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Thread: Advice requested for xover for old JBL D131 and new horn

  1. #1
    jonathan z
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    Advice requested for xover for old JBL D131 and new horn

    I'm working on a project that will update an old console based JBL system (this was probably not factory JBL but custom). It currently contains a D131 and 075 with a first order JBL xover. I've been informed that the xover is probably set for 2500hz. Coil and cap are unmarked so I cannot confirm this. The problem with the system right now is that the 075 produces a very erratic response and the midrange clarity is poor.

    I am replacing the 075 with a new modern compression driver (Radian 475) matched with a DDS waveguide. The combination has a usable frequency response of 800-20K presuming a 4th order xover at 800 hz. Otherwise, the range must be raised to protect the compression driver. I do want to cross as low as possible since I believe that will improve MR clarity.

    Based on the factory spec of the D131 (I do not have the drivers available for test right now) and the cabinet dimensions (2.52 cubic feet) the D131 will develop a very peaked response at resonance with a standard LR 4th order LP (according to Unibox). It seems in part that this is due to the size of the enclosure.

    I'd appreciate any suggestions for successfully integrating the two drivers.

    Also, is anyone in this forum familiar with the crossover setup used by Pi Speakers (LP 2nd order and hp 3rd order) and if so, what do you think? Obviously using that would force me to raise the xover frequency.
    Last edited by jonathan z; 12-10-2003 at 12:59 PM.

  2. #2
    Senior Member GordonW's Avatar
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    There's no reason why a D131 couldn't be used up to 1200 Hz, or even 1500 Hz. Heck, the original crossover was 2400-2600 Hz, depending on which box was in there...

    I'd say, maybe try a 1200 Hz asymmetric... second order LP and third order HP. That way, you can easily "tune" the rolloff of the woofer, by tweaking the "gap" (frequency spacing between the woofer and tweeter crossover points) or the Q (resonance peaking or lack thereof, electrically, for the woofer) of the crossover...

    Regards,
    Gordon.

  3. #3
    jonathan z
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    How to tune?

    Gordon: I'd say, maybe try a 1200 Hz asymmetric... second order LP and third order HP. That way, you can easily "tune" the rolloff of the woofer, by tweaking the "gap" (frequency spacing between the woofer and tweeter crossover points) or the Q (resonance peaking or lack thereof, electrically, for the woofer) of the crossover...

    JZ: Can you be a bit more specific or give examples? Are you suggesting letting the woofer have a low end peak and sliding the tweeter x point lower to compensate for it? Do you mean something else by tuning the rolloff?

    Thanks!

  4. #4
    Senior Member GordonW's Avatar
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    With a second order low-pass on the woofer, it's easy to tune the "shape" of the "knee" of the electrical rolloff... ie, whether it's a sharp, arbrupt thing, or a gradual sloping rolloff. This can be quite useful in taming rising or falling response drivers- use a low-Q rolloff (Linkwitz or whatnot, Q= .5 to .577 or so, maybe even lower) for a driver with a peaking or rising response, or OTOH, use a high-Q rolloff (Butterworth to Chebychev, Q greater than .707) to "prop up" a driver that's "sagging" at crossover (though you need to be careful, the peaking crossovers can induce bad impedence dips, which can make amps unhappy)...

    It's easy to change the Q of one of these lowpass crossovers- just calculate the values for any "normal" crossover, and log them down. Then, you can take ANY inductor and capacitor, that MULTIPLY to the same PRODUCT as the original cap and coil value multiplied together, and maintain the SAME crossover frequency, just with "knee shapes" at the crossover point. To lower the Q, use a larger coil and a smaller cap, to raise the Q, use a smaller coil and a larger cap.

    Geesh, anyone know any simpler way to describe this? It's one of those things I KNOW, but it's always hard to put in words, any simpler than this...

    Regards,
    Gordon.

  5. #5
    jonathan z
    Guest

    Thanks - I get it! (NT)

    Thanks - I get it! (NT)

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