Advice wanted: 2121 Recone or Not?

[pyonc]

Hi,

I need your advice on the desirability of reconing my 2121 mid-range drivers.
Here is the fact:
One of the pair was reconed with JBL factory kit in 2009, but the kit was 2121H, not 2121 (the right one in the attached pic ). The other one was reconed many years ago (maybe 2000), but I'm not sure if the recone kit was genuine JBL.
Given that one of them was reconed in 2009, it's too early to have it reconed again.
I checked their ohms with a digital multimeter. One reads 6.4, and the other one varies 6.4-6.5.

The reason I want to have them reconed is for matching balance of the pair.
I hear more loudness from the 2121 reconed with factory kit (though it's 2121H) than the other one. As a beginner, I just tend to believe these 2121, each reconed at different times and mismatched kits, affect the loudness balance.

Am I wrong here? If I'm wrong, I don't have to have them reconed?
Any advice would be welcome. I'll decide on the reconing, depending on your advice.
Thank you in advice.

[JeffW]

Personally, I'd get them both reconed so they'd be close to each other at least for now. Plus they'd at least look the same.

[pyonc]

Personally, I'd get them both reconed so they'd be close to each other at least for now. Plus they'd at least look the same.

Thanks for your kind suggestion.
Do you agree the mid-range unbalance is also affected by this different reconing?

[JeffW]

It could be, I don't know. But if you get both drivers reconed, you'll have eliminated that as a reason for any imbalance. Just looking at them, it's obvious that they are different. I wouldn't expect drivers that looked that much different to sound identical to each other. And if you only recone the older driver, then you run the risk that it and the current newest driver won't be identical.

[MikeBrewster77]

Thanks for your kind suggestion.
Do you agree the mid-range unbalance is also affected by this different reconing?

Yes. I don't know about an actual loudness difference (it's possible) but performance, timbre match, etc., would all be invariably impacted.

[pyonc]

It could be, I don't know. But if you get both drivers reconed, you'll have eliminated that as a reason for any imbalance. Just looking at them, it's obvious that they are different. I wouldn't expect drivers that looked that much different to sound identical to each other. And if you only recone the older driver, then you run the risk that it and the current newest driver won't be identical.

Thanks again for your kind reply. If I have to, I'd certainly get both of them reconed.

[pyonc]

Yes. I don't know about an actual loudness difference (it's possible) but performance, timbre match, etc., would all be invariably impacted.

Thanks. The difference is not that much, so actually you never notice the difference when you turn up the volume at or above 9 o'clock on your preamp (I hear them in biamp mode).
But when I compare each of them one by one, with the preamp mode switch in mono, I can hear the difference.

[grumpy]

Might consider a remag before reconing... Could also
test w/WT2 or equiv if you have access, to verify some
aspects of the driver's characteristics.

[HCSGuy]

Have you swapped the drivers between the enclosures and compared? You could have failing components in one or both of your crossovers due to age. If the louder speaker is the same regardless of midrange driver, you've got crossover issues. If the louder speaker moves with the driver, work on the driver. I've never had a 2121, so I'm not an expert, but that fat surround on the old one looks suspect. However, you've also got an Alnico magnet as a variable - I would get the old one reconed and also have the magnet recharged, by a reputable firm. Then, compare it to the newer one. When you do the comparison, swap enclosures again. If there is still a difference, have the new one reconed and recharged - have you been saving up for this? Do the two driver as close together as possible so you get same batch recones. Whoever does the recone can check JBL's inventory before you do the work. I would consider having them order two recone kits even if you're doing them staggered, as you will then have a spare in case one of them fails, or you'll use it to get a matched pair. However, I believe JBL's policy to authorized reconers is that they cannot sell recone kits without installation labor, so ask ahead of time. My JBL tech will let me take the recone kits, but makes me pay for the installation labor up front, so that I would be a fool to try it myself or take it elsewhere (especially since he's a master). I'd also keep your eye on Ebay for another 2121 to test, or put a call out on the forum to borrow one, though 2 way freight may be prohibitive. Good luck!

[pyonc]

Might consider a remag before reconing... Could also
test w/WT2 or equiv if you have access, to verify some
aspects of the driver's characteristics.

Thanks, grumpy. I don't have that equip, but definitely have them checked for remag before reconing at the shop.
By the way, each one of the pair reads at 6.4 ohm with a digital multimeter. I checked it last night.
I wonder if this ohm reading has anything to do with the issue of remag.
What do you think?

[pyonc]

Have you swapped the drivers between the enclosures and compared? You could have failing components in one or both of your crossovers due to age. If the louder speaker is the same regardless of midrange driver, you've got crossover issues. If the louder speaker moves with the driver, work on the driver. I've never had a 2121, so I'm not an expert, but that fat surround on the old one looks suspect. However, you've also got an Alnico magnet as a variable - I would get the old one reconed and also have the magnet recharged, by a reputable firm. Then, compare it to the newer one. When you do the comparison, swap enclosures again. If there is still a difference, have the new one reconed and recharged - have you been saving up for this? Do the two driver as close together as possible so you get same batch recones. Whoever does the recone can check JBL's inventory before you do the work. I would consider having them order two recone kits even if you're doing them staggered, as you will then have a spare in case one of them fails, or you'll use it to get a matched pair. However, I believe JBL's policy to authorized reconers is that they cannot sell recone kits without installation labor, so ask ahead of time. My JBL tech will let me take the recone kits, but makes me pay for the installation labor up front, so that I would be a fool to try it myself or take it elsewhere (especially since he's a master). I'd also keep your eye on Ebay for another 2121 to test, or put a call out on the forum to borrow one, though 2 way freight may be prohibitive. Good luck!

Thanks a lot for your counsel. Following your suggestion, I swapped the drivers between the enclosures, and compared. To my surprise, I found this time it's the one with the old one that sounded louder, not the one reconed with JBL factory recone kit as it used to be. So, I think this might be the crossover issues. How should I fix them? Any idea? Even if I get them reconed, this loudness unbalance issue will be still there if I don't fix this crossover issue, right?

[HCSGuy]

Just kidding, but as a final test, PIA as it may be, let's rule out room acoustics and your ears by swapping the entire speakers left to right. You've got 4343's, right? Shouldn't be too much trouble...After that, we'll try stacking them to eliminate the Z axis room acoustics and see if your hearing is the same with your back thrown out - just kidding on the stacking - don't do it, and I'm not liable if you do.:p

If you swap them and the loud speaker follows, then things get much tougher, but we probably isolate the crossover. There are still more variables - was it the midbass that was louder, or maybe the midrange on one side has a lower output, making one midbass seem louder - get the point? You may end up swapping all the drivers left to right, one at a time, until you've isolated the one component that stays with the louder speaker, no matter what else changes. Whatever that component is, it will need to be serviced - it may be the crossover or a midrange horn driver that is the issue. Keep testing!

[Eaulive]

Loss of magnetic strength is an issue on alnico magnets and it definitely affects the sensitivity of the speaker.

If both kits are original JBL they should sound the same, as far as I know the 2121 and the 2121H use the same C8R2121 kit.

They don't look the same so maybe one of them is not an original kit, check the back of the cones for any markings.

The dust cap on the left looks like it has a flange that rests on the cone, AFAIK JBL does not use caps like those.


---------------edit---------------

Forget it, I read you found your problem :applaud:

[pyonc]

Loss of magnetic strength is an issue on alnico magnets and it definitely affects the sensitivity of the speaker.

If both kits are original JBL they should sound the same, as far as I know the 2121 and the 2121H use the same C8R2121 kit.

They don't look the same so maybe one of them is not an original kit, check the back of the cones for any markings.

The dust cap on the left looks like it has a flange that rests on the cone, AFAIK JBL does not use caps like those.


---------------edit---------------

Forget it, I read you found your problem :applaud:

Thanks for your tip and advice.
The upper one has marking that reads 5/4/09, equipped with original JBL recone kit, but I'm not sure of the other one with some number on the back of the cone. I've no way of ascertaining whether the recone kit of this one is genuine or not, but its dust cap is definitely an aftermarket one. Pls see attached pics.

[pyonc]

Just kidding, but as a final test, PIA as it may be, let's rule out room acoustics and your ears by swapping the entire speakers left to right. You've got 4343's, right? Shouldn't be too much trouble...After that, we'll try stacking them to eliminate the Z axis room acoustics and see if your hearing is the same with your back thrown out - just kidding on the stacking - don't do it, and I'm not liable if you do.:p

If you swap them and the loud speaker follows, then things get much tougher, but we probably isolate the crossover. There are still more variables - was it the midbass that was louder, or maybe the midrange on one side has a lower output, making one midbass seem louder - get the point? You may end up swapping all the drivers left to right, one at a time, until you've isolated the one component that stays with the louder speaker, no matter what else changes. Whatever that component is, it will need to be serviced - it may be the crossover or a midrange horn driver that is the issue. Keep testing!

Thanks. By the way, you mentioned "same batch cones." in your previous reply. I still don't understand. Are you talking about them produced in the same month or year? Why are they important?

[speakerdave]

You might take both midranges to a JBL authorized recone guy who knows and cares about classic JBL and find out if the earlier recone is genuine JBL factory stuff. If it isn't, have just that one redone with a genuine JBL kit.

Demagnetization from being overdriven is not an issue with underhung voice coils; these midranges have underhung voice coils.

[ratitifb]

By the way, you mentioned "same batch cones." in your previous reply. I still don't understand. Are you talking about them produced in the same month or year? same production run (from my understanding)

==> i.e. same sourcing (suppliers), same raw material and same process of assembly for all the parts of the cone kits

[jbl_daddy]

The 2121 kits are still available from JBL, Gorden at Northwest Speaker Repair restored mine about a year ago. My money is betting that your speakers are ok, check all of your components and cables for a problem. Having these drivers restored can be costly. My 2 cents...

[pyonc]

You might take both midranges to a JBL authorized recone guy who knows and cares about classic JBL and find out if the earlier recone is genuine JBL factory stuff. If it isn't, have just that one redone with a genuine JBL kit.

Demagnetization from being overdriven is not an issue with underhung voice coils; these midranges have underhung voice coils.

Thanks a lot, speakerdave. The one in the upper pic was done by our forum member, aka edgewound who confirmed he had done the reconing. I see 5/4/09 on the back of the cone. As for the other one, I'm not so sure of the authenticity of JBL original recone, but its dust cap is 100% aftermarket. JBL dealers want me to get both of them reconed for some reasonable reasons, but you suggest getting only one of them, namely the dubious one, reconed, which I also think is good and reasonable. As for demagnetization of these drivers, I checked their ohms with a digital multimeter, each reading at 6.4. How should I interprete this reading? Does it mean they need remagging?

[Mannermusic]

The 2121 kits are still available from JBL, Gorden at Northwest Speaker Repair restored mine about a year ago. My money is betting that your speakers are ok, check all of your components and cables for a problem. Having these drivers restored can be costly. My 2 cents...

This is wonderful advice. There is a ton of stuff interacting here, including the room. I may be missing something, but have you tried tweaking the level controls individually to get acoustic balance at your seating position? There is nothing sacrosanct here, the control settings don't have to be exactly the same side to side - they are there to compensate for "anomalies." Of course, instrumentation would help, but a good ear is the final instrument anyway (if it sounds good it IS good) - I'd experiment and see if tweaking the mids for best balance does the trick. Turn off the horn and UHF leaving only the woofer and mids playing and balance the mids. Then bring in the horns and balance similarly and finally the UHF. A 4-way is really hairy so takes patience. In any case, my experience with this old gear is that stuff tends to move - mostly degrading connections (as per JBL daddy above), both solder and mechanical (especially RCA) and you just compensate as needed and keep cleaning with deoxit and checking for cold solder joints ad infinitum! Speaking of cleaning, have you cleaned/replaced the pots (L-pads) in recent time? There is your first crossover villain. I actually keep my crossovers sitting on top of the speakers so I can service/tweak easily. Not kidding. Hey, there are 8,645 connections and they are ALL 30+ years old. My additional 2 cents . . . :blink:

[pyonc]

same production run (from my understanding)

==> i.e. same sourcing (suppliers), same raw material and same process of assembly for all the parts of the cone kits

Thanks for your kind explanation. I've got to check them first with the JBL dealers before I ship the drivers.

[pyonc]

This is wonderful advice. There is a ton of stuff interacting here, including the room. I may be missing something, but have you tried tweaking the level controls individually to get acoustic balance at your seating position? There is nothing sacrosanct here, the control settings don't have to be exactly the same side to side - they are there to compensate for "anomalies." Of course, instrumentation would help, but a good ear is the final instrument anyway (if it sounds good it IS good) - I'd experiment and see if tweaking the mids for best balance does the trick. Turn off the horn and UHF leaving only the woofer and mids playing and balance the mids. Then bring in the horns and balance similarly and finally the UHF. A 4-way is really hairy so takes patience. In any case, my experience with this old gear is that stuff tends to move - mostly degrading connections (as per JBL daddy above), both solder and mechanical (especially RCA) and you just compensate as needed and keep cleaning with deoxit and checking for cold solder joints ad infinitum! Speaking of cleaning, have you cleaned/replaced the pots (L-pads) in recent time? There is your first crossover villain. I actually keep my crossovers sitting on top of the speakers so I can service/tweak easily. Not kidding. Hey, there are 8,645 connections and they are ALL 30+ years old. My additional 2 cents . . . :blink:

Thanks a lot for your insight. Actually I've never cleaned or replaced the pots (L-pads) since I bought 4343 used but in good contion from a forum member about this time last year, and I didn't check with the seller on this, either. Some member here also mentioned the possible crossover issues. Can you show me how to clean the L-pads?

[Eaulive]

As for demagnetization of these drivers, I checked their ohms with a digital multimeter, each reading at 6.4. How should I interprete this reading? Does it mean they need remagging?

The DCR (DC Resistance) has nothing to do with magnet strength, the only way to measure it is with a gauss meter inside the gap as far as I know.

The 6.4 ohms reading is normal for an 8 ohms speaker, depending on material (aluminum, copper) and the type of driver it could even go lower, 5 or even 4 ohms. This is the purely resistive value of the winding, the impedance is based both on resistive and reactive components.

[pyonc]

The DCR (DC Resistance) has nothing to do with magnet strength, the only way to measure it is with a gauss meter inside the gap as far as I know.

The 6.4 ohms reading is normal for an 8 ohms speaker, depending on material (aluminum, copper) and the type of driver it could even go lower, 5 or even 4 ohms. This is the purely resistive value of the winding, the impedance is based both on resistive and reactive components.

Thanks for exposing this audio beginner's ignorance on the relevance of the ohm to manet strength. Relieved to hear the reading is normal. I heard over time, the magnet strength of alnico units like 2121 is losing their power over time. 4343 with this 2121 unit first produced in 1976.

[4343]

The DCR (DC Resistance) has nothing to do with magnet strength, the only way to measure it is with a gauss meter inside the gap as far as I know.

The 6.4 ohms reading is normal for an 8 ohms speaker, depending on material (aluminum, copper) and the type of driver it could even go lower, 5 or even 4 ohms. This is the purely resistive value of the winding, the impedance is based both on resistive and reactive components.

IIRC,an impedance sweep can show the effects of lowered magnet strength. Normal strength will show a huge increase in impedance at resonance, while a weak magnet will not increase as much.

Measuring the T/S parameters would be the first thing to do to find out if the two drivers in question are indeed mismatched somehow.

[Eaulive]

IIRC,an impedance sweep can show the effects of lowered magnet strength. Normal strength will show a huge increase in impedance at resonance, while a weak magnet will not increase as much.

Measuring the T/S parameters would be the first thing to do to find out if the two drivers in question are indeed mismatched somehow.


Nice to know, but it's only visible by comparison. Is there a way to really measure the magnet strength by computing TS parameters for a single driver?

[grumpy]

http://www.woofertester.com/wt2product.htm

[Eaulive]

http://www.woofertester.com/wt2product.htm

Neat... did you test it?

[grumpy]

Works quite well... have had one for a few years. Nice augmentation to a
mic/pre/laptop analysis system for audio FR measurements.