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[David Morison]

The sketching needs to be done on horizontal 2 cycle semilog paper for the lines to be straight and the estimates to be correct. The conclusion of Gordon's explanation conflicts with that from David Morison's calculation, which predicts an 8 to 12 dB hole.

Hi George,
FWIW, I used WinISD which lets you model crossover filters separately from the driver response itself, so a little less work than breaking out the graph paper  .

Also, I realised I made an error in modelling the Linkwitz-Riley case, I've edited my post to correct that but the end result is that the hole isn't quite as bad as I first stated - ie 10dB rather than 12.25.

Sorry for misleading you,
David.

[Scott Wagner]

I think the point that most in this thread seem to be forgetting is that the "on paper" electrical crossover point is rarely the same once reality sets in (ie: acoustical crossover point).  Do yourselves a favor, and do some research on the subject.  In the meantime, go listen to the system in question.  You would certainly hear a 10dB hole in that frequency range if it were there.

[Tim McCulloch]

I think the point that most in this thread seem to be forgetting is that the "on paper" electrical crossover point is rarely the same once reality sets in (ie: acoustical crossover point).  Do yourselves a favor, and do some research on the subject.  In the meantime, go listen to the system in question.  You would certainly hear a 10dB hole in that frequency range if it were there.

We have a winner!

Our only wired tool for affecting the acoustic response of a transducer is modification of the electrical signal being sent to it.  The ~1/3 to 1/2 octave under-lap may well be what is needed.

Until JBL replies to George, or someone with a measurement rig and the chops to use it has access to this combination of speakers, we're speculating about the acoustic results of this crossover.

[George Friedman-Jimenez]

OK, JBL's systems engineering manager got back to me with some very useful information that directly answered my question. The 90 Hz -6 dB frequency of the low pass filter listed on the spec sheet is a typo. The actual -6 dB frequency of the internal LPF is about 100 Hz and the filter uses 2 cascaded 4th order (24 dB/octave) Butterworth filters with a measured summed slope of approximately 42 dB / octave. Also the -6 dB frequency of the loop out HPF is 113 Hz not 120 Hz. He sent me a graph of actual measured 715XLF, 710 and summed frequency responses and the actual acoustic crossover point is around 104 Hz. There actually seems to be about a 2 Hz overlap rather than the 30 Hz underlap suggested by the spec sheet. An additional graph he sent me of the PRX718XLF HPF electrical output (identical HPF to the 715XLF) shows a -6 dB frequency of 113 Hz for the HPF, slightly higher than the -6 dB frequency measured using the PRX710 acoustic output when fed the HPF signal.

Bottom line, there is only a 2 dB dip around 110 Hz, with the low end -6 dB point at 44 Hz in agreement with the spec sheet. This is an excellent summation plot, the dip will probably be completely inaudible (to me at least). You can't get much better than that, and I am now very anxious to demo the 710/715XLF system when I return from travel the week after next. I have found the 80, 100 and 120 Hz warble tones on Bink Knowles' Test CD to be very useful in listening for sub/top crossover holes or bumps and hope GC will let me play that at real life SPL when I demo the system.

Thank you to JBL for an extraordinarily helpful response from their Systems Engineering department, and thank you all for your comments. I will get back to you after I listen to the system.

[Chuck Simon]

That was extraordinary help with detailed information hard to get from most companies these days!

[George Friedman-Jimenez]

Well, I went to GC today, after calling ahead to confirm that they had both the PRX715XLF and the PRX710 in stock, set up for a demo. I specifically asked the Pro Audio salesperson on the phone if he was sure it was the PRX715XLF subwoofer not the PRX715 top speaker. He assured me it was the subwoofer. When I got to the store I was told the salesperson thought I meant the PRX715 top, that the PRX715XLF is not in stock at any GC, and would not be available for at least a week or 2. I will go back when they have them available.

[cliff truesdell]

Any new news Gorge?

[George Friedman-Jimenez]

I called around and went to Sam Ash where they had the PRX710/715XLF set up for a demo. I played a few songs from wav files on a digital recorder and the bass sounded very uneven. Then I played 80, 100 and 120 Hz +-40 Hz warble tones from Bink Knowles test files and the bass sounded very uneven again. Playing 40, 50, 63, 80, 100 and 125 Hz sine waves through the sub alone it sounded like a bandpass sub with a big peak at 80, and almost nothing at 40, 50 or above 100 Hz. Turns out they had the sub and top set up with an 80 Hz Driverack crossover, ignoring the 90 Hz 48 dB/oct LPF that is built into the sub and not defeatable. Incidentally, I did not hear any cooling fan noise at all from the sub although we did not stress it much.
Bottom line, it was impossible to judge the quality of the sub or crossover given the use of an external crossover in addition to the internal LPF on the sub. Looks like I may have to buy the damn thing to do a proper demo, and then return it if I don't like it. I hate that. Before I stoop that low, I will go find and listen to a Yorkville LS720P and a QSC K-sub.

Any other suggestions? I am looking for a very musical sounding powered sub with built in crossover that is very small, very light (preferably <70 lb ) for between sets djing in small clubs 50-100 people. Tops are EV ZX3s although I am looking to downsize to 10" powered tops for those small gigs. The hard thing to find is the sub, there are several decent candidates for 10" tops. For the sub, I am looking for relatively low extension around 45 Hz -6dB point but don't need very loud. My PRX618XLF sounds pretty good but is too big and heavy for my back and these small gigs.

[Gordon Brinton]

Any other suggestions? I am looking for a very musical sounding powered sub with built in crossover that is very small, very light (preferably <70 lb ) for between sets djing in small clubs 50-100 people. Tops are EV ZX3s although I am looking to downsize to 10" powered tops for those small gigs. The hard thing to find is the sub, there are several decent candidates for 10" tops. For the sub, I am looking for relatively low extension around 45 Hz -6dB point but don't need very loud. My PRX618XLF sounds pretty good but is too big and heavy for my back and these small gigs.

The  Danley TH28 seems to fit that order nicely. It is available in a powered version as well. It only weighs 61 lbs without the power amp installed and doesn't look like it has a big peak at 80Hz.

http://www.danleysoundlabs.com/danley/wp-content/uploads/2012/04/TH-28-spec-sheet1.pdf

[George Friedman-Jimenez]

Thank you Gordon. Yes, I had looked at that but have not yet called Danley as I imagine the powered version set up for portable use would be extraordinarily expensive. The TH28 as shown on the spec sheet looks like it is designed for installations not portable use. I will ask if it is available with handles, and with a pole cup on the large flat side to give it more stability supporting a top speaker, and if so, the price and weight of the powered version.
Has anyone used the TH28 for portable applications?

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