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View Full Version : 4412 Weak Bass (re foam issue?)



Paul_M
09-18-2018, 12:52 PM
Hello fellow JBL enthusiasts!

Hopefully this post can help other JBL folks as well. I purchased a pair of 4412 studio monitors new about 25 years ago. Noticed some foam rot on the 128h-1 woofers about 8 yrs ago (or so). Had them re-foamed by a factory authorized JBL rep. Haven't really used them mush over the last few years but ever since they were re-foamed I have noticed a lack of bass.

The foam surrounds are put on correctly (glued to the back, etc.). I have checked phase and polarity to the woofers on each side (L/R). Tested with 1.5 v battery (+ to + and - to - pushed woofer to back of frame so we are good there. Just swapped the polarity on the woofers last night to see if there was a difference and there was NO noticeable difference in sound. Seemed odd.

Started thinking that perhaps the crossover network but, and somebody please confirm, I am under the impression that the crossover really shouldn't cause any issues on the low end as the crossover is more or less passing thru the low end frequency unabated (perhaps a little low pass filtering?). So let me say that the surrounds are not very forgiving and seem pretty stiff. Not much movement at all when listening to material. I have plety of amp as I am using a Bryston 4NRB for the power amp. Even pushing a bit on the cone seems fairly stiff with limited movement. Nothing binding up (like voice coil) but just stiffer than I would think they should be.

Starting to think that the material (foam) used may have not been the correct material as the chap that repaired them was mostly into repair of JBL pro (PA, etc.) equipment for rock gigs. I don't know but what do folks think - possible foam issue here?
Also I have read that the original foam is long gone but some folks may have comparable foam (i.e. Rick Cobb perhaps?). Lastly any plugs for some service shops in the USA. I am on the east coast but can ship anywhere domestic US. Well thanks for reading and any comments would be highly appreciated. Thanks!

PM

SteveJewels
09-18-2018, 12:56 PM
I can't help you with you issue although I am very interested as I am looking for some 4412's.

Miller Sound in Lansdale PA is excellent for servicing JBL speakers.

RMC
09-18-2018, 02:34 PM
Hi Paul,

As far as I know JBL doesn't sell factory made refoam kits, only complete recone kits. So the foams installed are most probably aftermarket things.

Could it be that the refoaming fellow installed 128H refoam kits, or some generic 12" or for other jbl 12" ones, on your 128H-1 drivers instead of the proper specific ones? Consider the following:

Springfield speaker Web site: "This kit is made specifically for the black-coned JBL 128H-1 woofers found in the 120Ti, 4412 and 4412A cabinets. This kit does NOT fit white-coned JBL woofers"

Midwest Speaker repair Web site: "This kit will fit the JBL 128H-1 (Black Cone Only) woofers used in 120Ti, 4412 and 4412A speaker cabinets.

These drivers should have about 8 mm Xmax (almost 1/3") based on JBL T/S table so the cones should move a good bit back and forth when you press lightly on the cones near the middle. Maybe what you have installed are not the proper fit/performance ones? Regards,

Richard

Paul_M
09-19-2018, 08:50 AM
I can't help you with you issue although I am very interested as I am looking for some 4412's.

Miller Sound in Lansdale PA is excellent for servicing JBL speakers.

Thanks Steve - I'll give them a look.

Paul_M
09-19-2018, 08:52 AM
Hi Paul,

As far as I know JBL doesn't sell factory made refoam kits, only complete recone kits. So the foams installed are most probably aftermarket things.

Could it be that the refoaming fellow installed 128H refoam kits, or some generic 12" or for other jbl 12" ones, on your 128H-1 drivers instead of the proper specific ones? Consider the following:

Springfield speaker Web site: "This kit is made specifically for the black-coned JBL 128H-1 woofers found in the 120Ti, 4412 and 4412A cabinets. This kit does NOT fit white-coned JBL woofers"

Midwest Speaker repair Web site: "This kit will fit the JBL 128H-1 (Black Cone Only) woofers used in 120Ti, 4412 and 4412A speaker cabinets.

These drivers should have about 8 mm Xmax (almost 1/3") based on JBL T/S table so the cones should move a good bit back and forth when you press lightly on the cones near the middle. Maybe what you have installed are not the proper fit/performance ones? Regards,

Richard

Thanks Richard. Yeah def a possibility the wrong foam was used. Also thanks for the t/s info. I found the table you were referencing. Never knew that information was out there. Learn something new everyday - cool. Any chance there could be crossover issue? Or is this type of behavior (weak bass) not really associated with network issues (caps, etc.) ?

Robh3606
09-19-2018, 09:20 AM
Any chance there could be crossover issue? Or is this type of behavior (weak bass) not really associated with network issues (caps, etc.) ?

Most probably not it's usually about driver Fs and box tuning. The box didn't change which leaves Fs as the most likely culprit. Measurement is the only real way to confirm.

Rob:)

SEAWOLF97
09-19-2018, 09:40 AM
I would have gone with "out of phase" , but you checked that.

How about switch to MONO and see if it improves ?

Sub in another set of speakers and see if problem is upstream ?

RMC
09-19-2018, 11:13 AM
Hi Paul,

I forgot to ask in my first post since "Haven't really used them mush over the last few years..."

Any chance you might have changed speaker location/placement in the meantime, like from before and after refoam job done? Or something like raising the boxes off the floor on speaker stand/pedestal for example? These could really affect perceived bass response.

That's a pretty elementary question that we (including me) forgot to ask right at the beginning to eliminate from the start such likely issue. Similar to those "dumb" questions in audio gear manuals, troubleshooting section, where you're first asked if the device is properly plugged in a live AC outlet, signal cables connected in the right inputs/outputs, etc. It does happen to anyone...

As for a crossover issue, I wouldn't say impossible, but rather unlikely. Troubleshooting implies eliminating or progressing from the easiest or more evident going towards the more difficult or complicated issues...

Regards,

Richard

Paul_M
09-19-2018, 12:21 PM
I would have gone with "out of phase" , but you checked that.

How about switch to MONO and see if it improves ?

Sub in another set of speakers and see if problem is upstream ?

Yeah I have one more component to check upstream in the chain but have been subbing in components from a different working reference system, Last component to swap out is the amp but my gut tells me that ain't the issue either. Find out later tonight empirically:-)

PM

Paul_M
09-19-2018, 12:28 PM
Hi Paul,

I forgot to ask in my first post since "Haven't really used them mush over the last few years..."

Any chance you might have changed speaker location/placement in the meantime, like from before and after refoam job done? Or something like raising the boxes off the floor on speaker stand/pedestal for example? These could really affect perceived bass response.

That's a pretty elementary question that we (including me) forgot to ask right at the beginning to eliminate from the start such likely issue. Similar to those "dumb" questions in audio gear manuals, troubleshooting section, where you're first asked if the device is properly plugged in a live AC outlet, signal cables connected in the right inputs/outputs, etc. It does happen to anyone...

As for a crossover issue, I wouldn't say impossible, but rather unlikely. Troubleshooting implies eliminating or progressing from the easiest or more evident going towards the more difficult or complicated issues...

Regards,

Richard

All good points but nothing has changed in reference to positioning (etc.). So it really is an apples to apples comparison. I'm trying to use process of elimination in my troubleshooting to isolate the issue. By tonight I should have omitted everything in the signal chain but driver as root cause.

Thanks,
PM

Paul_M
09-19-2018, 12:39 PM
Most probably not it's usually about driver Fs and box tuning. The box didn't change which leaves Fs as the most likely culprit. Measurement is the only real way to confirm.

Rob:)

Hey Rob - thanks. I am tending to lean towards Fs as the culprit in this scenario. Been learning quite a bit about speaker design and T/S parameters over the last week or so. Good stuff. Spoke with a fellow at a shop in PA that does JBL repair and he is fairly confident that the cause is a stiffened (dried out) spider on the 128H-1 driver. That would affect Fs from my understanding. Stands to reason that could be the cause. Thinking of just getting a recone done on these and possibly a regauss of the magnets while we are at it. The recone would address the spider and the surround potentially not being correct all at the same time. Thoughts? Should I use the OEM CR8R128H-1 kit (which is still available according to JBL Northridge as of today). Also how would one go about performing a measurement to determine if Fs (due to suspension issues, etc.) is the cause (perhaps prior to doing the recone)? Would that somehow be using the xmax value spec?

Thanks,
PM

Robh3606
09-19-2018, 01:04 PM
They are ferrites, there is no need to get them re-gaussed. Re-cones are always the best way to go but are expensive which is why many re-foam even when cone kits are available. I will typically go for new cone kits if they are available and not ridiculously priced. To measure Fs you just need something like a woofer tester so a test box and software. You could probably do it with a resistance meter and a low power frequency source you can sweep from say 5-10 hz and up through about 100hz or so.


Rob:)

Triumph Don
09-19-2018, 06:09 PM
I have owned a pair of 4412's for years now. Refoamed them myself with Rick Cobb kits. IMHO from what I have read here, the foams and spiders are not the issue. Had these great 12" 3 ways powered by a few different amps and pre amps in both large and small rooms. Have used them in A-B comparisons recently with L100's, L65's [Jubals] and L100T's. Lots of different amps and pre amp combos from NAD, Sony ES, McIntosh, and Hafler [yes I have a little free time]. The lack of bass has to be from amplification [not good enough] and room placement. And size of the room. Just moved these to my son's new to him house, bigger listening area than my test room, needed a sub for help. Tried a 10' but a 12'' did the trick but a better amp and pre amp is coming for Xmas. My experience is with older JBL's, but the 2214 is by far my favorite 12 incher. Likes lot of good, clean power! I have brought several back to life with proper refoams, never have seen a spider issue.

BMWCCA
09-19-2018, 07:50 PM
Spoke with a fellow at a shop in PA that does JBL repair and he is fairly confident that the cause is a stiffened (dried out) spider on the 128H-1 driver.

Would that be Millersound in Lansdale?

brutal
09-19-2018, 10:04 PM
Just a cautionary note:

I can't say for the black cones, but the 128H white cones do come in two different sizes. Rick Cobb will have you measure them to be sure you get the right foam surrounds.

Paul_M
09-20-2018, 08:36 AM
Would that be Millersound in Lansdale?


Ah yes :-)

Ian Mackenzie
09-20-2018, 09:38 AM
http://www.audioheritage.org/vbulletin/showthread.php?10645-128H-1
https://www.google.com/amp/s/m.wikihow.com/Measure-Speaker-Impedance%3famp=1

Assuming the driver does not measure to spec you wonít know for sure if itís the spider or the surround but itís likely the surround based on service history.

If the driver does meet spec start looking else where ie swap out the preamp and power amp.

Order a Rick Cobb surround and other necessary tools & glues and either get someone to install it properly and nicely (not a rush job if you want a nice cosmetic appearance) or follow re foam instructions recently posted on the forums.

Re test the driver resonance as above after breaking in the driver with some usage.

I hope this helps.

If there is a forum member nearby who is well practiced in testing woofers it might be worth a road trip ( and a few beers) to figure it out.

BMWCCA
09-20-2018, 06:09 PM
Ah yes :-)
They have a great reputation for re-building vintage drivers with no factory support or spare-parts availability. Nearly every repair experience I've heard of from them seems to involve a replacement spider to create a somewhat Franken-speaker product. Not saying they aren't talented but they seem to always focus on spider replacement. Not saying that it's not possible to be the problem, but I find it hard to believe that the issue with your JBLs is the spider. :dont-know:

Robh3606
09-20-2018, 07:45 PM
Nearly every repair experience I've heard of from them seems to involve a replacement spider to create a somewhat Franken-speaker product. Not saying they aren't talented but they seem to always focus on spider replacement. Not saying that it's not possible to be the problem, but I find it hard to believe that the issue with your JBLs is the spider. :dont-know:

I find that kinda odd as a "worn" spider may be an issue as far as centering the coil but it certainly will not raise Fs if anything it will lower it. Almost every re-foam I have done has been within a couple of hertz from spec using Rick Cobb surrounds. Some have been lower which makes sense if the spider stiffness is lower from use as the two and cone mass determine the Fs of the driver. I have a pair of 128H-1 I did re-foams on will post the Fs results.

Rob:)

Ian Mackenzie
09-20-2018, 11:33 PM
That makes sense.

Maybe if the drivers are run on a sine wave at 30 hertz for a reasonable period they might loosen up.

You can down load an app on an iPhone to do that.

Robh3606
09-22-2018, 09:32 AM
Just ran one of the re-foamed 128h-1 and the T/S table has it as 20 Hz. My driver came in at 20.4 Hz so basically right on the money. This is cold with no run in at all from storage right to the measurement.

Rob:)