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Author Topic: QSC PLX amp's limiting for subwoofers  (Read 1315 times)

Paul G. OBrien

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Re: QSC PLX amp's limiting for subwoofers
« Reply #10 on: March 14, 2019, 09:54:33 pm »

What Yamaha mixer do you have? My general impression of their analog desks having used a few different models now and after having some time behind consoles like the A&H MixWiz is that the Yamahas don't have much clean headroom, you really can't put the channel strips or the main bus into the red at all as distortion increases dramatically, I generally try to keep everything peaking at a couple yellow/amber leds at most.

A DCX or similar is what a place like this really needs and the way you had it connected was ideal, I believe I read that the digital output from those Pioneer DJ mixers cannot be driven into clipping regardless what the front panel led displays indicate. The DCX can be software locked to prevent tampering of course but ideally all this rack gear should be locked away somewhere the DJs don't have access to, that is the only way you can be confident things won't get messed with.

P.S. What inputs are you connecting the DJM outputs to on the Yamaha? Are you using mic inputs or one of the line level stereo input channels?
« Last Edit: March 14, 2019, 10:00:38 pm by Paul G. OBrien »
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Benjamin Goulart

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Re: QSC PLX amp's limiting for subwoofers
« Reply #11 on: March 15, 2019, 05:59:20 pm »

Yeah I know what an amp pot does.  Thanks for the heads up, though.

I guess if the 3002 has a switching supply then that means it's got sort of thermal limiting built in that should prevent it from ever burning the subs out.  I guess that's a benefit if I choose to bridge it.

Not sure what you mean by digital clipping being worse than analog.  Some of the worst clipping I've heard has been on otherwise great sounding analog mixers.  Digital at least can't clip louder than its hard full scale limit.

I have the Yamaha sound board connected correctly, pad on, and levels tested at unity with highest levels the Pioneer can input into it.  Agreed that not connecting to a sound board properly is pretty common out there in my experience.

The DCX was definitely great in the rack, but I can't convince them to buy one.
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Mike Caldwell

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Re: QSC PLX amp's limiting for subwoofers
« Reply #12 on: March 15, 2019, 11:00:55 pm »

In what I've read here I think you have far more issues than singling out an analog crossover and mixer as the main problem.

Curious what analog crossover model do you have that has a limiter built into it.

Back in the day analog crossovers with output limiters for the most part were in the big boy league of the analog crossover world.
« Last Edit: March 16, 2019, 04:21:00 pm by Mike Caldwell »
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Chris Grimshaw

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Re: QSC PLX amp's limiting for subwoofers
« Reply #13 on: March 16, 2019, 03:37:55 am »


I guess if the 3002 has a switching supply then that means it's got sort of thermal limiting built in that should prevent it from ever burning the subs out.  I guess that's a benefit if I choose to bridge it.


No.
The amplifier has thermal limiting to protect itself. Nothing else.

If you need to thermally protect your drivers, you'll need an amplifier with some DSP that can detect the power levels being delivered, and attenuate to protect the drivers.

With the PLX3002 bridged, you could cook your subs without clipping. A sine wave just below the limiters would probably do that within a minute, but only if your circuit breakers can hold up. Sine waves mean there's a lot of power being delivered, but the peak voltages (which light up the clip lights) aren't particularly high.

Chris
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Benjamin Goulart

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Re: QSC PLX amp's limiting for subwoofers
« Reply #14 on: March 17, 2019, 02:00:36 pm »

What Yamaha mixer do you have? My general impression of their analog desks having used a few different models now and after having some time behind consoles like the A&H MixWiz is that the Yamahas don't have much clean headroom, you really can't put the channel strips or the main bus into the red at all as distortion increases dramatically, I generally try to keep everything peaking at a couple yellow/amber leds at most.



The Yamaha is an emx 5000.  There is a lot busted on it or super glued by some moron.  I took the crossover out of the path and used the Yamaha's crummy low-pass mono output filter and combined that with an old Roland mono graphic equalizer set to -12dB mode with everything above like 100 or 120hz down.  The bass actually has more tune to it and less boom, which is surprising, and the mids and highs on the tops are way better.  The roll-off of higher frequencies isn't as good as the Behringer crossover, but it's close enough probably with the combination.  I can also run some pink noise later and probably EQ the bass a little. 

The 40 hz filter on the top speakers' amp isn't doing a whole lot and there's a little content coming through them below 80hz using the high-pass on the Yamaha to test its effects, but everything sounds more clear, lucid, and nuanced than before.  There was sort of a glassy quality to the mids and it seemed like treble needed to be turned up too much to get the same info to present, but sounded obviously bright that way.  Last night it sounded more balanced and didn't need to be as loud to hear everything.

I know that the Yamaha is degrading the sound compared to the DCX when the Yamaha was out of the path, but the Yamaha has to be in the signal path to run the few bands that come through, so if that has to be there and in the path anyway I might as well try and use some of its so-so features and eliminate other stuff from the chain.

No.
The amplifier has thermal limiting to protect itself. Nothing else.

If you need to thermally protect your drivers, you'll need an amplifier with some DSP that can detect the power levels being delivered, and attenuate to protect the drivers.

With the PLX3002 bridged, you could cook your subs without clipping. A sine wave just below the limiters would probably do that within a minute, but only if your circuit breakers can hold up. Sine waves mean there's a lot of power being delivered, but the peak voltages (which light up the clip lights) aren't particularly high.

Chris

I meant if the amp is going to trip the breakers in the basement or is using a switching supply, both function as kinds of thermal limiters.  Switching supplies can't send out continuous deep bass sine waves at a constant volume.  It will start to sag on them after a few seconds.  And like you said, the breakers will also trip if too much power is fed for too long.  I'm trying to think of the QSC amp and poor power situation in this venue as glass half full.  The breakers have actually tripped before when the amp was previously bridged and the subs are still working, so I guess that should be a good sign.
« Last Edit: March 19, 2019, 01:59:43 pm by Benjamin Goulart »
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Paul G. OBrien

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Re: QSC PLX amp's limiting for subwoofers
« Reply #15 on: March 21, 2019, 11:36:16 pm »

The Yamaha is an emx 5000.  There is a lot busted on it or super glued by some moron.  I took the crossover out of the path and used the Yamaha's crummy low-pass mono output filter and combined that with an old Roland mono graphic equalizer set to -12dB mode with everything above like 100 or 120hz down.  The bass actually has more tune to it and less boom, which is surprising, and the mids and highs on the tops are way better.  The roll-off of higher frequencies isn't as good as the Behringer crossover, but it's close enough probably with the combination.  I can also run some pink noise later and probably EQ the bass a little. 
Was that a Behringer CX3400, that is one of the few analog crossovers I have seen with any kind of limiter built in. It's also not one of their better sounding pieces... like many of the analog mixers there always seems to be something not quite right with the sound, but it's still going to do a better job at crossovers than the old graphic EQ trick.

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Brian Jojade

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Re: QSC PLX amp's limiting for subwoofers
« Reply #16 on: March 22, 2019, 12:20:41 am »

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.


I agree with 100% of this.

As far as protecting gear from idiots, the safest thing to do is make sure you have way more speaker than amp.  If the RMS rating of the speaker is 4X the peak rating of the amp, it's pretty much impossible to burn up your subs even if driven into full square wave insanity.  Now, of course, that means leaving a lot of headroom on the table.  Matching RMS rating of the amp and the speaker will give you decent power and you'll be safe against all but the complete idiots that like to drive the system into solid clip.

If you're in control, having an amp that delivers 2X the RMS rating of the sub can be generally safe, but a little screw up can result in big expenses.  The difference in volume with that extra power is only a couple db.  Is the risk of damage worth that?
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Brian Jojade

lindsay Dean

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Re: QSC PLX amp's limiting for subwoofers
« Reply #17 on: March 22, 2019, 01:48:44 pm »

 it's obvious you don't have enough rig.
 obvious if you don't find some way to control it it's going to cost somebody some money and blame .
 see about getting more rig.
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Chris Grimshaw

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Re: QSC PLX amp's limiting for subwoofers
« Reply #18 on: March 25, 2019, 09:36:37 am »


I meant if the amp is going to trip the breakers in the basement or is using a switching supply, both function as kinds of thermal limiters.  Switching supplies can't send out continuous deep bass sine waves at a constant volume.  It will start to sag on them after a few seconds.  And like you said, the breakers will also trip if too much power is fed for too long.  I'm trying to think of the QSC amp and poor power situation in this venue as glass half full.  The breakers have actually tripped before when the amp was previously bridged and the subs are still working, so I guess that should be a good sign.

... and the SMPS myth continues.
A cheap switch-mode power supply won't sustain current. A good one will.

From the comments about the PLX3002 running subs, it sounds like the SMPS hasn't been designed to run full-power sine tones for hours on end. That's okay, though - the PLX amps still enjoy an excellent reputation for mid/high use, or sub use into higher impedances.
Having a power supply that holds up to 2ohm/ch full-power sine tones would add a lot to the price tag, for a feature that's rarely used.

Chris
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Re: QSC PLX amp's limiting for subwoofers
¬ę Reply #18 on: March 25, 2019, 09:36:37 am ¬Ľ


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