Page 1 of 2 12 LastLast
Results 1 to 15 of 18

Thread: LE 15 A changing impedance with time

  1. #1
    scorpio
    Guest

    LE 15 A changing impedance with time

    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,

  2. #2
    Senior Member Flodstroem's Avatar
    Join Date
    May 2004
    Location
    Norway
    Posts
    401

    Mine to have the same problem

    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)

    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
    Flodstroem

  3. #3
    Senior Member
    Join Date
    Apr 2003
    Location
    Richmond Hill, Ont.
    Posts
    4,682
    Quote Originally Posted by scorpio
    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 .


  4. #4
    Administrator Robh3606's Avatar
    Join Date
    Apr 2003
    Location
    Rocinante
    Posts
    7,984
    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

  5. #5
    Senior Member edgewound's Avatar
    Join Date
    Jun 2005
    Location
    Southern California
    Posts
    2,672
    Quote Originally Posted by 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
    I agree with that diagnosis....probably the lead wires. They can be replaced.
    Edgewound...JBL Pro Authorized...since 1988
    Upland Loudspeaker Service, Upland, CA

  6. #6
    scorpio
    Guest
    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...

  7. #7
    Junior Member Lakanta's Avatar
    Join Date
    Nov 2004
    Location
    Germany
    Posts
    29

    Zmax/DCR shifts

    [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)

  8. #8
    RIP 2011 Zilch's Avatar
    Join Date
    Jun 2004
    Location
    Berkeley, CA
    Posts
    10,036
    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....

  9. #9
    Senior Member jarrods's Avatar
    Join Date
    Feb 2004
    Location
    Australia
    Posts
    179
    Quote Originally Posted by scorpio
    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

  10. #10
    Senior Member edgewound's Avatar
    Join Date
    Jun 2005
    Location
    Southern California
    Posts
    2,672
    Quote Originally Posted by jarrods
    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.
    Edgewound...JBL Pro Authorized...since 1988
    Upland Loudspeaker Service, Upland, CA

  11. #11
    scorpio
    Guest
    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,

  12. #12
    Administrator Robh3606's Avatar
    Join Date
    Apr 2003
    Location
    Rocinante
    Posts
    7,984
    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

  13. #13
    Senior Member
    Join Date
    Apr 2003
    Location
    Richmond Hill, Ont.
    Posts
    4,682
    Quote Originally Posted by scorpio
    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 .

    Quote 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)
    - 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 .

    Quote Originally Posted by scorpio
    - 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 ? )

  14. #14
    scorpio
    Guest
    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'...

  15. #15
    Senior Member
    Join Date
    Apr 2003
    Location
    Richmond Hill, Ont.
    Posts
    4,682
    Quote Originally Posted by scorpio
    The question I have to solve for myself now, is it worth the cash,
    - Is it a lot of cash ?

    Quote Originally Posted by scorpio
    ( 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 .

    Quote Originally Posted by 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 .
    - 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 !

Thread Information

Users Browsing this Thread

There are currently 1 users browsing this thread. (0 members and 1 guests)

Similar Threads

  1. time delay problem JBL 4343 (3143 network)
    By B&KMan in forum Lansing Product Technical Help
    Replies: 30
    Last Post: 03-20-2007, 10:09 AM
  2. Standard Time
    By edgewound in forum Forum Feedback
    Replies: 4
    Last Post: 11-03-2005, 05:37 AM
  3. Surround break in time?
    By dblaxter in forum Lansing Product Technical Help
    Replies: 3
    Last Post: 07-06-2004, 07:23 AM
  4. BSS FDS-388 and 366 Omnidrive
    By jim henderson in forum Electronic Crossovers
    Replies: 18
    Last Post: 10-10-2003, 08:00 AM

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •