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sbjacob
12-28-2009, 11:26 PM
I have a few 2420 driver and one of the diaphrams looks different than the other 2??43413

speakerdave
12-29-2009, 12:11 AM
That looks like the phenolic diaphragm the 2470 and 2461 drivers were furnished with. Don't throw it away; they are not made any more and some people like them.

Check the 2470 data sheet in the library for some information on performance:

http://www.lansingheritage.org/html/jbl/specs/pro-comp/2470.htm

sbjacob
12-29-2009, 12:33 AM
I have three of these drivers and all have different DCR. The one in the photo is 11.3 and the other two are 6.5 and 3.8.

sbjacob
12-29-2009, 08:22 AM
Try this.....
JBL Drivers came with 2470-J & 2470-H Model numbers.
2470-J is 16 ohm Impedance
2470-H is 8 ohm impedance

Typical DC resistance is approx 80% of rated imedance = approx....12 ohm which is very close to your 11,3 unit, the 8 ohm unit would have approx 6.5 ohms DC resistance.

You also need to know if these heavy drivers have been adequately packaged for shipping to preclude high probabability of "de-magnetizing damage if they have been dropped on a concrete floor or miss-used. If the flux density is less than 18,000 or 19,000 quass, the high frequency reponse and overall sensitivity will be seriously compromised.

If you have the horns, you may want to see how well they work for you. Reselling them can be a very tricky thing. Your potential buyer is entitled to an accurate description of your merchandise.

I wish you the best. herky the cat
Thanks for the info the drivers have no major scratches so I assume they have not been dropped. Thanks

toddalin
12-29-2009, 10:38 AM
I have three of these drivers and all have different DCR. The one in the photo is 11.3 and the other two are 6.5 and 3.8.

What would cause one to be just 3.8 ohms? Seems kind of low.

sbjacob
12-29-2009, 11:57 AM
What would cause one to be just 3.8 ohms? Seems kind of low.
I am assuming someone put a 8 ohm diaphram in one of them as that would be within the tolerance for a 8 ohm as according to the spec sheet I found in the library the 16 ohm is 6.5 ohm + - 8%

toddalin
12-29-2009, 01:07 PM
I am assuming someone put a 8 ohm diaphram in one of them as that would be within the tolerance for a 8 ohm as according to the spec sheet I found in the library the 16 ohm is 6.5 ohm + - 8%

Don't think this would be the case.

Seems more to me that the 16 ohm impedience should go with the 11.3 ohm dcr, and 8 ohm impedience with the 6.5 ohm dcr. If the 16 ohm impedience is for the 6.5 dcr and 8 ohm impedience for 3.8 ohm dcr, how does the 11.3 ohm dcr fit in?

Beowulf57
12-30-2009, 07:45 AM
Don't think this would be the case.

Seems more to me that the 16 ohm impedience should go with the 11.3 ohm dcr, and 8 ohm impedience with the 6.5 ohm dcr. If the 16 ohm impedience is for the 6.5 dcr and 8 ohm impedience for 3.8 ohm dcr, how does the 11.3 ohm dcr fit in?

This is confusing to me as well...at soundspeakerrepair.com the "8 ohm" replacement for a 2420 (JBL-2425-8) notes that JBL specs this model as "JBL's DCR 3.2 +or- 8%"; the "16 ohm" version (JBL-2425-16) notes "JBL's DCR 6.4 +or- 8%".

Has anyone actually measured the AC impedance of these different diaphragms? I have "8 ohm" LE85's and "16 ohm" 2420's (all original diaphragms form the early to mid 70's) and they all measure 6.2-6.5 ohms DCR. So, the listed driver impedance changed arbitrarily, however the diaphragm was identical.

Beowulf57
12-30-2009, 08:17 AM
It seems that the 11.3 ohm rating is actually an average AC impedance for the 16 ohm version. See the 2420 curve in post #7 here: http://www.audioheritage.org/vbulletin/showthread.php?t=5213. (http://www.audioheritage.org/vbulletin/showthread.php?t=5213)

Also see this post on 2420 voice coil DC resistance:http://apollo.csd.net/vbulletin/showthread.php?t=2303

Thanks to 4313B:

D16R2420 5.9 - 7.0 ohms

D8R2421 2.9 - 3.5 ohms
D16R2421 5.9 - 7.0 ohms

D8R2425 2.9 - 3.5 ohms
D16R2425 5.9 - 7.0 ohms

toddalin
12-30-2009, 09:45 AM
It seems that the 11.3 ohm rating is actually an average AC impedance for the 16 ohm version. See the 2420 curve in post #7 here: http://www.audioheritage.org/vbulletin/showthread.php?t=5213. (http://www.audioheritage.org/vbulletin/showthread.php?t=5213)

Also see this post on 2420 voice coil DC resistance:http://apollo.csd.net/vbulletin/showthread.php?t=2303

Thanks to 4313B:

D16R2420 5.9 - 7.0 ohms

D8R2421 2.9 - 3.5 ohms
D16R2421 5.9 - 7.0 ohms

D8R2425 2.9 - 3.5 ohms
D16R2425 5.9 - 7.0 ohms

So how would one account for two drivers, both unopened with original diaphrams, both play, both are within 100 serial numbers of each other, one at ~3.5 ohm dcr and one at ~13 ohms?

Beowulf57
12-30-2009, 09:58 AM
So how would one account for two drivers, both unopened with original diaphrams, both within 100 serial numbers of each other, one at ~3.5 ohm dcr and one at ~13 ohms?

Internal contact corrosion? Those two drivers would hardly be expected to produce the same levels...would they? The 3.5 ohm measures like an "8 ohm," the 13 ohm might have a contact problem inside?