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Thread: JBL driver field coil conversion

  1. #31
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    Quote Originally Posted by Predrag Dukic View Post
    Element contents are usually expressed in wt. percents, not atomic..

    0.23 pct is a usual construction steel..
    For 1-1.4 Tesla gap field, nothing more fancy is needed.
    Armco iron goes up to 1.9-2.0 Teslas, and Permendur to 2.3-2.4 Teslas


    Yes, but not without any other alloying elements. No phosphorus or sulfur. That's not normal construction steel. Its just iron and carbon.

  2. #32
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    Well the alnico is going back in the woofers for now. I missed a very important piece of info. With the cast return structure and clearance for the voice coil going back into the structure. I'm left with very little room for the solenoid. So its back to the drawing board for the woofers.

    I will have to design a new return structure for then. Then buy all the material and machine the return structure. It will end up costing way more then I expected it too.

    But converting the 2440 is still going as planned.

    if I build the new return structure I'll design the whole driver from the ground up. So the only jbl part would be the frame means it won't be here. But I don't know if I'm up for it. Since there's a lot I don't know I would spend a massive amount of time in trial and error.

    Oh well, that's life.

  3. #33
    Administrator Mr. Widget's Avatar
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    Quote Originally Posted by NickH View Post
    But I don't know if I'm up for it. Since there's a lot I don't know I would spend a massive amount of time in trial and error.
    It is always good to know one's limitations.



    Widget

  4. #34
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    For now.


    I don't believe in those kinds of limitations. Its just a reason to learn how to do something I previously didn't know.

  5. #35
    Administrator Mr. Widget's Avatar
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    Quote Originally Posted by NickH View Post
    For now.


    I don't believe in those kinds of limitations. Its just a reason to learn how to do something I previously didn't know.
    Sure... just saying I wouldn't want open heart surgery performed by a second year medical student.


    Widget

  6. #36
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    Quote Originally Posted by NickH View Post
    Yes, but not without any other alloying elements. No phosphorus or sulfur. That's not normal construction steel. Its just iron and carbon.

    You are right, I didn't take notice of that fact. But existance of P and S is negative in any steel. So we can maybe say "very good" ordinary steel....

  7. #37
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    Quote Originally Posted by Predrag Dukic View Post
    You are right, I didn't take notice of that fact. But existance of P and S is negative in any steel. So we can maybe say "very good" ordinary steel....

    That's not necessarily true.


    Very true Widget

  8. #38
    Senior Member ivica's Avatar
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    Quote Originally Posted by NickH View Post
    Well the alnico is going back in the woofers for now. I missed a very important piece of info. With the cast return structure and clearance for the voice coil going back into the structure. I'm left with very little room for the solenoid. So its back to the drawing board for the woofers.

    I will have to design a new return structure for then. Then buy all the material and machine the return structure. It will end up costing way more then I expected it too.

    But converting the 2440 is still going as planned.

    if I build the new return structure I'll design the whole driver from the ground up. So the only jbl part would be the frame means it won't be here. But I don't know if I'm up for it. Since there's a lot I don't know I would spend a massive amount of time in trial and error.

    Oh well, that's life.
    Hi NickH,

    Any news about measurements with the phase-plug after closing center slits.?

    I am not so good in the construction, but I think that putting solenoid in the outer part of the magnetic circuits (as JBL have done using ceramic magnets) You will not have almost any limitations about the size and the power dissipation of the voice coil would be much easier.

    Regards
    Ivica

  9. #39
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    No I haven't yet Ivica. I decided Im going to get a real measurement mic to do it. I don't want to post inaccurate results.

    Nick

  10. #40
    Senior Member ivica's Avatar
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    Quote Originally Posted by NickH View Post
    No I haven't yet Ivica. I decided Im going to get a real measurement mic to do it. I don't want to post inaccurate results.

    Nick
    Hi NickH,

    any news ???

    regards
    ivica

  11. #41
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    Things are in a holding pattern. I've had a bit of bad health and haven't been able to work much. So money has been tight.
    Its nothing life threatening, just physically impairing. A Mic is on the buy list. Just has to wait for the money.
    It will all happen in do time. I'll let you know when I've got some info.


    Nick

  12. #42
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    Well its been almost a year but I'm finally moving forward on this. The only difference is I'm using altec 416 as the guinea pigs. I'm going with low resistance coils. I'm using 950 feet of 18 gauge wire. The first one I will have to experiment with the settings. I will also be potting the coils in the magnet pot with heat conductive epoxy. I know the big complaint with field coils is the heat generation which is why I'm using heat conductive epoxy.

    I originally thought about building a power supply using tungar recruiters, until I saw there prices on ebay. I'm going to use normal silicon fast recover diodes first off. I plan to sim the supply with a pi filter. The last stage will be a lt1083 constant current sources.


    The only issue is I don't have a magnetometer for measuring the field in the gap. I figured I can do it my making other measurements though. I'm after a much lower qes. Since I'm pretty certain the gap geometry is the same I might cathodyne moving assembly to a 515g cone.
    The drivers are actually GPA 416-8b and I was shock at what I saw when I removed the first magnet. The coil was glued in the cone at a 15 degree angle. Plus the coil wasn't centered in the gap. Since I've had these driver for quite a few years there's no warrantee. I'm doing 2 at a time so I've got 2 more to convert. Hoping there's no more surprises.

    I'll post some photos as I progress.

    Nick

  13. #43
    Senior Member louped garouv's Avatar
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    had you used these woofers conventionally prior to the conversion?

    ie. was the apparent assembly defect audible?

  14. #44
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    Yes I did use them. I never noticed an issue. I'm quite surprise I didn't.

    I'll try and take a few photos and post them.


    Nick

  15. #45
    Senior Member Steve Schell's Avatar
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    I was unaware of this thread, though I have corresponded with Nick a few times. A few general thoughts... it is feasible to convert many alnico drivers to field coil because their ancestors were field coil to begin with. The Altec 288 of 1945 began with the quite similar Lansing 284 of 1936. The Altec 802 (1945) and JBL D175 (1946) are lineal descendants of the Lansing 801 small format driver of 1937.

    When these drivers were converted to permanent magnets, minimal changes took place and are easily reversed. The Lansing drivers used a straight center pole that maintained the inner pole piece diameter all the way to the front plate. Alnico versions added a skirt to the center pole to match the outer diameter of the interior Alnico V ring magnet. Almost every modern f.c. converter gets this wrong and creates a too-wide center pole, leaving inadequate room in the pot for the field coil. The field coils fitted resemble a sewer pipe in cross section, which overheat and create far too few ampere-turns and inadequate flux density.

    Proper conversion involves removing the skirt from the pole piece, machining a new pole piece extension, and winding a field coil of proper dimensions. I have written to several converters and they seem to get it right once they know the full story.

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