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Wagner
09-12-2013, 05:54 PM
Hello,
Day before yesterday, I let something that should not have move me to action. A few of the wood screws attaching the mids in a pair of L100s would not tighten to my satisfaction. I involved myself in the pain in the ass game of reinforcing/repairing loose fitting screw holes in big particle MDF.
THAT'S another story but I did achieve decent results.

While undergoing this waste of time, and getting all up close and personal with a flashlight, I noticed something I had never noticed before about this pair of L100s. Aside from the fact that the screw holes are drilled absolutely TOO CLOSE to the edge of the cut out for an MDF of this type (I am actually surprised ANY of them would/could snug up from day one), once I had the drivers back in the boxes I could see (because I was really scrutinizing things) that the hole was cut (from the factory) so large as to never allow for a sealed install! Even WITH the standard JBL hollow O-ring gasket in place. This is only obvious on 1/4 of the circumference as the drivers are also not truly concentrically mounted with the cut-out. Just a sliver but a "leak" nonetheless.
(I asked about these in another thread, my particular pair never had any).
Just typical rapid fire assembly work I would assume (they were popular after all! :bouncy: )

The hole for the midrange is cut larger than it would have to be if only for the driver itself, to allow for the insertion of the cardboard chamber for the mid. The way the wiring is handled also leaves something to be desired, just an awl punch (very "get it done" type workmanship) and then the big glob of silicone sealer around the wound. I see this sort of heavy handedness in a lot of speakers from the '70s, especially Klipsch stuff built during that decade; must've been all the weed! :crying: It's functional, good enough.

So, my question (without inviting all the L100 ribbing hopefully :)) is how detrimental is this to sound and response, as obviously the LE5-2 is an open back driver?

The only simple solution I can see, short of reworking the front baffle (which isn't going to happen with me and L100s) and mounting the mid's chamber in some other fashion (the chamber also precludes the ability to easily switch over to T-nuts) would be to seal the joint with a caulk type sealer. Going that route would put more strain on those crummy wood screws and holes so now we're back to square one.
(like I said, self inflicted worry and waste of time)

Is this even worth pursuing? Any suggestions?

Thanks,
Thomas

Also, after noting the problem with the one box, I examined the other one very closely. It suffers the same problem, just not as visible. I can find it easily though with a slip of paper. Guess this is why this pair (and I am assuming many others) did not have O-ring seals installed on the mids. There is nothing for the O-rings to seal to. This is a LATE production pair of L100-As. Not typical fit and finish for anything JBL I have ever owned (especially the damn wood screws). Even the countersinks in the LE5-2 frames are all different in depth and size, Slight, but still inconsistent.

NickH
09-12-2013, 07:28 PM
If I recall correctly it doesn't matter on mid range drivers unless it communicates with the whole cabinet. It really only matter with woofers.


But I'm not an expert though.

subwoof
09-13-2013, 02:58 PM
2 schools of thought - if the air mass compression behind the mid isn't correct, it *can* change the sound

BUT the other ( realistic one ) is that, as you noticed, particle board is shit wood, machining it that close is a joke, mass-producing speakers of that quality is an oxymoron and that hindsight is actually 20-20

:)

Since the crossover networks are so primitive ( look ma - just 2 caps ) and the cabinet sealing so random, it makes little diff BUT for the rattle effect.

What I like to use is a "string" like compound used in auto body repair made by 3M. It's gray, looks like a string of red licorice and is sold in auto body supply houses. It doesn't harden, seals and prevents rattles and is easily worked/removed. works much better that that joke fish paper as on the similar size studio/control monitors.

Handy to keep around for all kinds of other uses. Keeps china plates on the shelf, reduces large wood doors from slamming and great gag for the lazy kids who shuffle their feet...TRIP...

If you find yourself removing those mids a lot you can use a forstner bit and make a clean 1/4 hole through the baffle and wood glue a hardwood dowel in the space. Let dry, sand / paint and drill a correct size pilot hole for the screw. I have done this for some of the older studio monitors.

:cheers:

sub

badman
09-13-2013, 04:07 PM
If I recall correctly it doesn't matter on mid range drivers unless it communicates with the whole cabinet. It really only matter with woofers.


But I'm not an expert though.

That's not accurate. The space available to any driver (that doesn't have it's own sub-enclosure on it's frame) determines the stiffness of the enclosure. This changes the frequency response at the bottom end of the driver's response. In this case it may not matter, but certainly many of the larger JBL systems with dedicated mids need the highpass filter action from the mid enclosure, which would be compromised by any substantial airleaks (small airleaks aren't a biggie in speakers, unless they whistle). Substantial can vary but it's definitely something to watch out for.

To solve an oversized cutout, one can glue strips of veneer to the inside edge of the hole. I hate the truncated frames of the JBL LE series drivers, pain in the neck and a half.

Wagner
09-13-2013, 05:45 PM
2 schools of thought - if the air mass compression behind the mid isn't correct, it *can* change the sound

BUT the other ( realistic one ) is that, as you noticed, particle board is shit wood, machining it that close is a joke, mass-producing speakers of that quality is an oxymoron and that hindsight is actually 20-20

:)

Since the crossover networks are so primitive ( look ma - just 2 caps ) and the cabinet sealing so random, it makes little diff BUT for the rattle effect.

What I like to use is a "string" like compound used in auto body repair made by 3M. It's gray, looks like a string of red licorice and is sold in auto body supply houses. It doesn't harden, seals and prevents rattles and is easily worked/removed. works much better that that joke fish paper as on the similar size studio/control monitors.

Handy to keep around for all kinds of other uses. Keeps china plates on the shelf, reduces large wood doors from slamming and great gag for the lazy kids who shuffle their feet...TRIP...

If you find yourself removing those mids a lot you can use a forstner bit and make a clean 1/4 hole through the baffle and wood glue a hardwood dowel in the space. Let dry, sand / paint and drill a correct size pilot hole for the screw. I have done this for some of the older studio monitors.

:cheers:

sub

Yes, I like the 3M stuff too. I have the black as well as some of the similar stuff that Parts Express carries.
Unfortunately I can't dowel the damn hole as it's only an 1/8 of an inch or so from the cardboard cup for the mid! (where the mid's cup and hole for the mid meet)
One was so close to the edge it has already torn out (big chip missing on the inner driver hole edge) just from being rammed home at the factory (the original install).
Like I said, could be straightened out properly but a whole lot of effort and labor.

Thomas