LE 15 A changing impedance with time

[scorpio]

OK, I admit posting on this before, but no workable answer received and the situation is changing...

I got a pair of cosmetically perfect LE15A about 18 mths ago, they had similar voice coil resistance at the time ('bout 6.7 Ohms if I remember right). With time, one started to behave strangely, loosing some apparent efficiency. Also, it's coil resistance went up to 9 Ohms, and testing in free space showed that it had 'lost' it's expected impedance max at resonance frequency.

Now it is extending to the 2nd driver also loosing it's peak when tested, but the coil resistance is unaffected. This leads to some difference in L-R dynamics even when working on the L-pads setting to balance the levels.

THere is no sign of voice coil rubbing or anything, someone suggested that the magnets might be loosing power, but I can't see how that would affect coil resistance and lead to the loss of the resonance peak (yet I dont understand much of the physics of these things).

This is another attempt to fish for ideas on what could be going wrong. Soundwise, I love it (despite the slight L/R difference), but I am hesitant to pay another extra hundreds euros for replacements if it's to get the same problem. Any suggestions anyone?

For info, I use them in combination with LE85 and with original L200b networks (now completely rebuild with new and better components), in home made boxes and circular horns.

Thanks for any suggestion,

[Flodstroem]

Hi scorpio

I have a LE15 pair behaving the same as yours but worth.
I made a decision for a time back to re-cone these speaker and didnt
thought ov it after that. Until now when reading your post.

That pushed me to grab the ohm-meter and check for any new changes from last measurement. Jeesus, the DCR had change a lot. This is the full history for these speakers:
first measurement, DCR: 7.7 for first and 7.9 for the second speaker :)
second measurement, DCR 7.7 and 10-11.7 for the second speaker :(
todays measurement, DCR 8.7 and 23.7 for the second speaker (ca a year later from second) :biting:

I didnt thought of it the first and second time, but now. This must have to do with bad connections at the voice coils magnet wire to the connectors flexible leads, also there must be a corrosion issue involved due to age.
These two LE15A have the old lance-alloy foam surround and there is a LE15-III stamped at the rear of the cones. I decided long time ago to have them re-coned, but if there is a way to fix this problem, I would be happy.
But Im afraid we had to re-cone these speakers to get them in full working order and to get the right JBL specs back again.
Sorry I wasnt to any help, scorpio

Regards

[Earl K]

Also, it's coil resistance went up to 9 Ohms, and testing in free space showed that it had 'lost' it's expected impedance max at resonance frequency.

- Some Observations ;

(a) Zmax Shift ;
- I know from experience ( with le14a's ) that the Zmax peak ( of the ancient Lans-a-Loy surrounds ) will drop lower & lower / as the Fs shifts upward . This is caused by the surround hardening ( & drying up ) .
-You didn't state which sort of surround your speakers have and you didn't state whether or not there was a measurable shift in Fs ( measured in free space ) .
- JBL Service Specs ( 1976 ) lists a range of healthy Fs as 14 to 20 hz with accompanying Zmax figures of " infinity " to 200 ohms ( respectively ) .

(b) DCR Coil Resistance ;
- JBL Service Specs ( 1982 ) for the le15a state that a "healthy" coil is in the DCR range of 7.6 - 9.2 ohms.
- older JBL Service Specs ( 1976 ) for the le15a state this "healthy" DCR range as 8.1 - 9.5 ohms.

- I would remeasure both of your woofers Fs to see where they are now .
- You may need to refoam the surrounds, if you have the older Lans-a-Loy types, which will always stiffen up over time .

:)

[Robh3606]

It sounds like a solder joint fatiguing and going resistive with time. How are the tinsel leads on them??? May be there is a problem where the tinsel and voice coil are attached???

Rob:)

[edgewound]

It sounds like a solder joint fatiguing and going resistive with time. How are the tinsel leads on them??? May be there is a problem where the tinsel and voice coil are attached???

Rob:)

I agree with that diagnosis....probably the lead wires. They can be replaced.

[scorpio]

Thanks all for your replies, at least I can fell a little 'better' :( that I am not the only one to have problems.

Yep, I had the leads checked, and they were replaced on the 'defective' first speaker by a guy who is supposed to know what he's doing, it did nothing to help, but I might try to have that checked again. I can't say if the Fs peak has shifted, it's more like it's disappeared completely...
The surrounds are foam, excellent conditions, supposed to have been replaced by the proper parts about 1 yr before I got them - it's not stiffening.

Around here; the most knowlegeable folks believe that the loss of Fs peak is due to demagnetisation.

Problem is that I have not yet been able to find a place to have these checked and remagnetized/sorted out properly in Belgium - I am looking for one in France at the moment, but no luck.
To be honest, I love the sound but I am starting to get tired of dumping money into lousy components (I already changed the HF driver from a 2426 (now at least that was a MAJOR improvement) to marginally OK LE85 to having now mint NOS LE85) - I'm thinking of getting a pair of Iconic reproductions of 515 (heresy...), I would get an extra 8 dB in the process...

[Lakanta]

[QUOTE=Earl K]- Some Observations ;

(a) Zmax Shift ;

(b) DCR Coil Resistance ;

Hi,
Zmax can shift quite heavy, because in normal use the resonance drops. Spyder and surrounds get 'normaly' more flexible. (I worked in the quality control of a louspeaker manufactor and the 'Lifetime test', 100h at Pmax, shows incredible drops)

But.. Don't forget that the DCR depends also on the temperature you measuring! (Winter in the garage versus summer)

[Zilch]

I often have difficulty getting reliable DCR measurements on the old style terminals, and resort to measuring on the solder lug side. Ohmmeter measures using wimpy currents that can't break through any film or oxidation. Contacts gotta be clean for good readings.

Oh, and it's gotta be QUIET when measuring, too.... :p

[jarrods]

someone suggested that the magnets might be loosing power, but I can't see how that would affect coil resistance and lead to the loss of the resonance peak (yet I dont understand much of the physics of these things).


Magnet strength has zero effect on the DC Resistance of a coil. It's reaction to AC excitation is a whole different ball-game.

cheers,
Jarrod

[edgewound]

Magnet strength has zero effect on the DC Resistance of a coil. It's reaction to AC excitation is a whole different ball-game.

cheers,
Jarrod

If the cone/voice coil is wobbling in the gap as you measure the DC resistance, the measurement will change....but I'd agree with you.

[scorpio]

Seems like I finally found someone in France who is really competent to do a remag on these, and has all the necessary material to recone with original kits if needed.

Can anyone confirm that a remagnetisation is a good thing to do? I have been told occasioanlly that it's not bringing the speakers back to specs and that remagnetisation does not hold long (dont understand that from a physics standpoint though) - but that was from professionals in sound reinforcement applications, not home based hifi applications (specially that I use low power triode amps).

Thanks,

[Robh3606]

They would charge the magnets and pots after assembly to bring them up to spec. A 2440/375 can only be assembled this way. This being the case why wouldn't a recharge work??? I would think it should last as long as the original providing it was done correctly and the driver doesn't go back into the same application that removed the charge in the first place. How are they doing it?? If you can't measure the field in the gap how do you know the magnet is recharged up to it's original factory condition?? Is there some property in an Alnico Magnet that limits the effectiveness of a recharge???? Sounds kinda fishy to me.

Rob:)

[Earl K]

Can anyone confirm that a remagnetisation is a good thing to do?
- Someone else will need to answer that with a testamonial gleaned from personal experience. I've only had compression drivers remagged .



I have been told occasioanlly that it's not bringing the speakers back to specs and that remagnetisation does not hold long (dont understand that from a physics standpoint though)

- I've been told ( by my reconer ) that there are different types of magnetizers.

- I would want to do more research into what type JBL and/or Orange County use.
- Whichever type it is, they do seem to be particularly suited to remagging Alnico magnets of all gauss levels.

The following is just rumour ;

(a) Fast-Charge :Some remaggers' zap the magnet with a huge quick-charge and loadup the magnet to its' 100% capacity. The weight/charge capacity of the alnico magnet dictates the final gauss level in the gap.

(b) Slower-Charge: Another type is able to dial in the charge starting at a lower setting
- Since the le15a has a pretty low gauss level in the gap , I would want to know that any magnetizer can dial in the proper gauss level . It's possible the alnico slug magnet in your speakers could hold a larger charge than the le15a specs call for . A larger gauss level would be a problem because it would skew the T/S specs. On the other hand; Maybe the magnet ( as dictated by weight ) is already holding it's maximum charge . This is another area requiring more research .



- but that was from professionals in sound reinforcement applications, not home based hifi applications (specially that I use low power triode amps).

- Well "Pro" SR users of alnico products ( I'm more or less one of these ) regularly overdrive the snot out of alnico woofers. It's well documented that alnico can lose its charge by being overheated ( over-driven ) or suffering sharp hard-knocks ( such as found when "Hire" companies move their gear around in trucks that are not "AirRide" ).

- My final analysis ? More home-work is required before any remagging is to be done by a facility just saying "yup, we can do it" .

:) ( not much help , really , am I ? )

[scorpio]

The facility will measure the field in the gap before doing anything else. Way I understand it, they can't dismantle the pot, the whole frame has to be treated, that's why they need to de-cone/re-cone as well .

Thanks for the input on not re-gaussing too high, I'll call them again and check, and ask about their process as well. Might give me more clue about their expertise (although I found a number of local testimonies that they know what they are doing...).

The question I have to solve for myself now, is it worth the cash, or do I buy new speakers and forget about it... I don't want to buy another pair of LE15A which would only be marginally more expensive (yes, it's that expensive when there is only one place in the country to do it!) only to find that they are lousy as well...

Sometime I wish I didn't like music, or could just live with a MP3 - there might be something good after all with 'progress'...:barf:

[Earl K]

The question I have to solve for myself now, is it worth the cash,
- Is it a lot of cash ?



( snip ) or do I buy new speakers and forget about it... I don't want to buy another pair of LE15A which would only be marginally more expensive (yes, it's that expensive when there is only one place in the country to do it!) only to find that they are lousy as well...

- I think all the world of my ME150s woofs. Harman US sells them for @ $415.00 . The 1500FE is another evolutionary stepup ( so I believe ) from the ME150 ( though they have completely different gap topologies ) .

- I consider them a nice blend between a 2235H and a le15 type .

- They are an overhung coil operating in a deep gap. MMS ( cone weight ) is like a 2235 ( @ 150 grams ) but the deep gap is similar to a le15 .

- Subjectively, their sonic "weight" is much closer to that of a 2225H ( @ 105 grams ) / but with much better articulation than either the 2225 or the 2235.

- I'd first consider buying either of these speakers before the 515-8G ( if you can exist with a 800hz crossover point , or lower ). The 515 is decidely pretty light-weight in the woof-woof department .


The facility will measure the field in the gap before doing anything else. Way I understand it, they can't dismantle the pot, the whole frame has to be treated, that's why they need to de-cone/re-cone as well .


- This sounds like the "fast zap process". I've been told that the cone and frame need to be removed from the main magnetic assembly , ( that basic "circuit" consisting of the cast-pot / top-plate / & internal alnico slug magnet ) because the initial "surge-power" is enough to blow the cast aluminum frame through the roof if left attached ( StarWars Rail-Gun Effect ).

- If my guess is correct, your cost is dictated by all the labour necessary to dismantle & then rebuild the whole construct .

- Are they able to degauss first ( before dismantling the woofer ) ?

<> Tough Decision Ahead of You ! :)

[scorpio]

Where can you get these? I dont even see those refs on the JBL components page, can they be ordered separately?

[Earl K]

Where can you get these?

- You'll need to find a euro-based Harman/JBL retailer to order these for you.


(snip), can they be ordered separately?

- Yes ( the ME150H has a part # I can dig up if you really need it ).
- Obviously, the 1500FE isn't in stock right now ( at least according to this avenue of access )

:)

[Flodstroem]

Hi
Earl K

The following is just rumour ;

(a) Fast-Charge :Some remaggers' zap the magnet with a huge quick-charge and loadup the magnet to its' 100% capacity. The weight/charge capacity of the alnico magnet dictates the final gauss level in the gap.

(b) Slower-Charge: Another type is able to dial in the charge starting at a lower setting
- Since the le15a has a pretty low gauss level in the gap , I would want to know that any magnetizer can dial in the proper gauss level . It's possible the alnico slug magnet in your speakers could hold a larger charge than the le15a specs call for . A larger gauss level would be a problem because it would skew the T/S specs. On the other hand; Maybe the magnet ( as dictated by weight ) is already holding it's maximum charge . This is another area requiring more research . :hmm:

Hmmmmm. :blink:
When charging a magnet you must apply a high enough magnetic pulse so the whole magnet is saturating. This mean that all the small magnetic parts/domains in the material (in this case Alnico) has to be charged/magnetized.



Originally Posted by scorpio
I have been told occasioanlly that it's not bringing the speakers back to specs and that remagnetisation does not hold long (dont understand that from a physics standpoint though)

If for some reasons the charging pulse was to weak, then the most difficult domains (the strongest) to magnetize would be unmagnetized. Such a magnet will "unload" much faster than that of a saturated magnet due to the weaker domains (in this case, the one and only) will be more easily unloaded/demag.

When charging a magnet the most common procedure is to first make it saturate (high enough magnetic pulse power) and next stepp is to unload the weakest domain in the magnet by a controlled small reversed magnetic puls. This will yield a magnet that will last for a long time and will hardly demagnetize.

When measure a magnet you could measure total flux or you could measure flux density. Two total different ways to specify a magnet, especially loudspeaker magnet assembly.

The LE15 hasnt any weaker or smaller magnet than for example the 2440/375. What you see in the JBL´s specs is density (in Gauss or Tessla) and the difference you noticed is because the LE15´s magnetic gap area is far more bigger than that of the 375 due to thicker top plate. Because of the bigger gap area the Gauss readings flux density has to be lower (9 kG) than that for the small area 375´s gap area (flux density approx 20.5 kG). :applaud:

When I measured an old 2440 for flux density I could state that it had demag down to 19kG (±0,5% accuracy in Gauss meter). Will be a 7.3 % demag. This I think was due to ruff handling as it had marks on the outside of the assembly who could verify that it could be the case. Alnicos are very sensitive to mechanical impacts. :bash: :(

Make it any sense?
:cheers:
Regards