Upgrading from 2421B to 2425J

[badman]

The Radians are ok replacement diaphragms, but not an upgrade to a JBL aluminum diaphragm. If you want to upgrade get an Altec driver. JBL did not do anything with the one inch phase plugs for 50 years or so. Altec have worked with the phase plugs a lot.

They would be used as an upgrade to the JBL Ti phragms in 2426h.

[frank23]

I would consider changing from 2421+alu to 2425+titanium a downgrade. I have had a number of horns, drivers and diaphragms over the years and always returned to the 2420+alu combination for home sound quality.

I just read a great research paper on beryllium diaphragms as compared to aluminium and titanium and it clearly shows that aluminium has about the same waterfall as beryllium whereas titanium has a worse waterfall. Alas the researchers do not mention the exact brands / types they have tested.

Another aspect that is tested is surround fatigue. The titanium diaphragms tested show great fatigue after an endurance test of 100hours at 25W pink noise from 500-5000Hz. I must admit that this seems quite a severe test to me to say the least, at least when considered from a home audio perspective. The beryllium diaphragms do not show fatigue. But the difference has nothing to do with diaphragm material. The difference is because the titanium diaphragm surround is titanium whereas the beryllium diaphragm surround is a polymer of some sort. And as you might know aluminium shows far greater fatigue problems than titanium, that is why JBL changed from aluminium to titanium. So I expect my old aluminium diaphragms to by now also show fatigue problems in the surround. The fatigue caused a 5% distortion figure after the endurance test wheras before it was about 0.1%, same as the berylllium ones with the polymer surround. But the polymer surround stayed at 0.1% and did not get worse.

The fatigue test was only published on the titanium and beryllium diaphragms. I suspect they did not publish those figures for the aluminium diaphragms, because they may have used Radian aluminium diaphragms that use a mylar surround and the fatigue tests might just have turned out just as well as those of the beryllium diaphragms with their polymer surround, which is not good for the beryllium manufacturer as he wants to state beryllium is best… :-)

So I have just ordered a pair of Radian alu+mylar diaphragms, for an optimal combination of "alu waterfall" and "mylar fatigue free" just to see how they sound being fresh and hopefully stay fatigue free with use. The only problems I can think of with these Radians is that the surrounds are not the optimal termination for the aluminium. But JBL also uses polymer surrounds with their own beryllium diaphragms, so there must be something ok with them...

[martin2395]

Hi Frank,
I was only considering replacing the motors but kepping my D16R2421's as I'm not a fan of titanium dia's. Replacing the alu 2421B to ti 2425j's would be indeed a side/downgrade.

[ivica]

....... But JBL also uses polymer surrounds with their own beryllium diaphragms, so there must be something ok with them...

IS IT ????
As I have remembered, JBL 4"-Be has Be surrounds

Ivica

[martin2395]

Found this pic:
http://www.audioheritage.org/vbullet...1&d=1189868511

Well, it could be mylar.

[frank23]

IS IT ????
As I have remembered, JBL 4"-Be has Be surrounds

Ivica

No, it has a kapton surround, here I found it:
http://www.audioheritage.org/html/pr...logy/435be.htm

While the switch to beryllium addressed distortion and extension issues, it would not address the requirement for high output since a surround formed out of this same material has much less fatigue resistance than the titanium it replaced. Doug addressed this in a very innovative way, with a unique composite design. He specified a kapton surround that would be immune to fatigue failure. However, previous composite designs always proved problematic due to the need to have the diaphragm, voice coil former and surround all attach at the same point. Doug solved this issue by having the kapton surround formed into a deep well just before the point where it attaches to the diaphragm. The voice coil was dropped into this well, which eliminated the need for a former, and thus, only the surround had to be fastened to the diaphragm. This resulted in a very robust design with reduced moving mass.

[ivica]

No, it has a kapton surround, here I found it:
http://www.audioheritage.org/html/pr...logy/435be.htm

Hi,

for 3"-inch diaphragm You are right, but here I have mentioned ONLY 4"-inch JBL diaphragm suspension

example JBL 476Be driver.



Regards

Ivica

[JMD]

Hello,
I bought an aftermarket diaphragm for JBL2425/2426 16ohm to driver JBL2421 of JBL4345 loudspeakers. I am disappointed, because a measurements THD are very bad compared to the original membranes. THD 2nd and 3rd are about 20dB to 30dB worse than the original driver. Another problem is the decreasing sensitivity towards the lower frequency of about 15 db at 1kHz. It looks like as though the coil does not have correct dimension. Although the view of the membrane seem to be the same. I will try to check and possibly correct the membrane.

Best Regards

Bohdan

[martin2395]

Why bother with aftermarkets, they are rubbish.

The only good and worthwile replacement dia's are Radian - model 1225-16 (for 16 Ohm drivers)

[JMD]

Thank you very much, I did not know this product. Do you have any measurements?

Thanks

Bohdan

[JMD]

My measurements aftermarkets are similar to those...
http://www.audioheritage.org/vbulletin/showthread.php?31988-JBL-4425-Questions-JBL-2416H-vs-RAdian-1225-8&p=325397&viewfull=1#post325397