D123 design.

[castlesteve]

I`ve picked up a pair of JBL D123 12" drivers and want to design a cabinet and pick a high frequency driver to compliment the 12s. I`m thinking a large BR or possibly an MLTL design. What do you think about using a small fullrange driver to do the highs?
Does anyone know where i can find T/S parameters for the 123s? I`ve tried searching without results.
Steve.

[kevinkr]

You might find them, depending on the version right on the JBL Professional website. There is a spreadsheet you can download.
Just watch out for all of the poorly documented variants. Early 16 ohm versions are not covered, but many of the parameters are reasonably close. (The rest you can measure.)

The 075 might be a good, if expensive match for this driver if it can be crossed over at >2kHz.

Kevin

[tomtt]

http://www.jblproservice.com/pdf/Thi...Parameters.pdf
`````````````````````````````````````````
toward the bottom of page 4

[Josephjcole]

Originally posted by castlesteve
I`ve picked up a pair of JBL D123 12" drivers and want to design a cabinet and pick a high frequency driver to compliment the 12s. I`m thinking a large BR or possibly an MLTL design. What do you think about using a small fullrange driver to do the highs?

I think a small fullrange driver can work quite well here. I did something similar with a 12" Eminence and a 4" fostex driver. The 12" is in a MLTL with the fostex in a small sealed enclosure inside. I ended up with a single capacitor on the fostex which combines with its natural rolloff for a 3rd order crossover at 300Hz, then a standard 3rd order crossover for the Eminence. 300Hz was the -3dB point for Baffle Step losses on my cabinet so it all worked out pretty well. Hopefully I'm not just tooting my own horn, and this is helpful information for you. But for some blatent horn tooting I've attached a picture of them, sorry can't really help myself.
Joe

[GM]

Greets!

Indeed, 'FR' drivers are a good solution! I've literally built a house full of this two way layout for others back when RadioShack, LaFayette, and Olson Electronics combined had a large selection of relatively inexpensive/high SQ drivers to choose from. Even did a few D123s using JBL's recommended 5.5 ft^3/40 Hz Fb alignment in a tower design (aka MLTL now) back before T/S ruled, which lines up well with their published specs, so appear to be accurate enough. The 123's DC needs to be damped though, since even a low XO (I used 500 Hz/2nd since it could be bought cheap enough pre-assembled) didn't stop it from being excited.

GM

[castlesteve]

Thanks for the replies guys. I wonder Joseph how you managed to match up the fostex to the eminence as i would have guessed the eminence is much more efficient than is the small fostex single driver.
I have been warned about the dust cap issue by another gent who advises using 3 coats of damar to damp the cone. Is this cone issue something that can be dealt with or would i be better off looking at other drivers. I love the coherence of single drivers but don`t like the frequency limitations and the lack of dynamics. So this idea of using the 123s was to keep much of the seamless sound of the fullrangers and add bass and slam.
Steve.

[GM]

Greets!

You're welcome! The dammar will work, but if these are original, I wouldn't do anything that's not reversable, which Dammar isn't. You can cover the driver with layers of grillcloth or similar, adding them until it sounds smooth in the XO transition BW. If you just want to damp the DC's break-up modes, then suspend a thick, felt disc in front of it. I don't recall how big a one I used, maybe 4" and 0.032"? piano wire. Anyway, you will have to experiment with thickness/diameter since I don't think the 5/8" thick carpet underlayment I used is still available. On some 'fullrange' speakers several years ago I used 3/4" thick polyfil sheeting to flatten the whizzer's and surround's high Q resonances.

Anyway, the D123s have good enough resolution for mating to ultra low Q FR drivers in the 500 - up Hz BW, so Fostex, Lowther, etc., horn drivers and pad them as required. You'll still have enough dynamic headroom to go 'live' if you have the power, though maybe not at front row center of some performances.

GM

[Josephjcole]

Steve,
My Eminence are only about 94dB/1m/1w. The fostex are somewhere between 90-91 dB. So with baffle step losses not much attenuation is needed at all. I do have a little bit of attenuation through some l-pads that I have hooked up to them (I've been too lazy to replace the l-pads with fixed resistors), but not much at all. If you went with a slightly more effecient fullrange driver you wouldn't have to pad the JBL's down at all.
Joe

[castlesteve]

Ok i like the sound of this. I have a pair of Fostex FE167e. Would these work on an open baffle crossing to the 123s. If so, where would you recommend crossing and how steeply? I also have a pair of FF85Ks but i think they may be a little small for open baffle use and they are only 88db.
Steve.

[Josephjcole]

Well, I'm not that familiar with the D123 or the fe167e, but two approaches come to mind. Obviously a lot of this has to do with how low the 167s roll-off, and how loud you like to listen. First option is what I ended up doing is pushing the crossover down to where the driver naturally starts to rolloff and just add 6dB/octave at this point with a capacitor. Then match the roll-off of the fostex with your woofer. This might limit your SPL's due to overexcursion of the 167, but may not be a problem....
The other option is to push the crossover up to the point where you can do a honest first order crossover. I'm not going to even try to guess a frequency but there looks to be quite a bit of play in this as both drivers are quite wide-range drivers.
Joe