JBL 4" core drivers (2445 etc.): modifying space between diaphragm and phase plug

[ivica]

OK.
For 2446, 2450 etc., which are later (younger) models, it's consistent. Probably there was also an evolution in respect of 2441 from 3 to 4 slits.

I've come accross this paper from 18sound http://www.eighteensound.com/staticC..._procedure.pdf (no publicity intended) which gives an idea of the tolerances in the manufacturing process of their drivers; the end user is meant to use shims, where necessary, in order to reach the optimum distance of 0,8mm +/- 0,05mm (!). Drivers and diaphragms bear indivual marks so as to achieve this optimum value... Impressive.
I've seen on original JBL diaphs also some handwritten marks...

Hi Elac310,

Many thanks for the info from the 18-SOUND. It is really very nice explanation from 18-sound corp.
It would be nice if we an get something like that from JBL too, or , may be, from some of the AH-forum members, who are "near" the JBL technical stuff.

It seams that such explanation "uncover" the meaning of the hands written numbers marks on JBL compression drivers top-plates

regards
Ivica

[Mr. Widget]

Advertising here makes no sense AND isn't allowed.

Advertising is allowed in the appropriate Market Place area... it's just best left out of the other forum areas as it tends to derail the topic.

Back on the topic of this thread... I'm not sure how JBL does it, but I was at the Meyer Sound factory and I watched as a technician measured every diaphragm installation by hand and followed up with acoustic measurements. To maintain precise performance, they often place custom shims beneath the diaphragm individually for each driver... they do not consider their drivers field serviceable for this reason. In the case of Meyer, I don't think maximum extension is as important as unit to unit consistency and overall reliability.

I know that JBL also performs acoustic testing on their drivers before they are approved, but I don't think their tolerances are quite as tight as a boutique manufacturer like Meyer.


Widget

[1audiohack]

The metal phase plug also rings.

They do?

[ivica]

..... I'm not sure how JBL does it, but I was at the Meyer Sound factory and I watched as a technician measured every diaphragm installation by hand and followed up with acoustic measurements. To maintain precise performance, they often place custom shims beneath the diaphragm individually for each driver... they do not consider their drivers field serviceable for this reason. In the case of Meyer, I don't think maximum extension is as important as unit to unit consistency and overall reliability.

I know that JBL also performs acoustic testing on their drivers before they are approved, but I don't think their tolerances are quite as tight as a boutique manufacturer like Meyer.


Widget

Hi Mr. Widget,

Do You have any knowledge about JBL "numbering marks" that are visible on the top plates of their (for sure) 4"-diaphragm drivers ( 2441, 2445, 2446, 2450, 2451) ?

Regards
Ivica

[NickH]

Doesn't jbl teach how to adjust diaphragm offset in there tech school? A friend of mine has a close friend who went to jbl for this training. He said that jbl teaches how to set the diaphragms offset from the phase plug by shims on the mounting flange of the diaphragm. Now I didn't hear this first hand. But it make since that they would.

Can any of you trained jbl gurus confirm this?

Some metals do ring, some don't. I though jbl uses zinc for there phase plugs. You would be pretty hard pressed to get any resonance from zinc. Maybe with an induction heater though, lol.

[ivica]

Doesn't jbl teach how to adjust diaphragm offset in there tech school? A friend of mine has a close friend who went to jbl for this training. He said that jbl teaches how to set the diaphragms offset from the phase plug by shims on the mounting flange of the diaphragm. Now I didn't hear this first hand. But it make since that they would.

Can any of you trained jbl gurus confirm this?

Some metals do ring, some don't. I though jbl uses zinc for there phase plugs. You would be pretty hard pressed to get any resonance from zinc. Maybe with an induction heater though, lol.

Yes, that would be nice, but here on the forum, about such "things" SILENCE is USUAL answer.

Only very useful info that I have seen has been written by subwoof

http://www.audioheritage.org/vbullet...l=1#post354792

".........
Originally Posted by subwoof

Well there are a few tricks for each mounting method - in general for the older pin diaphrams drill out the locating holes from 1/8 to 9/64 and that should give you plenty of room to "wiggle" This is a common procedure around here..

For the shoulder types, the dia is held in position at each mounting screw position against a small portion of a shoulder that is milled into the top plate.

Take a NEW, decent size tool steel *right angle* edge ( as would be found on a wood chisel SIDE or machinists block ) and shave down the diaphragm where it meets this shoulder. DO NOT USE A FILE, SANDPAPER, KNIFE EDGE, LAWN MOWER or something LAYING AROUND...Buy it NEW if you aren't a machinist!

Because slight offsets and material size differences when making the diaphragm assembly happen ( it is a sandwich if you look close ), there are burrs / waste material / etc etc.

By shaving down these areas ( ALL EIGHT ) the diaphragm can move around. Bingo...be sure to remove the burrs you CREATE !

NOW THAT YOU KNOW THIS - it is IMPORTANT to have a low distortion oscillator to align the diaphragm - AND THE DRIVER must have a nominal air mass load at the exit otherwise you will get false response / harmonics that will FOOL you..

NOTE: methods for aligning the NEWER Nd drivers with the dia glued into the back cap are not yet hacked. Someday I will find a need but HELLO, they never blow up right??

Anyways I made a wood box with various holes with wood dowels for guides and lined it with insulation.. stuck a cheapo microphone in the side and connected to the oscilloscope and compared in/out. Wearing earplugs and headphones you can actually hit a driver with some decent power to see if there's any breakup but it REALLY pissed off the cats.

Worked great until I gave it away to a tech and never bothered to make another.

.................."

Regards
ivica

[NickH]

Maybe jbl has them sign a non disclosure agreement.

[ivica]

Maybe jbl has them sign a non disclosure agreement.

May be You are right, but there are a lot of possibilities that such 'secret' to be uncovered.

regards
ivica

[1audiohack]

I really doubt its anything like that. I think we're attempting to assemble a very old puzzle. How long has it been since JBL consumer has used a four inch driver? Most of the folks that worked there are long gone and pro and consumer seem to separate further and further as the years go by.

Another consideration, machine accuracy and work holding have come a long long way and it is likely the kind of tweaking that used to be required is no longer needed.

I have a pair of tweaked (supposedly belonged to one of the senior engineers at JBL) 2450's with SL diaphragms in them that extend unusually high as verified by PWT measurement some time ago. They sound really good in the house. I have the PWT set up again and am going to explore them to see what's up as they also sound like breaking glass when played super loud (out doors on big horns) and if I was to hazard a guess it would be that the diaphragm is in very close proximity to the phase plug assembly. It will be interesting to find out.

I do know from experience that pushing and pulling and fussing with these usually brings about better performance and I have even had a couple of driver/diaphragm combinations I couldn't get happy at all and had to scatter them around and or not use them.

Barry.

[Elac310]

I have a pair of tweaked (supposedly belonged to one of the senior engineers at JBL) 2450's with SL diaphragms in them that extend unusually high as verified by PWT measurement some time ago. They sound really good in the house. I have the PWT set up again and am going to explore them to see what's up as they also sound like breaking glass when played super loud (out doors on big horns) and if I was to hazard a guess it would be that the diaphragm is in very close proximity to the phase plug assembly. It will be interesting to find out.

Barry.

Very interesting !
Yes, please, do let us know of what tweaking you manage to identify on those drivers.