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Thread: Driver Magnet Size

  1. #1
    jimd
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    Driver Magnet Size

    Does anyone out there know the magnet size/weight of the smaller 175/2410 and the larger 275/2420/85 1" throat drivers? I was once told they all have the same size magnet, just different magnetic pots, I wonder about that.
    While ALTEC would tell you these things in their spec sheets, JBL only listed the magnetic structure weight, not the true magnet size, which is what I want to know. So, if you have that information, please let me know, JIM

  2. #2
    Dang. Amateur speakerdave's Avatar
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    According to my listing from the late 70's the LE175 and LE85 have different flux densities, 16000 Gauss vs. 19000. Since they use the same diaphragms, and therefore have the same gap geometries, that means the magnets on the LE175 are smaller. The field intensity is the number that matters here, I think.

    David

  3. #3
    jimd
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    Dave, this my not be the case. They may have the same magnet, the larger structure of the 275/2420/85 may be more efficient and raise the flux density.

    The ALTEC 802 had 1.2Lbs of alnico ring magnet. The LE 175 is exactly the same exterior dimension as the 802. The two drivers, except for cosmetics are nearly identical in performance. This is not to be unexpected since the diaphragm dies for both companies were made by Mr. Lansing. The suspension tangents in the surrounds of the respective drivers do run in opposite directions, however this is only because Mr. Lansing wanted to distance his later driver from that of his former employer, ALTEC. The 2420 size pot was an attempt to improve on the smaller unit, and was, from my experience, an almost unpreceptable difference.

    So, I continue my quest for the magnet weights of the two drivers. JIM

  4. #4
    Senior Member Steve Schell's Avatar
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    I believe that the 2420 does have a larger Alnico V magnet than the D175, but this is going from memory. Generally in a compression driver, when it is well designed like a JBL, the entire return path will have at least three times, and often four or five times the cross sectional area of the inner pole piece at the voice coil gap. This places the operating point of the material in the return path well down the BH curve and the material used is not terribly critical as long as it is quite low in carbon and has low reluctance. When this is the case (usually is), the rest of the circuit is far from saturation and adding more material would not materially alter the flux density in the gap, which is pretty much determined by the magnetomotive force of the magnet and its ability to saturate the pole tips. If there is enough magnet then exotic pole tip materials like Permandur will permit a higher saturation flux density, but I don't think this is the case with any of these JBL drivers.

  5. #5
    jimd
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    Thank You Steve,

    You are probably right, I wonder if anyone anywhere knows the magnet size short of dissembling a driver and measuring it. Might be my only option. JIM

  6. #6
    RE: Member when? subwoof's Avatar
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    size matters....

    I have a single 2420, LE175 , 2440 and K140 alnico magnets that are weak in my "later" pile in the shop corner...

    I will bring them home tonight and post pics / dim.

    BTW the 2420 mag is bigger that the 2410 if I remember.



    sub

  7. #7
    jimd
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    Sub,

    Thank you for your offer. Too bad you live so far away, I have access to a magnet charger. Was just about to think about buying a dead one to take apart myself,if you want to sell one, let me know.

    On another note, as most of you know, I grew up working with ALTEC products and know them inside out. I have had some contact with JBLs, but not near as much, and as such am not near as versed in the product. I know, the 275 and LE85 are identical. The 2420 is similar and has a different back cap, what is the difference between the 2420 and 2421? Never too late to learn, JIM

  8. #8
    Senior Member edgewound's Avatar
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    The only difference in the 2420 and the 2421 is the stock diaphragm. 2420 had half roll suspension, 2421 has diamond embossed suspension.
    Edgewound...JBL Pro Authorized...since 1988
    Upland Loudspeaker Service, Upland, CA

  9. #9
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    Quote Originally Posted by edgewound
    The only difference in the 2420 and the 2421 is the stock diaphragm. 2420 had half roll suspension, 2421 has diamond embossed suspension.
    - That's half right .

    - The 2420 (like the 2410 ) had a tangential ( swirled ) surround .

    - The 2440 has the half roll suspension . ( Also, all the phenolics use a form of the half roll suspension )


  10. #10
    Senior Member edgewound's Avatar
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    Quote Originally Posted by Earl K
    - That's half right .

    - The 2420 (like the 2410 ) had a tangential ( swirled ) surround .

    - The 2440 has the half roll suspension . ( Also, all the phenolics use a form of the half roll suspension )

    Sorry ...pre-senior moment...I guess it's better to be half right than totally wrong . Thanks for the correction, Earl...shoulda confirmed before I hit the submit button.
    Edgewound...JBL Pro Authorized...since 1988
    Upland Loudspeaker Service, Upland, CA

  11. #11
    RE: Member when? subwoof's Avatar
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    it's the same...old song

    The magnets are the same size. Surprise surprise...



    sub


    Quote Originally Posted by jimd
    Dave, this my not be the case. They may have the same magnet, the larger structure of the 275/2420/85 may be more efficient and raise the flux density.
    Attached Images Attached Images  

  12. #12
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    More Efficient Return Circuit ?

    Wow,

    - Great info Sub !

    - I think anyone interested in magnetic circuit design will find this all very intriguing .

    - Are you able to get an accurate weight on those magnets ?



  13. #13
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    Quote Originally Posted by edgewound
    Sorry ...pre-senior moment..
    Think nothing of it .
    I have those moments all the time, just about always before my second cup of coffee.


  14. #14
    RE: Member when? subwoof's Avatar
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    the bathroom scale lies....

    Well dealing with 75lb+ amplifiers and even heavier speakers, a less than 10lb accurate scale is something I have yet to install in the shop.

    However I will pop by the post office and use the "accurate" one in the customer service area.

    Assuming the residual magnetism doesn't set off alarms or ruin the scale, we shall know soon.

    sub

  15. #15
    Senior Member Steve Schell's Avatar
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    Surprise, surprise!

    Well gee whiz, I stand corrected. Thanks for the great picture, subwoof.

    This brings about a new series of questions. What is the spec for flux density for the two driver types, and what do they actually measure in practice? I've got a whole bunch of rusty surplus 2420 bodies, though I'm a bit thin on 175s; only have a couple of them in vintage systems. I will have to measure their flux density and report back.

    The ALE company in Japan makes a line of Alnico compression drivers, several of which seem to have really humongous magnet structures. Their best line features the use of Permandur for the entire return path, which up to now has seemed to me to be a pure extravagance. Maybe not!

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