View Full Version : loss of efficiency on one driver

02-06-2005, 09:49 AM
My system is based on used JBL LE85, I have a clear difference of efficiency on one channel which is due to the driver, not the electronic upstream. Since these are Alnico drivers, is there any other explanation for the loss of efficience than partial demagnetization?

Also, for the more knowledgeable or better equipped ones out there; if this is due to loss of magnetization, would it result in a significantly different response curve between the drivers? Judging by ear only, I cannot sense a difference in that respect, but then...


Earl K
02-06-2005, 10:13 AM

If the red wax seals are missing from the back-caps' screw holes - then you might have different impedance diaphragms in those old magnets.

What's the DC resistance of each voice coil ?

02-06-2005, 02:03 PM
Earl, I can't measure the impedance, but the voice coil resistance is roughly similar at both at about 6.5 Ohms,

Mr. Widget
02-06-2005, 02:36 PM
Are there L-pads or other adjustments as part of the speaker's internal crossovers? Frequently their failure or getting dirty is the cause of this type of problem. If you are certain it is the driver, it can be that the phase plug or diaphragm is out of alignment. It is possible that the magnet has lost strength, but I would look at that as the least likely scenario.


02-06-2005, 02:52 PM
No, there is nothing between the amp and the driver as I use line level filters.

I played these for a while without the back cover, as it gives a little more 'air', but stopped as I realized that it was not good, the aluminium colored ring started to move vs the black driver pot, and I had to struggle to put the screws back when fitting back the voice coil cover, so for what I understand, I may have a mis-alignment issue? Is there a way to check if the voice coil is well aligned (sorry if this is a stupid question, but I am a newbee concerning these!)

thanks much

Mr. Widget
02-06-2005, 03:13 PM
That aluminum colored ring is the top plate and it is attached to the phase plug. It sounds like it shifted. You will need to have the driver completely rebuilt. :( It will need to go back to the factory and be demagnetized, realigned and remagnetized.

On second thought, the top plate shouldn't have been able to slip that easily, so maybe you needed to remagnetize them anyway. You should probably have both drivers recharged.

Sorry for the bad news.


Earl K
02-06-2005, 04:42 PM
Is there a way to check if the voice coil is well aligned (sorry if this is a stupid question, but I am a newbee concerning these!)

It's not a stupid question. If the top plate has shifted some how , then the gap that the voice-coil sits in, will be out of round. A "binding" coil will have reduced sensitivity. To check the "gap" for roundness ( concentricity ) you'll need to take your driver(s) into an authorized JBL service center.

A JBL service center will have the "Blessed" set of JBL gap gauges. They'll run the .031 gauge size around inside the empty gap to check it for concentricity.

As Mr. Widget has pointed out - your drivers may need remagging . Have a JBL professional determine this for you.

I don't suppose you removed the "Bug-Screen" in another attempt at getting some more "air" from these drivers ? If so, you may have a new "visitor" hibernating inside that driver ( spider / piece of metal ). It is winter you know and even the little critters need homes. :blink:


02-07-2005, 12:58 AM
Widget, Earl, thanks a lot, even if not good news... I'll see if I can find anyone over here in Belgium able to do this, anyway I was thinking of replacing those with either 2" or (should I dare to say this on this forum....) TAD 2001 or something similar, looks like I may have to do that earlier than expected.


Maron Horonzakz
02-07-2005, 06:56 AM
Try switching VC diaphrams first. One might be defective.

02-07-2005, 02:30 PM
Interesting, but don't try this at home... Checking after reading all the above, I realized that the back chamber screws were very tight, which is not normal. Checking further, the diaphragm was stuck in the gap... The back plate was in fact slightly out of alignment, so I figured I had nothing to loose, I (very!!) gently hammered the back plate in the proper position (guessing it by the screws become easier to fit) - guess what, the diaphragm could be removed from the gap again...

Fitting everything back together, I recovered the lost efficiency on that driver (as much as I can detect by ear, so it's probably between 2 and 4 dB's! Beginner's luck some may say.

I chceked further and realized that, one thing after another; the support for the drive (a simple wooden block) had moved and that all the weight of the driver plus part of the weight of the horn was supported fully by the back chamber, that's what may have moverd everything out of alignment, one more lesson learned and something to remember for the future!

Anyway, I am quite happy, the magnet strength may not be the best anymore, but the musinc and the dynamics are certainly there!

Thanks again for your great inputs, it drove me into doing this crazy stunt, but it sure worked!

Earl K
02-07-2005, 03:18 PM
Very Glad you found a remedy to your problem. :)

My reconer ( he's been around a long time ) has indicated that the top plate in those older alnico structures is just held in place by magnetism. Your experience would seem to confirm that statement.

Your thoughts about how this "gap-binding" first occured is certainly interesting and does make sense. There must have been quite a lot of cantelevered force in play to cause that shift ( especially with the back-caps' bolts still installed ).