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View Full Version : Is it worth it to rebuild an old JBL cabinet...?



fjhuerta
03-20-2004, 09:09 AM
Hi all!

I've been thinking about a nice project to spend a couple of months with.

I own a pair of Danish made JBL LX-66 speakers. I like their sound, and would like to make them even better.

On further examination, I found the cabinet to be one of the weakest I've ever seen. It's not even made out of MDF - it's just particleboard. There's no bracing inside.

The speakers have 2 8" woofers and have quite a lot of bass. I'd assume such a weak cabinet, combined with so much bass power, is not exactly a recipe for uncoloured sound.

My plans are to build a new, heavily braced 1" MDF cabinet in the shape of the L-250 JBL speaker, and use the original crossover and drivers for the LX-66, while mantaining internal volume constant.

I calculated the internal volume of the LX-66 as being 107L. Then, after measuring the 4" port on the back, I arrived at a 40 Hz tuning point.

I'd also like to drop said tuning point at least 5 Hz. My goal would be having a 30 Hz tuning point, at the expense of efficiency. I always thought the speakers were a bit underdamped, anyway.

Do you think this would be a worthy project? I don't know - the 26Ti tweeter is not recognized as one of the best, and the original speaker is in good shape. I'd like to build it because I got a new low power tube amp, and think the JBL's would be ideal (with their high efficiency rating - around 94 dBs!).

Also, I've been looking for the T/S parameters for the 408G woofer driver, but haven't been able to find it, so I can't determine the speakers' response after re-tuning the cabinet, so I'll have to test them by ear.

Any opinions will be very welcome!

GordonW
03-20-2004, 10:18 PM
My only qualm with this idea, is that there may be some crossover re-tuning (ie, changing of values, etc) needed, to compensate for the change in baffle shape and size. The response of the woofer and mid will probably change, just a bit.

However, really... the advantages of making a REALLY SOLID, quiet (from an acoustical "ringing" of the cabinet standpoint) cabinet, probably will outweigh the negatives. I've never seen a case, where a MORE rigid cabinet DIDN'T improve sonics.

As for tuning- no reason not to try the lower Fb. Just be sure and leave yourself the option of going back to the factory tuning, or something intermediate between factory and the initial lower Fb you test- don't glue or otherwise fasten the port tube into the box, until you're SURE you have it where you like it. Some of the "underdamped" behaviour, may be more the "box talk" from the wimpy box structure, than an inherent woofer tuning issue... so make sure you're not wedding yourself prematurely, to a change that may not need to be made.

Regards,
Gordon.

fjhuerta
03-22-2004, 09:36 AM
Gordon, thanks for your help!

I changed my mind. For the sake of simplicity, I'll be building the cabinet in a rectangular shape, so I guess the crossover re-tuning won't be so necessary (building a pyramid-shaped speaker takes a lot of effort).

As for the tuning, I'll start with 30 Hz and work my way upwards... I intend to keep the original cabinet handy, in case something goes wrong.

Thanks again!

Javier.