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  1. #1
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    JBL driver field coil conversion

    I have decided to convert some alnico jbl driver to field coils. I thought I would document my travels on here. I'm doing both 15" woofers and 2440's. I started by taking apart the magnet assembles and taking measurements. From there I calculated to size of the coil. I'm going for a middle of the road approach and I'm using 18 gauge wire. After I had the magnets out I started machining the pole pieces of the 2440. I attached a pic. This was kind of the point of no return, LOL. Once the material is off I cant put it back on.
    I'mIm going to analyze some of the shavings to get the chemical composition of the steel using. Luckily I work in a metallurgical lab and I can do that kind of stuff.I'mIm going to use 1018 steel. Steve Schelrecommendeded 12l14 steel but after looking at mill certs I decided to use 1018. It has less alloying elements in it. The carbon is about the same as 12l14 too. Plus its a little cheaper.

    Name:  2013-05-28 08.35.11.jpg
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    I hoping to buy a 11 pound spool of 18 gauge wire this weekend so I can start winding coils for the woofers. The woofer coils will be 1 1/2" tall with an outer diameter of 5 1/8". The inner diameter is 3 1/8". It will be about 1000 turns of 18 gauge.

    Now Im sure some of you guys will give me a hard time for this. But remember, the change over to perminent magnets was for cost reasons and not quality. Plus I find it fun. The drivers are d130's and if I recall, the first d130's were filed coil drivers. But I am going to do some experimentation with different cones, voicecoils, and spider. Just to see what ts parameters I get.


    More to come as I proceed.


    Nick

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    Im surprised in the chemical analysis of the pole piece. There 1 % carbon. But that all thats in there.

    Name:  2440 pole piece.jpg
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    Senior Member ivica's Avatar
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    Quote Originally Posted by NickH View Post
    Im surprised in the chemical analysis of the pole piece. There 1 % carbon. But that all thats in there.

    Name:  2440 pole piece.jpg
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    Hi NickH,

    Very interesting work You are ready to do. It would be interesting to see the results.
    With mentioned 2440 driver, I think much can be done in its improvements by improving the phase plug, not the magnet itself.Here I think about fist to introduce 5 slits instead of 4, then some different slots width, even some suggestion was to put some acoustic absorption material in the phase-plug center. Lot of that can be find on Internet and US patents.

    Regards
    Ivica

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    I know the ones on the 2440 are Bakelite. The plugs on my 2445's look to be metal. Im going to try and do a temporary mod on one of the 2445 phase plugs tonight. I have some rope claulk that's the consistency of clay that Im going to seal some cotton into the 2 center most ports with. It doesn't harden so it will be easy to remove if I don't care for it. Ill report back. Maybe If I feel energetic Ill cart it outside and do some measurements this weekend comparing it to an unmodified 2445.

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    Senior Member ivica's Avatar
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    Quote Originally Posted by NickH View Post
    I know the ones on the 2440 are Bakelite. The plugs on my 2445's look to be metal. Im going to try and do a temporary mod on one of the 2445 phase plugs tonight. I have some rope claulk that's the consistency of clay that Im going to seal some cotton into the 2 center most ports with. It doesn't harden so it will be easy to remove if I don't care for it. Ill report back. Maybe If I feel energetic Ill cart it outside and do some measurements this weekend comparing it to an unmodified 2445.
    Waiting for the results...

    Regards
    Ivica

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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.

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    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

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    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

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    Quote Originally Posted by NickH View Post
    Im surprised in the chemical analysis of the pole piece. There 1 % carbon. But that all thats in there.

    Name:  2440 pole piece.jpg
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    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

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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.

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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....

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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

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    Ivica your getting ahead of me. But that's next on agenda. But machining a phase plug is quite a bit more complex then what I'm doing now. But I wonder what would happen if I drilled a hole right thru the center.


    All the fancy stuff is to tell me what metal will work to carry the field.


    But what is wrong with the phase plug on the 2440 to begin with? There are quite a few people who like the driver.

    I'm doing the field coil conversion for the tweekability in the drivers.

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    Starting with about the 2446 IIRC, JBL went to a Coherent Wave Phase plug or something like that. Not going to hurt anything experimenting on a 2440.

    What PMI technique is that, XRF? Looks like nearly pure iron, ought to be the mutt's nuts for machinability. I deal mainly with different grades of carbon steel, lots more trace elements to keep track of, but we only PMI HA stuff like stainless or SB material.

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    Mod done. I did it how I said I would. Cotton and moretite caulking rope. Very easy to undo if I want.

    I will post a pic tomorrow.

    Nick

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