Printer Friendly Forum Posts - benefits of split coil design

The Diamond Hex speakers are a pretty impressive speaker too. I would stick with them unless you are capable of changing the crossovers to compensate.

Here's how Dan Wiggins described split coil and compared it to XBL^2 (used in the Mpyre mid):

"Split Coil Design
The original concept to this approach was patented in the early 1970s, and used off-and-on in a few drivers here and there. As I hinted at earlier, we're actually licensing XBL² to the owner of this patent; they've used both topologies and decided that when all things are considered, XBL² simply works better. If that's not the biggest "plug" for the superiority of XBL² over the Split Coil, I don't know what is...

Anyway, Split Coil basically operates as XBL² but in reverse; rather than having dual gaps with a single coil, you have dual coils with a single gap. Here's the basic design:

You have two voice coils - each wound in the same direction - and seperate them. You space them such that there is ~50% of the gap height between the two windings. Then you make each winding about 50% of the height of the gap as well (so the winding:separation:winding:gap ratio is 1:1:1:2).

As the voice coils - on their common former - move up, an equal amount of the upper winding leaves the gap as enters the gap from the lower winding. And it continues to move that way until the lower winding starts to leave the gap. So you get some good stroke!

The advantages of this approach are:

1. Long stroke. You can get long stroke from a total voice coil height that's not too much longer than the gap height itself.

2. Flat BL. You do get flat BL, just like XBL².

3. Royalty free. The patent on this approach expired in the early 90s.

There are several downsides, though:

1. Higher moving mass compared to XBL² from the dual sets of windings. This is actually the biggest reason that the split coil inventor is moving to XBL². Much lower mass for a given stroke.

2. Higher inductance versus XBL² because of the multiple turns. And again, this brings with it lower bandwidth, greater flux modulation.

3. EXTREME sensitivity to winding length and winding spacing. Get the spacing off by a turn of wire, and you get a pretty significant peak in the BL curve center. Likewise, make it a shade too long in spacing, and you get a big dip. Much more sensitive than XBL² to winding errors. Winding tolerance is much harder to control versus machining tolerance, as it takes a LOT more effort to get proper stacking for the correct lengths.

4. Low efficiency. Because you're purposefully avoiding a good chunk of the gap, you trade off high flux (in the gap) for low flux (fringe). You can get high overall BL, but it's not very efficient at flux utilization, being somewhere between 25% and 50% efficient in terms of flux usage.

5. Motor size. Because the overall voice coil effective length is much longer, you simply need to have a deeper/taller magnet stack. And that brings the issues with rocking, etc. with it.

6. Shorting-ring-versus-flux tradeoff. I should have brought this up with the other motors, too... If you want to use a shorting ring, the best place to use it is in the gap (where the steel is closest to both sides of the voice coil), and then plate the pole in its entirety. However, that means you widen the gap, so you lose flux. So do you want lower inductance, or lower efficiency? That's often the tradeoff."