JBL driver field coil conversion

[NickH]

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.




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

[NickH]

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

[ivica]

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

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

[NickH]

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.

[JeffW]

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.

[NickH]

Hey Jeff. That's a eds detector on my labs scanning electron microscope. Its very similar to xrf. I was trying to find a picture of the" coherent wave phase plug". No dice. I figured it would machine like crap, but its not to bad. It got a percent carbon and a trace of copper. The copper wasn't enough to put it in the spectra. I'm planning to use 1020 steel. The carbon under a very low and so are the other alloying elements. The only better choice would be 1008 but I can't find any.

Steve Schell told me 12l14. 1020 has less extra elements in it then 12l14, and carbon to. Even less then my sample. And I did clean them before testing them. The picture is of a shaving at around 750x in the electron microscope.


Nick

[JeffW]

Oh, the ol' scanning electron microscope! Now there's a cool toy I'd like to check out.

Yeah, making a Coherent Wave Phase Plug would be a PITA. Get your field coil design down on a 2440, then snag up a pair of the newer drivers with the newer phase plug and slap a field coil mod on them. Sounds like you are already in contact with The Man when it comes to field coil drivers, that's a good way to get going.

[NickH]

Yep, the sem is a handy toy, albeit pricey. We have a cheap one. Its was only $125000. There's another reason I picked the 2440. Ferrite driver aren't conducive to the conversion. It is possible but the coil would be a big one. The alnico's are easier, much easier.

[ivica]

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.

Hi NickH,

Owing to some works that emphasized that the diaphragm center of the compression driver produces the larges distortion and phase incoherence I think that making hole in the phase-plug center would be bad idea. Anyhow in order to check the results of your tedious work You have to 'establish standard' measuring procedures, so the results can be comparable between each upgrades done. At least not only the FR response, but THD too.

I think that some improvements can be done to mentioned 2440 driver mainly around its phase plug. Even JBL from model 2446 (2450,2351, 2452) has mentioned that they have improved sound phase coherency by adjusting the lengths (and the shape) of the horns in the phase plug slits.
If taking that in mind and some other work, I can suggest (among lot of the others) :

1. US Patent US20060034475 ( http://www.google.com/patents/US20060034475 )
2. US Patent US20080192972 ( http://www.google.com/patents/US20080192972 )

I believe that some improvements can be get (especially at home listening levels where maximal efficiency is not of the greatest importance).

Unfortunately the production of such precise and 3-D shape product as phase-plug is fare to be easy, but just because "of that" it is much more challenge.

Regards
Ivica

[NickH]

Thanks that's some good info. I printed them out and Ill read them. If only patents were a little easier to read. Its like reading the tax code, LOL. At least there's pictures to see what they try to explain in verbiage.

[ivica]

Thanks that's some good info. I printed them out and Ill read them. If only patents were a little easier to read. Its like reading the tax code, LOL. At least there's pictures to see what they try to explain in verbiage.

Hi NickH,

The forms of the patents are not usual reading, but the point is relatively easy to find out. In one it is written that the central part of the phase plug is filled with some kind of absorption material (or foam) in order to absorb noise and distortion produced by center part of the diaphragm (that is why I can not suggest to make center hole through the phase plug), while on the other patent there is suggestion how to place slits, and what would be the expectable width of the slit in order to keep the pressure (speed) constant over the whole phase plug surface, in that way (under correct geometry) produce in-phase planar wave front in the driver throat (reducing sound cancellation due to the different phase of the wave front produced by different slits-horn)

Regards
Ivica

[NickH]

Yes I know. I can interpret them. I've done it many times. I haven't looked over the 2 patents yet. I did print them. Im going to look over them a little better after I post this thread. And do it more thoroughly tonight when Im home. But what you've said about blocking the center portion of the phase plug seems like an easy experiment to try. I could pack the center 2 paths with cotton and see how it sounds. Maybe use a few dabs of tacky glue to hold it in place. If I hear an improvement, use some silicone. But I would leave one unchanged for comparison.

[NickH]

Ok I printed out a few trees worth of phase plug reading material. Some new, some old. Should keep me entertained for a while. At least until I start to wind.

[ivica]

............ But what you've said about blocking the center portion of the phase plug seems like an easy experiment to try. I could pack the center 2 paths with cotton and see how it sounds. Maybe use a few dabs of tacky glue to hold it in place. If I hear an improvement, use some silicone. But I would leave one unchanged for comparison.

Interesting idea. I think that it would be "better" to block (if possible) center exit part then to block slit on phase plug 'top' (near the diaphragm). But some measurements before and after "the operation" would be welcome .

Regards
Ivica

[NickH]

I was thinking that. If its blocked by the diaphragm I would think it would cause some issues with reflection. But if I fill the cavity with a sound absorber and seal the bottom, makes more sense.See what you did? Got me going on a damn tangent, .Well I'm waiting to buy copper anyways. Should change the thread title to filed coil conversion and phase plug modification. If a mod sees that can the change the title?I'm thinking I might try this on a 2445 first. See if I can tell the difference. I need some new diaphragms though, soon.does anyone know what the phase plug on a 2446 is made from?Nick