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.
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.
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
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
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
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
had you used these woofers conventionally prior to the conversion?
ie. was the apparent assembly defect audible?
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
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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