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Old 20th April 2005, 02:57 PM   #1
diyAudio Member
Join Date: Feb 2002
Location: Germany
Default More field simulations: T-pole (Vifa classic series)

Next thing I took apart was an old Vifa M25WO, probably WO41 I got from ebay overstock and that had noticable clicking when driven with LF sines (so I got a free replacement).

On taking of the bottom plate (heating on kitchen stove), I found that the magnet was poorly centered, and slightly covering the gap area on one edge.

This was one of the old designs, with a turned pole piece that is inserted into the stamped back plate. It is heavily undercut, which makes for a more symmetrical field. Also, the undercut region is in saturation, which is nice in that it lowers the inductance and inductance variation with travel, and in that it keeps field modulation from spreading, at least if the AC modulation is small compared to the DC saturation.

Note that I estimated the gap width at 2 mm (I yet have to take of the moving parts to measure the inside diameter of the top plate. The gap might be 1.75 mm, which would make for a slightly stronger field inside the gap.
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Old 20th April 2005, 03:07 PM   #2
diyAudio Member
Join Date: Feb 2002
Location: Germany
Default field symmetry

The next picture shows the field along the coil (14 mm long) + nominal travel range (+/- 4 mm) + 2 mm (I was originally mistaken about the linear travel).

Notice how nicely linear and symmetrical the slopes are down to about 20% of the maximum value.

Note that I had to displace my curves by 0.2 mm with respect to the center of the top plate to get symmetry after mirroring (the pole plate is flush with the top plate at its lower side and extended by about 1.5 mm on its top side).

The coil in rest position would extend from 5 to 19 mm on the x-axis. As it moves, the change in Bxl per mm will be a couple of %.

This would be a beautiful design for a midrange, i.e. with a coil that is a couple of mm shorter and hence has less linear travel.

Two words of caution:
- I should really try to find out how to form the scalar product of the B-vector and the coil z-axis.
- The coil might be missing a couple of turns on the outer layers at its top end to compensate for the asymmetry, I can tell when I have taken everything apart.
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