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Thread: Affordable Quality Compression Drivers?

  1. #1
    Administrator Mr. Widget's Avatar
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    Affordable Quality Compression Drivers?

    I am starting this thread because of an OT post in another thread that I think is very important and I am not sure it has been properly addressed before.

    If we DIYers are not using the moderately priced B&C, Beyma, or BMS drivers, we frequently are using second hand drivers or "new" drivers sourced from eBay. This is reasonable... especially if you want 375s or 2441s or any other vintage JBL as buying them new from JBL would be rather difficult.

    The problem is that loudspeaker drivers are precision instruments and compression drivers require particularly tight tolerances. Having bought secondhand drivers and even those with recently replaced diaphragms with factory fresh replacements... the results are not always up to spec. Especially if the person doing the work is inexperienced. The quote below has been edited to only discuss this topic, but I think you'll get the idea.

    Quote Originally Posted by 4313B View Post
    Were they swept for spurious resonances afterwards? ... it was the usual PITA to get the diaphragms installed correctly. Unfortunately it isn't as simple as screwing down a bunch of bolts. BTW - even the "modern" 2435's and 435Be's need similar tweaking to get rid of all the spurious resonances, especially with the surround creep issues.


    My point is, a 2425H or 2421A, or any other compression driver for that matter, can sound pretty harsh if it isn't set up correctly. I've got a pair of 2421A's right now that sound like crap because the diaphragms are "off". I'll have to take them apart and then eff around with them for awhile until they sweep real nice.
    The point I'm trying to make is that buying a secondhand compression driver is a real crap shoot... I have measured dozens and it is rare that a secondhand pair are within several dB of each other or even their specifications... new drivers purchased through "normal" channels will be within a dB or two of each other and have the specified harmonic distortion levels...

    My recommendation is to use new drivers when possible and with vintage gear either work with a top flight repair center or send them back to the factory... we've discussed red wax seals before, but simply screwing in a pair of new diaphragms isn't all there is to factory spec either.


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  2. #2
    RIP 2010 scott fitlin's Avatar
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    And I DO agree, BUT, they want to do DIY creations, and the only way to learn..................

    Some will thoroughly research, and some will be better than others, and a FEW will become ACES!

    Part of the hobby.
    scottyj

  3. #3
    Senior Member ratitifb's Avatar
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    JBL recommendations

    Except measurements that some of us can carry out, does JBL tech center have the ability to match a specific speaker motor to a selected diaphragm to improve voice coil centering at high level ?

    Does JBl work in that way when re-dia a 2426 driver ?

    Is there some recommendation to increase a bit the diameter of the 3 fixing holes on the plastic frame of the 2421 dia ?

    Is there some recommendation about torque when screwing the dia ?

    Thx

  4. #4
    Administrator Mr. Widget's Avatar
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    Quote Originally Posted by ratitifb View Post
    Except measurements that some of us can carry out, does JBL tech center have the ability to match a specific speaker motor to a selected diaphragm to improve voice coil centering at high level ?

    Does JBl work in that way when re-dia a 2426 driver ?

    Is there some recommendation to increase a bit the diameter of the 3 fixing holes on the plastic frame of the 2421 dia ?

    Is there some recommendation about torque when screwing the dia ?
    The fact is, I don't know what the JBL factory procedure is for repair of vintage drivers, but some years back I sent them a pair of 2441s with cracked throats. They told me that the diaphragms that were in them were no longer up to spec and recommended that I change them... I let them replace them. The drivers I got back looked as beat scratched up as they did when I shipped them, but with new crack free throats, newly magnetized and aligned gap, and new diaphragms the drivers performed as new. I was very pleased.

    ...and another anecdote reported elsewhere in these DIY threads.

    Some years back while Zilch and I were measuring the performance of a variety of JBL 3" diaphragm drivers and horns we discovered some of his "new" eBay sourced 2435Hs were measuring quite badly. They were sent back to JBL where they were rebuilt and voilā. Now they were truly new drivers.


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