"Magic number" inside JBL compression driver/ diaphragm

[gibber]

I visited a friend and measured B on a brace of JBL drivers, remagged some. I saw that a few were probably re-gaussed previously because they had B values marked inside, but that writing was on some of the 10-odd opened drivers only. The "magic" numbers, however, could be found on every driver and every fram, no exception. Hence i believe they were written at manufacturing time. I saw them on old and new drivers, ferrite, neo and alnico. Does anyone know about them or can provide a link to info on the meaning of JBL's "magic numbers"?

Attached a few shots. The "+1" fram came from the "-3" driver which looked like never opened (but i bought it 2nd hand...). Numbers i saw range from -3 to +3

Any clues?

[Don C]

This is only a guess. Maybe they were matching diaphragms with an extra turn of coil with the magnet housings that are a bit weaker.

[Steve Schell]

Yes, these numbers appear on all sorts of JBL compression drivers, and look sloppy on an otherwise precise product. I believe they reflect factory measurements of the phasing plug height in comparison with the surface of the top plate. Despite tight tolerances of the parts, small variations stack up to considerable errors. These numbers guide the diaphragm installer in shimming to achieve the correct spacing of diaphragm to phasing plug. Too close and the diaphragm could hit the phasing plug in normal use, too far and some top end response would be lost to acoustical capacitance of this "front chamber."

[1audiohack]

Hi Steve!

ive never seen shims. I would guess that the diaphragms are similarly sorted?

[gibber]

Yes, these numbers appear on all sorts of JBL compression drivers, and look sloppy on an otherwise precise product. I believe they reflect factory measurements of the phasing plug height in comparison with the surface of the top plate. Despite tight tolerances of the parts, small variations stack up to considerable errors. These numbers guide the diaphragm installer in shimming to achieve the correct spacing of diaphragm to phasing plug. Too close and the diaphragm could hit the phasing plug in normal use, too far and some top end response would be lost to acoustical capacitance of this "front chamber."

Thanks for this input, Steve -- You're right about the looks & it seems a very plausible explanation. Some drivers have more than one number written on them, up to four is what i've seen.

Hi Steve!

ive never seen shims. I would guess that the diaphragms are similarly sorted?

Yes, they are -- two of my pictures show frams, not cores. I haven't been able to upload all pics i wanted, but a 1.5" SL driver that looked never redone and had a mark of "+3" (and this corrects an error in my previous post) on the pole plate came with the pictured SL fram that carries "+1". I can't guarantee it was never opened, but it looked untouched ... however, i wonder why JBL don't ask for the value on the magnet when supplying replacement parts (??)

[Maron Horonzakz]

Is the magnet a replaceable part ???

[Steve Schell]

1audiohack, I'm thinking of the little square bits with the holes, sandwiched between mounting ring and top plate. You can see their footprint in gibber's last photo. On a very early 175 diaphragm I have they look crude and homemade. Gibber, they may indeed have measured and sorted diaphragms at the factory, but I cannot confirm this.

[1audiohack]

Man!!! That's just one more thing I don't want to have to think about! :^)

[JuniorJBL]

Man!!! That's just one more thing I don't want to have to think about! :^)

you are close enough that I could just aim the "remove-just one more thing I don't want to have to think about-inator" at you and all will be well!!

[1audiohack]

DO IT! Pull the trigger, I need the rest!

[Steve Schell]

That image makes me want a cigarette.