Why is one of my jbl d120fs so much quieter than the other?

[leboamp]

Hoping someone might be able to shed some light on this for me... I have a pair of jbl d120f speakers in my 1968 twin. When I bought the amp, the speakers had been reconed with the wrong cone. I wanted to restore them so I had them reconed by a reputable place that claims to be able to restore these to their full vintage glory. When I got the speakers back and installed them, one was noticeably quieter than the other. I sent it back and he said something about the spider being defective since it was collapsed. He redid the job and sent it back. I just reinstalled it and it's still quieter than the other one! Is it possible that the quieter one is normal, and the louder one is the anomally? What would cause the difference in volume? I compared both speakers with a WGS HP reaper and the the louder of the two jbls is a great match with the reaper. The jbl in question was again noticeably quieter. I checked the ohmage and they are both right around 8 ohms. Figured I'd see if anyone here has any insight before I reach out to the place that did the work. Thanks!

[RCGPNY1]

Since vintage JBL's used Alnico magnets, it's not beyond possibility that one of the magnets "degaussed" slightly. Most pro recone shops can reenergize alnico magnets...

[Cantplay]

Check with meter, same voice coil?

John

[Teleguy61]

Original D120 parts are no longer available, so most places use K120 parts, which is a different voice coil former, made of kapton, as in K120.
Higher heat resistance, slightly different tone, but works fine.
Other than that they are using after market parts and they can be variable.
Variations in output are more likely spider and cone stiffness, and may even out some with use.
If the spider is not installed correctly, the cone can't move freely, and while it may not be distorted it may be quieter due to less motion.

[Scumback Speakers]

My guess is with the magnet not being fully charged. When you separate an alnico magnet from it's plates it loses it's charge. Then it needs to be recharged to work correctly again.

And there is also the spider / cone / other parts that can play into it as well.

One of my frames showed up without a charge once. I guess they goofed before they sent it. It put out about 1/2w, or about the same volume as a mouse fart x 2.

I took it to Orange County Speaker Repair, they charged it and it was fine, full and loud afterwards. Sometimes weird stuff happens, but the magnet losing it's charge is what I'd check first.

[cbass]

When you have a big company recone your speakers isn't the magnitezeer/demagnitizer what you are paying a premium for? I mean reconing a speaker is a pretty fastmnprocess for someone that knows what they are doing.
I put a new bolt on basket on a PV scorpion speaker so I'm an expert reconer now.

[RCGPNY1]

Since vintage JBL's used Alnico magnets, it's not beyond possibility that one of the magnets "degaussed" slightly. Most pro recone shops can reenergize alnico magnets...

I think I said it right away

[rooster]

Here's a simple test. Take a fully charged nine volt battery to each speaker. Make sure you can see the cone movement clearly. Attach the battery such that the cone moves forward, and then reverse it to see the cone move backwards. Is the movement equal in both directions? Is this true for both speakers and is the distance equal for both speakers? And do they both measure 8 ohms?

Here's the thing. When the reconer said the spider collapsed on the first go'round, that was BS. What he should have said is that he built the speaker with the voice coil lower in the magnet structure - such that the speaker moved forward OK, but failed to move backwards very well - because the voice coil was sitting too low to begin with. Do you understand?

Here's another thing. Even if a speaker isn't broken in, you can take a nine volt battery to the speaker and do this test I just described. If the cone does not move equally in both directions, get rid of the speaker or get it reconed. It was not built correctly and will never change its flawed character, even after ten days of break in. It's a crappy speaker.

Jim and RCGPMY1, (and especially Jim because you're a VERY smart man in regards to speakers), seriously, what are the odds that a USED JBL speaker with an alnico magnet of THAT caliber could EVER demag? Ha, I will never see it in my lifetime. This is a poorly built speaker, BOOM, I said it. What's remarkable here is that you even got one speaker from the reconer that was acceptable.

Do the test, you'll understand.

[RCGPNY1]

Actaully there is quite a probablilty for a JBL to degauss. There are many reasons why this happens. One is just age, and large voicecoil speakers with shallow alnico plugs can cause a 3db(or more) loss. This is a quote from Vancouver audio speaker clinic:For example: Low frequency drivers with 4-inch voice coils (JBL D120, D130, D140, LE15, 2205, 2215, 2231, Gauss, or TAD drivers) can lose 2~3dB, becoming 'dull' or 'slow' sounding, unless they have been grossly abused, in which case they more and sound dreadful. Drivers with short Alnico slugs and 3-inch voice coils, (JBL 2213, 123A, etc) are effected more severely, so typically show 3dB sensitivity loss, while small alnico woofers (JBL 116A, 125A, 127A etc) are commonly 4dB down after quite modest use. Conversely, speakers with higher coercivity ( taller Alnico slugs) and short voice coils, such as Tannoy dual concentrics, Altec and EV woofers, rarely suffer unless they have been badly abused. Users report that Altec 416, 515, and 604 drivers become 'mushy' after heavy use, but most of their loss of definition is due to fatigued cones. Remagnetization can restore the magnet, but cannot address cone fatigue.
So...as improbable as it seems...it does indeed happen

[Scumback Speakers]

Rooster: I've been pretty lucky with my alnico magnet speakers, never having one lose it's charge. But it can happen thru various accidents, or mishaps for sure.

I've got two JBL's, never had a problem with them (G-125 late 80's model). As for what else might make that happen, there's a list of things that could do it, but usually you know because they involve abuse or carelessness.

It sounds to me like it was either the recone done wrong, or the magnet lost it's charge. Without it in front of me, I'm guessing, but those are the two most obvious causes.