below that room-rattling point gave me a headache by the end of the night.  Since the subs and room are producing far more distortion than the amp does prior to the clip light coming on at those last two volume notches on the amp, I'm thinking there's not a whole lot of point trying to get more power.

I was thinking about bridging, but after last night I'm feeling less enthusiastic about that.  In fact, those last two notches past 3 o'clock on the amp's volume controls have me slightly worried.  I'm wondering if maybe that's about the same additional 6dB on the digital mixer's rear pad I've changed and perhaps I should go back to the -12dB setting.  Everything's currently going through a Yamaha sound board, so if they really want to mess stuff up, they can always start moving sliders and dials up on the board.  The -6dB pad setting on the DJ mixer seems a really nice fit for the Yamaha, though.  Even with the DJ mixer double redlined, the Yamaha isn't clipping inputs at unity on its gain/trims.  I might tell the other DJ and put a note not to touch certain things on the board and not to go past 3 o'clock on the QSC amp at the bottom of the rack and just leave everything where it's at on the DJ mixer pad, etc.

As for my analog crossover I put in the signal path, that's after the digital DJ mixer. 

Signal chain:  DJM-800 --> Yamaha sound board --> my crossover with that limiter built in -->  the amps. 

So if the DJM-800 clips, it's going to clip prior to the crossover's limiter.  No point in putting the limiter threshold on the crossover earlier than the DJM-800's own hard brick wall clipping at 0dBFS.  I already have a sticker on the top of the DJ mixer to stay out of the red, and it clips at double red.  The Behringer crossover with the limiter does not have great midrange, though, and the old Biamp Advantage crossover I bought the venue last year that has much much better midrange quality unfortunately turns out to only have 12dB/oct Bessel filters that aren't really filtering out enough highs for the subs and lows for the tops as I'd like.  I'm sort of in a quandary on that.  I might just try using the crummy sub filter on the Yamaha and activate the 40hz filter on the top speakers' amp and just pull the crossover out of the chain entirely.  The midrange on the bar side's feed of the Yamaha board sounds more lucid, so that Behringer seems to be smearing those mids.

At one time, I had the Yamaha sound board out of the main signal path and was just using it for the amps on its rear to run the booth-facing speakers.  I had the DJM-800 booth out going to the Yamaha board just for booth-facing speakers.  People have killed too many other amps in this venue such that they're in short supply -- like amp outputs wired into amp inputs sort of stupid stuff.  Got a big dead Crown and a distorting QSC not in use right now in there.
 Lately we've starting running bands, though, so it's more convenient to just have the DJ mixer running into the Yamaha board.  The Yamaha degrades the sound noticeably and so does any additional analog crossover I've added in the system to some degree or another, even if I'm not offended by the mids on the Biamp Advantage... its bass isn't amazing, for instance.  About the only really great component I've put in was the Behringer DCX2496 I used as the DAC and crossover from the DJM-800's SPDIF output when the Yamaha board was out of the main signal chain and just running the booth speakers.  That was a substantial upgrade from the DACs on the DJM-800, the crossover & delays are incredible sounding with no degradation, and was just a joy to have.  Got nothing but compliments.  I became too nervous having something like that of mine in the amp rack, though, around the sort of people that come in and out of that place, though.  You should have seen what they did to my big Antari machine.  The venue's DJM-800 has stuff someone intentionally broke on it and someone poured super glue in their Yamaha board at one time.
« Last Edit: March 14, 2019, 05:47:58 pm by Benjamin Goulart »
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Rob Spence

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Re: QSC PLX amp's limiting for subwoofers
« Reply #9 on: March 14, 2019, 08:31:35 pm »

Generally the ďamp volume controlĒ is just an attenuator on the input signal.
If you want it quieter, either turn down the source or the amp. Both have the same effect. It isnít the knob position that is adding distortion, it is simply driving the amp too hard.

The PLX 3002 and 3102 are not great amps for subs. They simply donít have the power supply for it.

For me, I almost always have the input knobs on my amps WFO. That way, the worst that can happen if someone messes with it is that it gets turned down.

If it were me, I would turn up the amp, and stop running the mixer EVER into the red. Think red = bad. In the digital world, you NEVER want to get close to clipping. Once you run out of ones, bad things happen to the sound.



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Re: QSC PLX amp's limiting for subwoofers
¬ę Reply #9 on: March 14, 2019, 08:31:35 pm ¬Ľ


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