Peerless CSC and Vifa TC assymmetrical layers of VC windings

[capslock]

I took apart a Peerless 850128 (CSC217R, moving parts identical to HDS205 except for 4 vs. 2 layer VC) and a Vifa TC14 proto with fiberglass membrane. The trick is to soak them in hot soapy water for a couple of hours. The glue for the surround/basket and spider/basket joints turns whitisch and you can carefully peel off the parts (it's not PVA because it does not really turn soft, maybe some latex dispersion). With the CSC, you'd better take off the lacquered paper dust cap first, which is done by short soaking and use of a scalpel blade.

Anyway, in the Vifa, the bottom end of both windings where ending right on top of each other. On the top end, the 2nd winding ended 1.2 mm before the 1st, which is a lot considering there is only +/-3 mm overhang.

The CSC has a nominal overhang of +/-5.5 mm. The 1st and 2nd windings end flush on both sides. The 3rd and 4th are flush with them on the bottom end but on the top side, the 3rd ends about 2 mm early, the 4th 3.5 mm.

This will effectively make for an assymmetrical Bxl-curve. With the pole piece not being undercut and the pole plate being extended by no more than 0.5 - 1 mm, the stray field will be more extended on the bottom side, hence the assymetries of B-field and winding (l) distribution would appear to add.

I have never seens this strange winding technique in any driver with a vented spider, i.e. one where one could look at the top end of the VC without taking the thing apart.

[capslock]

I took the driver apart, measured the geometry of the parts and calculated the field with Maxwell 9 SV. I assumed 1008 steel and a Ceramic 5 magnet (Ceramic 8 does not make a huge difference).

Here's a 2D-rotational symmetric plot of B. The rotational axis is the non-existent pole piece vent.

[capslock]

The green graph is the B-field along the center of the gap (i.e. r=16.875 mm) and from -14 mm to + 14 mm above the center of the top plate.

Hence, 14 mm = vertical center of gap and center of voice coil,

5.5 and 22.5 mm = first and last winding of VC when in rest position,

0 and 28 mm = first and last winding of VC when displaced by +/- x_lin

[capslock]

The purple graph is the B-field along the same line, but with top and bottom reversed. This is essentially a symmetry check. As you can see, the symmetry is not too good. The stray field below the gap is stronger than the stray field above the gap.

It is interesting to see that you can get a flat B curve only along the inner 3.9 mm of the 6 mm gap. At the top and bottom end of the gap, the field is already down to 0.8 T = - 15% of the plateau.

[capslock]

Even without having done the integrations of the force along various positions of the VC (if anyone's mastered the Maxwell calculator menu, drop me a line), I am beginning to understand how it works:

I was intially confused because I was looking at absolute B and wondered why the top end of the VC, which has a lower density of turns than the center and bottom end, would be placed in the area of the weakest field.

But the secret is in looking at delta force integrated over the full length of the VC. Consider the VC to be in the rest position, i.e. extending from 5.5 to 22.5 mm, initially. Now, let it move inwards (downwards) so that it extends from 0 to 17 mm. As it travels these 5.5 mm downwards, the lower end sees a slowly decreasing field, but the upper end sees a strongly increasing field. So it is the asymmetry in the rate of change of the stray field that is causing potential problems. Now if the upper end of the VC has a lower winding density, this offsets the strong increase of B.

The asymmetrical winding technique helps to symmetrize the inductance curve, too, even if this is probably a second order effect. As the coils moves down, there is more steel enclosed in the VC, acting as a core. With the lower winding density, the increase in inductance with inwards travel is not as marked as it would be with uniform winding density.

[capslock]

First of all, I shaved 0.5 mm off the radius of the pole piece to make room for a copper cylinder. The caused the plateau to drop from 0.93 T to 0.81 T. At the same time, the flat region became wider, 5 mm compared to 3.9 mm before. Essentially, I took the top off the B curve, the sides didn't get much better.

Next I simulated what would happen if I reduced the magnet height from 20 to 15 mm. I happen to have a stash of 5 mm dia, 10 mm long Neo 42 magnets. On the quick, I took Neo35 in the simulation (the difference is like 10-15%) and 5 mm 1010 steel washer used as a spacer.

First thing to notice is that the field in the gap dropped from 0.8 to 0.4 T, even with a solid ring of 5 mm of Nd! The length of the Nd magnet does not contribute too much, but the area does. If I were to buy new Nd magnets, I'd probably go for 5 mm length and greater diameter and use two 5 mm steel spacers.

Anyway, the field now looks much more symmetrical!

[capslock]

As you can see, the top 50% of the B curve look a lot better now. The outer stray field can probably be symmetrized by carving out the pole piece, similar to the construction of the Usher 9950 tweeter (cross-section at Northcreek) or the XLS subwoofers (there is also a cross sectional view some place on the d-s-t site).

Getting the field back up to slightly below 1 T will take an awful number of Nd magnets. I should consider reusing the original 20 mm magnets and simply glue a 5 mm steel plate onto the top of the pole piece to obtain the extented pole piece.

Does anybody know how much magnetization a ceramic magnet looses when the return circuit is opened and closed again (assuming it was orginally charged with the circuit in place)?

[DanWiggins]

Excellent info! And Maxwell V9 is nice, isn't it? The 3D with transient is really cool, but $$$!

For steel, it's probably better to estimate 1010 grade of 12L14; those seem to be most common with overseas production (I'm 99.9% sure the steel in those drivers comes from India/China). Also, probably want to look at Y30 grade magnet; Y35 or Ceramic 5 isn't used too much.

One thing to note with Neo - this is one case where height is important. I've found if the Neo is thin (say less than 6mm) a good amount of the flux simply shorts back around itself, through the air. Thicker magnets are needed here, as well as more area.

You can definitely see the impact of cutting back to add a copper shorting sleeve on the pole - a ~10% loss in flux. This is why Peerless and many others will simply place a ring down below the top plate, and leave the gap itself untreated.

Lastly, about the winding length differences. A difference of 1.2mm is pretty big, but not that uncommon; typically you'll see between 2 and 4 turns differences, unless you ride the voice coil winders really hard to actually wind them properly stacked (and truncated).

Dan Wiggins
Adire Audio

[DanWiggins]

Originally posted by capslock
But the secret is in looking at delta force integrated over the full length of the VC. Consider the VC to be in the rest position, i.e. extending from 5.5 to 22.5 mm, initially. Now, let it move inwards (downwards) so that it extends from 0 to 17 mm. As it travels these 5.5 mm downwards, the lower end sees a slowly decreasing field, but the upper end sees a strongly increasing field. So it is the asymmetry in the rate of change of the stray field that is causing potential problems. Now if the upper end of the VC has a lower winding density, this offsets the strong increase of B.

The asymmetrical winding technique helps to symmetrize the inductance curve, too, even if this is probably a second order effect. As the coils moves down, there is more steel enclosed in the VC, acting as a core. With the lower winding density, the increase in inductance with inwards travel is not as marked as it would be with uniform winding density.

I missed this earlier... Yes, many people forget that BL is both B and L - flux and voice coil - and that it is the total integrated flux over the voice coil. Making the windings assymmetric can linearize or at leas symmetrize the BL curve.

This is the foundation for TC's LMT motor stuff - add extra windings/layers at the end of the voice coil, and a few extra in the middle as well, to kind of create a reverse XBL^2 motor; rather than two or more high flux regions, you have two or more high L regions.

Downside, of course, is incrreased Mms, Le, and loss of B because of the thicker gap required for the extra layers...

Dan Wiggins
Adire Audio

[ShinOBIWAN]

Wow guys, I consider myself a techie but at least 50% of what your talking about went straight over my head

I suppose on the plus side I understood half whats been said