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View Full Version : Question regarding DCR on JBL drivers and basses.



magger
07-24-2006, 05:18 PM
Hello forum,

I am "restoring" a pair of 4333A I've had for quite a few years now. They have always been sounding great ever since I got them, but during some free time I finally pulled myself together to take the basses, drivers and tweeters out for further investegation. I started measuring the DC resistance on all the units. Here are the results:

2231A: one measuring 6,0 / 6,1. The other 6,3 / 6,4
2420: one measuring 7,2 / 7,3. The other 5,7 / 5,8
2405: one measuring 6,2 / 6,3. The other 6,4 / 6,5

The 2231A are 100% original. On both the cones it writes "2231" on the underside. They were refoamed at a qualified JBL shop about 1 years ago and they both sound great.

The 2405 are also with the original blue diaphragms and both sound great. Though, I noticed when I had them apart that on one of the diaphragms there were 2 tiny tiny "lowerings/holes", like if you had touched the diaphragm with a small needle or something like that. It's not holes all the way through the diaphragm at all, just 2 tiny lowerings. Perhaps 0,02 mm lowerings or something like that. But as I mentioned, they both sound great. It shouldn't be a problem I take it?

Well, so far so good for the 2231A and 2405. My real concern here are the 2420. First of all they measure very differently. The 7,3 vs. the 5,8 I think is too big a difference. One of the 2420. The one measuring 5,8 has that red paint on the screws on the backpanel. I can see the previous owner has tried to unscrew the back panel, but with no luck. All 4 screws are totally ruined on the top, so it is impossible to just loosen one of the screws just 1 mm. Therefore I take it that this driver has never been taken apart and has the original alu D16R2420 diaphragm. Apart from that it sounds fine, and has always sounded fine to my ears. I have never noticed any kind of distortion etc. not even at high levels. But my concern here is that I read somewhere on this forum that a healthy D16R2420 diaphragm should measure betweem 5.9 and 7.0. Could this be a problem with a DCR of 5,8 on this driver?

The other 2420 driver measures 7,3, which is more than the 7,0 a healthy 2420 should max. measure. I took the driver apart and saw that the diaphragm does not look like an original D16R2420 diaphragm. As far as I am concerned the original D16R2420 alu diaphragm have a "silver" or "shiny metal"-like look. Am I right here? The diaphragm has the excatly same shape as the D16R2420, but it is grey, infact what I would call dark grey. There was no text on the diaphragm. At least that I could see. Hmm, this diaphragm have the same shape, but not the same looks as I know an orignal D16R2420 should look like. Apart from that it measures 7,3 DCR. Could this be some kind af cheap/bad aftermarket diaphragm? Or which diaphragm could it be, with the same shape of the D16R2420, but with that dark grey color?

I know that the sound of the drivers naturally is the most important thing. I must admit that I have not ever noticed any great difference on the sound of the speakers. This really puzzles me. How can a pair of 2420 that measure that differently, and most likely have different diaphragms sound so relatively much alike? Or at least what I can hear. With these dfferences I would expect them to sound much more different. The only real difference I can hear is that the driver measuring 7,3 plays slightly lower than the one measuring 5,8 at the same volume. The difference is hearable if you practically put your ears directly into the horn. I have just turned the L-pad about a mm up to compensate for the lower volume.

What are your thoughts regarding the 2420 situation? Is it me having very bad ears, as I am not able to hear a radical difference? Also, what do you think about the DCR on my 2231A and 2405. Should they be OK? As I mentioned i have never had any problems with them.

Just before posting this, I have another question regarding DCR. I also own 3 2441's where I will be using 2 of them for a future speaker project. The DCR situation with these are even more strange. They all have the original 2441 diaphragms. One measure: 7,6/7,7. Another measure: 6,1/6,2, and the last measure 8,0/8,1. I have tested them all and to me they all sound fine on a 2350 horn. But 3 so different measures should result in, at least noticeable, different sound, should it not?

Finally I can tell I have been searching this forum over and over again for threads regarding DCR, replacement diaphragms etc., but I feel i must fully describe my situation in order to get some inputs, and hopefully some answers regarding my situation. :)

Earl K
07-24-2006, 05:50 PM
Hi Magger,


- I suspect that you have a D16R2425 diaphragm in that 2420 which you opened up . This type of diaphragm is titanium, which definately looks darker than the bright & shiny aluminum type. This newer diaphragm also had a slightly higher DCR figure ( 5.7 to 6.9 ohms ) . The 2420 was 4.8 to 5.8 ohms .
- Can you take a picture of this diaphragm and post a picture here ?

- JBL ( in 1982 ) listed the DCR of a healthy 2441 as being 6.4 to 7.8 ohms .
- I don't have any modern reconers info to see if those numbers were modified . JBL does tend to slightly change things over time .
- If in your shoes, I would use the two 2441s' that measure the closest to each other ( in DCR & listening or RTA tests ). Then I wouldn't worry about it anymore . :)

speakerdave
07-24-2006, 05:57 PM
How you measure matters. I have found there are much larger variations when making measurements by using the probe to stab a plated surface of the terminal. I get much more consistent results by putting the probe into the spring loaded terminal and wiggling it a little bit with the pressure on.
Then take the reading. If you have not done this I suggest you go back and redo all of your measurements.

Except for the difference between the 2420's, I do not think the DCR differences are great enough to worry about, and Earl has addressed the 2420 question.

David

Robh3606
07-24-2006, 06:33 PM
What shape are the surrounds on these diaphragms?? Are they diamond or tangental?? If they are tangental they are original if they are diamond they are both replacements.

Rob:)

grumpy
07-24-2006, 07:10 PM
I'd be a bit more conservative in my guess regarding what's original, in that if
the suspension is tangential (see attached pics) vs. the diamond pattern
adopted later, that your diaphragm is at least similar in design to the original.

Original 2420 diaphragms are not inexpensive, so if you want to keep these up,
you might want to start saving (or find out if you don't mind the change to a
different sounding and perhaps less dear diaphragm... a much less costly
experiment).

Meanwhile, it sounds like you've already worked around the only small audible
issue (level matching). A fine pair of speakers indeed. :)

Cheers,

-grumpy

(no, the pics aren't JBL ... just visual examples)

magger
07-25-2006, 09:29 AM
First of all thank you all for your replies.

Earl K: Unfortunately I have no digi cam at the moment, so I cannot take any pictures of the dark grey diaphragm. As I mentioned it looks excactly like the original D16R2420, but is clearly dark grey and none-shiny, but has the tangental surround. Anyway, it's not that important, as I cannot live with to different diaphragms for the 2420's in my beloved 4333A's, even though the sound is not radically different. It is only a matter of time before I must find a solution. Either changing the dark grey to an original D16R2420, or some other solution.

David: I agree with you on the way to measure the DRC. I have experienced very different measurements on the same diaphragms, whether I measure directly on the diaphragms or using the terminals. In all the cases where I am able to measure on the diaphragms I always do that.

Rob and Grumpy: My dark grey diaphragm looks excactly like the diaphragm in the brown box, just not shiny.

Well, apart from that I am relieved to hear that I should not worry too much about the DCR difference between my 2231's and 2405's, nor my 2441's. As I already mentioned they all sound fine and so on, but anyway it is a relief to hear that the DCR difference should not be worried too much about. Nevertheless I take that the optimal would both of the speaker's basses, drivers etc. measure EXCACTLY the same in DCR.

Regarding my 2420 problem, I was wondering if any of you could help my find out which kind of diaprhagm I have in a spare 2420 I bought a few years ago. This diaphragm clearly has the diamond surround, but apart from that I cannot be 100% sure about which kind it is. My thought here is perhaps to buy one more of these diamond surrounded diaphragms, and change both the diaphragms of my 2420 in my 4333A's so I get 2 that are excactly alike. I read somewhere that the diamond surrounded diaphragms should be better sounding than the tangental ones. The diaphragm I talk about here measures 6,3 / 6,4 DCR and sounds fine on a 2307 horn.

Here are the pictures. Any idea what this diaphragm might be?

Mr. Widget
07-25-2006, 09:46 AM
Either changing the dark grey to an original D16R2420, or some other solution.You won't be able to buy new 2420 diaphragms as they are no longer made... the JBL replacement is the 2421. You can certainly find used 2420s on eBay and elsewhere but that is a real crap shoot, especially with diaphragms.

The 2421s have a slightly smoother and more extended response... so replacing your diaphragms with a pair of 2421s is considered a positive upgrade, but they are quite expensive so many people opt for the 2425 diaphragms... these are significantly less expensive, but to some sound a bit harsh or metallic and brittle.



Rob and Grumpy: My dark grey diaphragm looks excactly like the diaphragm in the brown box, just not shiny.From your description it sounds like one of the after market Be diaphragms from Asia... but I really doubt that. Could it simply be oxidation?


Regarding my 2420 problem, I was wondering if any of you could help my find out which kind of diaprhagm I have in a spare 2420 I bought a few years ago. It looks like it may be a 2421 diaphragm... I have installed them a couple of times but I must confess I didn't really study them... so I can't be certain.


Widget

edgewound
07-25-2006, 10:21 AM
Here are the pictures. Any idea what this diaphragm might be?

Yes...that would be D16R2421. Red dot means 16 ohm....green dot would mean 8 ohm.

magger
07-25-2006, 01:13 PM
Great! Thank you for the help.

At least that is good news. Now I "just" need to get another D16R2421, then I have 2 2420 with D16R2421 diaphragms to install in my 4333A's. I would, by what I have read, consider this to be a positive upgrade on the sound. At least it must be better than the 2 2420 I havde now with 2 different diaphragms.

When I hopefully get my hands on another single D16R2421, I will try biamping the 4333A's with my UREI 5235 crossover with 800 HZ cards in it. Will indeed be exciting to try. If doing that just slightly improves the sound as it is now using the internal crossover function, I will be happy :)

Zilch
07-25-2006, 01:30 PM
A local machine shop will have the tool necessary to get the screws out. They deal with frozen fasteners routinely. Don't let 'em hit TOO hard, tho; they're alnico magnet drivers, after all.

Replace with like type Fillister-head screws....

herki the cat
12-29-2009, 04:29 PM
A local machine shop will have the tool necessary to get the screws out. They deal with frozen fasteners routinely. Don't let 'em hit TOO hard, tho; they're alnico magnet drivers, after all.

Replace with like type Fillister-head screws....
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Consider this approach, which is increasingly valid today: Responding to a 2006 post at end of Dec 2009 regarding corroded screws on the back cap of two 2420 drivers is very charming since many of these gems are still floating around. The first consideration is: "Do not use any impact tools on the four screw heads. "Worse yet, do not attempt to drill out the scerew heads or you will have a nasty pile of iron chips clinging to the voice coil gap area.

I stared during three months at two 2470's, which have motors identical to 2420's, & i decided to drill a tiny 1/16 dia hole, 1/8 inch deep into the screw head to acommodate a 1950 style, three dollar, 1/16 inch screw-extractor nib taking care to continually remove the drilled-metal chips out frequently with a match stick size magnet.

This turned out extremely well.
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cheers, robbi the cat