When I connect a passive subwoofer to a Lab Gruppen Lab500 amp and this amp to mix 8 of the Qu-16 mixing table with all the faders of the equaliser of mix8 above 150 Hz completely down, will that result in a well enough working low pass filter?
no
No, the GEQ bands are quite wide.
The PEQ could be somewhat abused for the, with a shelf doing some serious cutting, but it would still not be “right”. A crossover applied post output is the ‘correct’ solution, other options might kinda, sorta, work(ish) if you’re lucky.
“Well enough”?
While far from ideal, it won’t hurt the subwoofer or the amp.
I do this all of the time into (powered 1200 watt B52’s) and non powered Subs using Lab Gruppen 4k amps
However I cross at about 100 hz
all faders (except 30hz left at 0db) 40 through to 100 I raise up. 125hz I leave at 0 (sometimes right off). the rest of the faders I bring down to off.
Not technically Ideal however it works fine.
Use your ears and be careful and actually go and look at your driver cones to make sure you are no pushing them too hard in those conditions.
Dhak - why not the PEQ?
Yes for sure I would use the PEG
I have been using the graphics only because I come from that era where graphics were/are used to artificially create crossovers
I’m not the only one that does this…
I have seen some huge JBL systems use this method.
I guess hence: people have been asking for Low pass filters on the outputs in QU.
I’m a little irritated about using the GEQ (or even shelving PEG) to mimic some kind of crossover. I’d not only expect issues from phase shifting, but both the PEQ and the GEQ only cut the unwanted signal by -12dB (or -15dB?, not sure). While this surely won’t hurt the subs, it may hurt your top, when trying to mimic a high pass (or low cut, however you prefer to call it).
When talking about “large” systems there should be some bucks left to add a proper crossover (cost way less then any repair)…
Irritated?
I think we were/are calling it low pass filter.
I have never had to repair any 18"subs. (only from Mr DJ’s when left unattended) 
I always run a comp/limiter after EQ on bottom end.
Its 12 db on the QU.
low pass filter… passes lows, cuts highs
high pass filter… passes highs, cuts lows
any of my subs would sound like crap if I didn’t have very specific control of the frequencies that they produce.
I use a 48 db per octave cutoff.
They sound wonderful!
I can not imagine using any kind of EQ, parametric or graphic instead of proper crossover settings.
All that being said, a man’s gotta do what a man’s gotta do
Use what you have and use it to it’s best.
so I’ve done some plots of actual Qu EQ responses, after checking out the plots you’re free to decide if PEQ/GEQ could be used as a proper LPF/HPF replacement. Cutoff always set to 1kHz for better readability.
Since only four attachments are allowed, this is split into four posts.
1-FlatResponse: simply verify my digital loopback is clean and properly leveled
2-1kHz-HPF: Response of the channel HPF
3-PEQ-1kHz-HPF: Shelving PEQ simulates HPF
4-PEQ-1kHz-LPF: Same for Low Pass
5-PEQ-1kHz-LPF+1kHz-HPF: Sum when using shelving LPF+HPF as a crossover, watch the significant drop in the center
6-PEQ-3kHz-LPF+333Hz-HPF: Sum of PEQ 3kHz LPF and 333Hz HPF, somewhat better
7-GEQ-1kHz-Cut: Cut a single band using the GEQ
8-GEQ-1kHz-HPF: GEQ simulates a HPF, please note the non-flat stop-band and that lowest frequencies still feed through without any attenuation. This would be a killer for any top cabinet (too much low frequency energy).
I’m skipping the plot for the GEQ LPF, since it would be same with frequencies above 16kHz not attenuated
If you are going to the expense of buying a QU series board and running decent quality amps, it seems somewhat pound foolish to skip using a proper good quality crossover to extend the life of your amps/drivers and produce a better quality sound for your audience.
Lee
Very interesting Andreas, thanks. Pretty much what I expected to see.
Sending too much HF to a sub is usually not going to damage anything, the other way around is.
Just put any decent crossover on the output or sub send. Use a driverack. I have used a driverack just to get a sub output before. It is not that difficult or expensive.
Not sure the signals paths match well enough to use the HPF and an antiphase version of the original signal 
Id be interested by the output of an external crossover…
Ok, here are three plots from a real crossover (LEM XO202) set at 200 Hz. As you can see from the plots, the numbers on the front do not necessarily match what you get, so measuring may be wise before using in production.
LPF and HPF plots look somewhat expected, for me very interesing was the LPF+HPF plot which is nearly flat (about +/-1dB) up to the highest frequencies.
Setup was:
- PC via USB into Channels 31/32 (Qu32) those are send to Mix2 Output
- Mix2 Out to XO input
- XO LPF/HPF into Channels 1&2
- Channels 1&2 send to Mix1
- Mix1 master used to level at 0dB overall and recorded via USB
So this also gives an overall impression of both DAC and ADC quality of the desc, and don’t forget the LEM is some analog gear in the middle.
Very different from the eq based plots…
Sure, since this is a crossover which is built to give a flat response when summed and not a shelving PEQ or cluster of notch GEQs. Sadly there’s no spec in the manual about the rolloff, but from the plots its 24dB/octave. The Qu-GEQ has about 12dB/octave the Qu-PEQ shelving filter some 3dB/octave.
Yes, I did expect it to be better, but even so…
The figures are useful to quote, since you have the charts up… What’s the HPF dB/octave?
(Feeling lazy, and want a consistent number recorded