View Full Version : 375 questions

06-18-2003, 07:45 PM
Greetings to all!,
Over the years I have acquired a few 375 drivers from the late 50's to the late 70's,and noticed some differences,

picture1-I have this 375 that has marking on it and some of the later models do not,

what do these markings mean?

2-can the year of manufacture be found with the number on the diaph?

and my last question is

-on the earlier models, on the inside of the cap(bell)it looks like felt was used to line the cap,while on later models foam was used,why the change,was foam better than felt?,are there any differences in the overall sound using the felt vs foam?


06-18-2003, 07:47 PM
picture 2

Earl K
06-19-2003, 03:36 AM
Hi jblwolf

Well, I don't know if foam is better than felt. I do know it's easier these days to source foam . Can't help on matching age to diaphragm numbers.

The 4 numbers , matching the 4 directiomal axis (NSEW) represent ( in .01" increments - I believe ) how far off from "book" the gap width actually measures . I've never seen these numbers climb into the "4"s , so I imagine that's when QC at the factory kicks in and "rejects" the driver for retail release .

I've never seen a "tolerance number" on the phase plug like your picture shows - but it makes sense that it's also checked .

regards <. Earl K

06-19-2003, 09:01 AM
Thanks Earl,
I'm hoping someone out there in the forum has insight in to these questions,I've been hooked on JBL for years and didnt have any where to go to find some of answers for questions that have bugged me for years.as for the foam in the cap,the real down side is when it goes bad,what a mess it can make,I have noticed the sound seems more mello/not has bright when the cap has the foam in it,I'm wondering if anyone has noticed the same thing,I have a few compression drivers that have marking(mostly the late 50's-early 60's stuff)on the phase plug & mounting plate,all of the drivers that I have from the late 70's-80's dont have any markings-did JBL change the standard or come up with a better way of putting them together?WOLF

Earl K
06-19-2003, 10:24 AM
Heh Wolf
Maybe all your later magnets have perfect gaps. I have one or two like that. All my 2440/1 drivers are late 70's vintage. One just never knows about some of these things. Manufacturing tolerances might have become tighter leading to the unmarked series of drivers you have. Or, another explanation could be that they were hand selected for sale to a large touring company-customer like Clair Bros. / ShowCo / A1 Audio / Maryland Sound / Audio Analysts / ,,, any of the larger companies who might have had the leverage to get " hand-picked lumber" . Do you know the "pedigree" of your drivers ?
Re; foam/felt,,, there's so much open-cell foam around these days , that I just find something handy and cut an appropriately ( oversized ) circle from it. I use a friction fit since I believe the glue originally used caused the chemical reaction that led to the original "particulant" problem . Appropriate thick "felt" is more difficult to find. I've kept a casual lookout for it around here ( Toronto ). I thought being in the land of snow & ice, that it could easily be found as felt snow boot-liners. I must be a couple of decades out of step or in the wrong region or I last searched in July. :D
Maybe Winnipeg is hoarding the available supply waiting for their next ice-age . ;)

regards <> Earl K

06-19-2003, 01:48 PM
I would have to say 95 percent of the drivers I have came from local estate sales or the original owners that got them at a local dealer and built their own systems,theses are all home models,not pro models,I think as time went on,JBL changed the standard and found "close enough" was good enough,I have seen a few"b"stock items that should have never left the factory,Wolf

06-19-2003, 07:41 PM
He was that a Toronto verses Winnipeg snipe???

Earl K
06-20-2003, 04:04 AM
Hi John
Yes , I guess it was. But at least I know where Portage & Main is , about the "Golden Boy" statue and Louis Reil .
My memory of "Felt Boot Liners" comes from buying them at the old "Army & Navy" stores that have disappeared from the Cdn west .

regards <. Earl K

06-20-2003, 10:13 AM
What is the specification for the gap width (on the 2440s & 2441s)?

06-20-2003, 10:19 AM
There is a United army surplus store just up the street from the golden boy. They were never connected with the stores out west but they have the same concept and they have the liners you speak of

Earl K
06-20-2003, 11:14 AM
Hi JohnH

According to a JBL note from 9/77 ( rev. A ) they were

.031" for the le85/2420, le175/2410/2460/2461/2470

.036" for the 375/2440

.043 for the 2480, 2482, 375AB, 375(F?)H

I just looked at a "Service Procedure" note from 8/82 . It mentions that some older compression drivers " may also be found to have narrow gap top plates and current (1982) diaphragms will not fit these units properly ". It doesn't say what type of compression drivers or what the older gaps measurements were. Just some more unsubstantiated info to support the notion why the older stuff does/did sound different/better ( tighter tolerances ?) .

regards <> Earl K

06-20-2003, 05:03 PM
Hey Earl,
Can we find out what the gap width and flux density is for a early 60's 375 is,verus say late 70's-early '80 375-2240 is?this may give some insight to why changes were made,I remember buying a diaphram(I think it was for a Blue 375-1962??) around 1984 that had the diamond surround and it would not fit in the gap,at the time I was told I needed to order the "old style"part,the replacement did not have the diamond surround and it dropped right in- Wolf -by the way still no responce to the other questions,has the forum been stumped?:confused:

Earl K
06-20-2003, 06:41 PM
Hi Wolf
A lot of the people that post here listen to 1.75" diaphragmed compression drivers vs the larger 4" type. I, among others would love to know the actual gap sizes for these older 375 drivers. This info was apparently lost at JBL in the big earthquake of 199? .
Now, the fact is , you seem to have a good cross-section of these vintage drivers ; ( meaning - it may be up to you to do the measurements & let us know ,,, not exactly what you wanted to hear , I imagine ) . Automotive supply houses will sell spark plug gauges that are non-magnetic in the appropriate sizes.

There is a spec sheet in the library that states some tech info including the magnets' flux level . I don't know the year of publishing, but it's still the most concise description of a 375 (http://www.lansingheritage.org/images/jbl/specs/home-comp/375/page1.jpg) that I'm aware of .

All my 2440/1 variants have stock foam/felt laminated pads. The foam is glued to felt that is in turn glued to the back-cap.

regards <> Earl K

06-20-2003, 09:08 PM
Hi Earl,
Your right,I really didnt want to hear that,my blood pressure hits the roof every time I take one apart,the pics I posted were from what I was told is a 1965-375,and it is back in service,at this time I'm using all of the older 375's(60's stuff) I have and it may be awhile before a get a chance to do some measurements to find the gaps,I did a little looking around the site and found some info,per bulletin b-1 1954 flux density for the 375 was 21000 gauss,that was also the case for 1956,I couldnt find any info for the 60's but by1973 the flex density had changed to 20,500,but still no gap info for early models,I have always been a fan of the 4" driver,and a 537-500 or 509 horn,I really never care for the smaller horns like the HL93,seemed to compress the sound,like it coundnt breath while the 500 or 509 really opened up the mids,like you right there,I'm still playing around with my 6 channel surround,I did finally settle on 2 le15,2 375 and 4 077 for my front channels,I hope to finish the rest of the channels before to long and post the the pictures,Best Regards -JBLWOLF:)

scott fitlin
06-20-2003, 09:15 PM
I have two 2440 drivers, and the rest of my stuff is 2441. Now one day I went to take out the original 2440 diaphragm and I put in its place the 2441 phragm. In the 2441 driver its a snug fit when you put the phragm in. On the 2440 assembly it slipped right in!

So I listened to this 1973 vintage 2440/w 2441 phragm and I swore it had a sweeter sound to it.

Does the 50,s and 60,s stuff have yet still a different sound?

06-20-2003, 10:06 PM
HI Scott,I also asked the same question but from the home product side,I think it does sound different, I know using larger horns is like night and day,I'm hoping some one can post some of the gap info for the 50's & 60's drivers,some changes were made that did change the sound/part swaping of the 60's to 70's drivers,I did have a voice coil come loose on a 375-glue failed(didnt sound right at low levels) but I caught it before it was damaged and putting glue on a tooth pick did the trick and that was over 10 years ago and still sound fine,at that time I also tryed to put a 2440 dia in this 375 but it wouldnt fit the gap,but it did line up with the alinement pegs -JBLWOLF

06-20-2003, 10:40 PM
Howdy All,At customers requests we have installed all versions of the larger diaphragms into all of the different motors(including some generic replacement diaphragm models)The only items that never quite fit were due to the locating pins being widened at the tops of the pin.This caused the first point of contact with the diaphragms to be snug.I have never experienced a gap that was smaller than the .036 (except in broken units).I believe that if JBLwolf has a driver that he believes to have a unique gap size to please measure the gap and also the locating pins.I also would be curious to see if there is an oddball gap size.Ok Mr. JBLwolf its up to you!!!! Let us know.Best regards,Oldmics

scott fitlin
06-20-2003, 11:01 PM
It was always told to me that the 2441/2440 driver wewre one and the same! The difference was the diaphragm. When I used one of my 2440 drivers with a 2441 diaphragm in it, the sound was very different from my 2441,s! It was sweeter. Softer sounding! If I had heard more high end off the 2441 vs 2440 I would have attributed that to some degaussing of the magnet!

But the 40 driver just gave the 41 phragm such a nice sound! It wasnt as forward sounding as the 2441. And the two 2395 lenses I got with the 2440 drivers came out of West Point. They had been in the Auditorium system there. They were dated 1973 west point Academy! The dates on the diaphragms matched the paper taped to the conical horn! I opened em up and looked.

Conversely, with the 2440 driver and the 2440 diaphragm I didnt like that sound! I found it a bit too bright for my taste. But I love 41 phragms in these 40 drivers. go figure. thats why I ask what the 50,s and 60,s stuff sounded like.

06-20-2003, 11:22 PM
The only real way to tell whats up with any of the old drivers is with quantifiable analysis tools. Using some sort of frequency producing and capturing device(such as an Iasys) a distortion meter (to assess diaphragm condition) a gaussmeter (to assess the motors magnetic condition)and of course a pin tool or gap gauge set.Fun stuff!!!! Best regards,Oldmics

Mr. Widget
06-21-2003, 01:00 AM
Hi Scott,

I replaced the diaphragms in a pair of 70's vintage 2440 drivers with 2441s and as Oldmics said, I had to ream out the guide pin holes as they were too snug to fully seat. Once fully seated the drivers were swept with an oscillator to insure no rubbing and then they were a pair 2441s.


06-21-2003, 07:30 AM
:eek:OH MAN, I just finished putting all this stuff together in the last few months,last week I finishing the last set of 375's,but now you got me wondering about this odd ball thing,at this point I cant bring myself to pull them all apart again,(not good for my blood pressure)but the next time I do,I'll gap them and see what I have-WOLF

Earl K
06-21-2003, 07:43 AM
Hi Wolf
I agree, it's WAAAY more fun to enjoy the music than to analyse it .
- That's my excuse for the slow pace of getting beyond building test enclosures for my MTM project

( Good looking system in your avatar )

regards <> Earl K :)

06-21-2003, 09:23 AM
Further regarding the 2440/2441. I read that the throat screens in these degrades the sound, that they are only there for outdoor use. If so, can the screens be removed without removing the throat horn or other major surgery? Can it just be pulled out?

06-21-2003, 10:59 AM
HI Earl,as for the avatar pic I have since added a second 077(on top),I played around with all kinds of combos,2 075's,1 075-1 077,up,down and around around before I got to using 2 077,they sounded the best to me,the room they are in is 28'by34' with 8' ceiling,the rear channels use the le15 537-500 horns with 2 075's,that Im still playing around with.and I fully agree with you about it's way more fun listening than testing but sometimes to get there you have to analyse to get to the fun,
To John H,I wouldnt mess with the screen,its there for a reason and on a final note,I have some screens that are flat and some that are domed,I was going to ask another question about the dome vs flat screen but,I'm going to leave that alone and set back and enjoy some listening for awhile,Best Regards to ALL-WOLF:p

Earl K
06-21-2003, 12:10 PM
Hi jblwolf
537-500s , now there's a unique horn/lense . In the last 3 years I've only seen 3 pair come up for sale on eBay ( from this continent ) .

JohnH ; bug screen removal ? I wouldn't do it. There are a myriad of things or changes that will accomplish more sonic enhancement than screen removal. ie ; playing around with different horns/ ( BIG CHANGES there ), looking for nuanced changes as found by swapping out different types of capacitors-inductors from the playback chain / different electronics / room treatments / etc. . Bug screen removal on a 375 would be non-reversable unless done during a complete dismantling/demagging of the driver at the factory . I'm not saying there isn't something to it. I just know its way,way, low on my own list of "must try this" . Also, I've never seen that "mod" enhance the resale value when such a device is resold on eBay - in fact , just the opposite. The lowest dollar value, TAD 2001 sale I've tracked, was on just such a beast. Just my opinion .

regards <> Earl K