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Thread: Jbl 2421a

  1. #1
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    Jbl 2421a

    Has anyone had experience of using the 2421A compression driver at its lower recommended crossover frequency of 800hz? The driver in question is part of a 4430 active restoration.

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    Senior Member grumpy's Avatar
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    Active experience? Only active with a stock 4430 with the passive MF/HF equalization intact and 5325 4430/4435 cards.
    Matching response curves and amplitude levels was fairly critical for the most benefit.

    Re pushing the crossover lower (and perhaps steeper),... my thought would be to go conservative with an unobtanium diaphragm vs trying to push it for some potential audible benefit. Ti diaphragm? Go nuts Then there's the inherent response ripple noted in

    http://www.audioheritage.org/html/pr...bl/4430-35.htm

    And some detailed info here:

    https://www.pispeakers.com/AES_v31_n6_p408.pdf

    but I expect you've already dug into those. Interested to see how your project comes along!

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    Administrator Robh3606's Avatar
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    If those are original Aluminum diaphragms I would be very careful running them to the lower 800Hz minimum. Depends on how hard you are going to play them. Do you know the history of the speakers/ drivers and how many hours and how hard they were used? That matters with Al. They are getting on 40 years old.

    One of the reasons JBL went to Ti was issues with the new diamond surrounds and failures with the then aluminum diaphragms. Aluminum fatigues much faster and the fatigue and failure rate is directly tied to excursion. Those diaphragms are no longer made and you should really take extreme care with them. Replacements will be hard if not impossible to find.

    If you want to run the drivers to 800Hz you just get Ti diaphragm's and store the Al in the boxes. That is if you are comfortable changing them out.

    Rob
    "I could be arguing in my spare time"

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    Thanks for all contributors thoughts and expertise so far, along with any in the future. All gratefully received.

    I believe JBL speced. the 2421A as usable down to 500hz at reduced power. Although as used in the 4430 the 2344 horn only has the potential to fully load the 2421A down to 800hz. The 4430 passive crossover is quoted at 1khz. From my calculations of the resonant frequency of the first leg of the hf network, for the 2421A, it appears to be closer to 1.6khz. The LF section is closer to 900hz, (from my calculations). In the 4430 crossover the frequencies above around 3kHz are boosted by close to 6db per octave to correct for the combination of the 2344's directivity and the 2421A mass roll off. So you're looking for about 12-15db gain by around 15 kHz to obtain an extended hf response. That's achieved via a damped series resonance, which also gives some adjustment of extreme hf. In my view the boost needed at hf is probably the limiting factor re distortion in the extreme hf in this design.

    Considering the limitations of a passive crossover, the 4430 had a following that extended it's shelf life to close on 20yrs. No doubt, in my view, greatly aided by the quality of the original transducers. But the possibilities utilising DSP have substantially moved (if not uprooted) the goal posts since 1980.

    So my take on the 4430: I believe there's benefit to be had from a fully active crossover which reduces the higher frequencies the 2235H has to handle and lowers the crossover point of the 2421A. Not yet sure where that will leave the LF crossover, or what the 2421A will sound like working down to 800hz. But with DSP generated 24db/octave slopes as the starting point and the option to apply multiple corrections to each drivers in band response, I think it will interesting to find out. As the speakers I'm restoring aren't intended for a studio environment where higher sound levels are much more the norm, the risk of excessive driver excursion of the 2421A near the lower crossover is less of a possibility. I believe such a project is worthwhile because of the intrinsic quality of these transducers. Something which I think can be better highlighted via digital processing and some additional attention paid to cabinet acoustic damping.

  5. #5
    Dang. Amateur speakerdave's Avatar
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    Do you think it's possible that to say you are restoring a pair of speakers and using the old CD diaphragms may be delusional? Your friends above have tried to discourage you from embarking on the Titanic. Bon voyage! In case you run into trouble, I have a pair of 2421 aluminum diaphragms I bought for my 4345's several years before the move of production to Mexico. PM me and maybe we can find a fair price. I never installed them because I started fooling around with TAD 200x and never looked back. I believe 2002's may be available if you can find them; I don't recommend buying used TAD 2001's because diaphragms for TAD legacy drivers are not available. Member Guido has posted a circuit for 2002's on that horn in the 4435. That reminds me. If you want to improve the woofer- horn handoff in the 4430, consider putting a 2234 in it, or even (much) better, 2216nd's, and EQing the bottom instead. And speaking of moving goal posts, although I have not heard that woofer, I believe, based on testimony here and elsewhere, and given your confidence in EQing digitally, the 2216 would work well in that cabinet, albeit with perhaps a change in tuning necessary.

    I wish you good luck with your project, but I suggest you follow the advice you've been given.
    "Audio is filled with dangerous amateurs." --- Tim de Paravicini

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    I've been listening to 2344 horns with 2426 drivers driven from DSP crossovers almost every day for the last decade. One of the first things that became apparent is that the 2344 is not a good sounding horn when used much below 1200 hz even with a 4th order high pass slope. 48db per octave sounds even worse but then it usually does unless you need to do some phase hocus pocus. If you want to reinvent this wheel consider a 10-12" mid driver between the woofer and the horn.

    Quote Originally Posted by Epplerd View Post
    ...The 4430 passive crossover is quoted at 1khz. From my calculations of the resonant frequency of the first leg of the hf network, for the 2421A, it appears to be closer to 1.6khz. ....

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    Recognize that the 1kHz crossover frequency was also chosen because that is the point at which the horn and woofer exhibit the same dispersion pattern which helps with imaging.

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