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pjanda1 9th November 2011 03:05 PM

Wild Burro Field Coils!
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Some of you may have seen this pair floating around at RMAF. I'm the lucky guy who got to bring them home! Dave Slagle designed and built the motor. Of course, the speaker really isn't specifically a Betsy or BetsyK, as the Q depends on the voltage at which you run the motor. You'll also notice the black cones. I've haven't posted it yet, but all new Betsys and BetsyKs have the new black cone. This pair sounds great in my OBs. I'm putting about 16V through them with an old HP lab supply. In addition to the FC/permanent magnet difference, this pair benefits from a much fancier, copper plated pole piece. All of the soft parts (cone, suspension, voice coil) are identical to a regular Betsy or BetsyK.

These aren't available for sale yet, nor can we nail down a price. It certainly is possible to build more.

Wild Burro Audio Labs - DIY Full Range Speakers

6L6 9th November 2011 03:51 PM

Ooohhh!!! Completely awesome!!

:D :D :D :D

Pano 9th November 2011 04:00 PM

Cool! :up: :up:

kevinkr 9th November 2011 04:19 PM

Very nice! :D

badman 9th November 2011 05:03 PM

cool stuff. Would love to hear what a nicer motor on your well-chosen cone etc sounds like.

ejfud 9th November 2011 08:06 PM

Pretty dang cool!

Andrew Eckhardt 9th November 2011 08:12 PM

Where does 16 volts put the total Q and how much current does it draw?

Einric 9th November 2011 10:12 PM

Looks Really Cool
What is the purpose of the field coil?
What benefit does it give?

pjanda1 9th November 2011 10:23 PM

You know, I honestly haven't measured this pair. I think they're drawing a bit over an amp. Judging by efficiency, I think the Q was in the Betsy range at around 12V. They can take up to about 18V before heat becomes an issue. But, current and voltage are of course dependent on the wire gauge. I think Dave wound these with either 18g or 20g, as it is what he had on hand. If we do a production unit, I think it would be nice to make them work with a commonly available laptop smps.

Those three nuts cap threaded rods that hold the can together. Those diminish the winding window a bit, but make the can less expensive than drilling and taping the tube (its 1/4" thick pipe). These are just prototypes, and we're trying to figure out if this is enough copper or if we need to get more turns in there. Obviously, I don't need a super low Q on my open baffles. But, some folks may want to use these in BLHs, so it will be good if we can crank the flux up pretty high. There are many potential ways to balance performance and cost. Machining is a big cost, but so is wire.

Doesn't the copper look cool? All of the FC parts (aside from the wire) are steel, but copper plated. This is how they came from the plater. They could be polished, but I like this look and it certainly fits with the Slagle/Jackson/Emia aesthetic.

Wild Burro Audio Labs - DIY Full Range Speakers

pjanda1 9th November 2011 10:34 PM


Dave can be a frustrating guy. I think he has a theory of why field coils sound better, but he doesn't like anything that sounds like marketing. He would advise that one listen to it and see for themselves.

Generally, keep in mind that the current flowing through the voice coil will, to a degree, affect the flux of the magnet. Different magnet materials can be affected differently. When I mentioned to Dave that I had read someone who proposed that field coils can lower distortion through this mechanism, he didn't disagree. But, he also pointed to a very, very expensive field coil fullranger and suggested that they may have designed the motor in such a way that it exaggerated this flux modulation. So, the jury is out on why field coils sound different, just as there is no agreement as to why different magnet materials (alnico, neo, etc) sound different. Some claim that they don't. Dave is a huge believer in field coils. The "listen and see" approach works fine if you are NYC and can go over to a friends house. In Scottsbluff, NE or Bozeman, it might be harder to find a comparison!

I was hoping someone would ask how they sound. I tend to be a bit skeptical, and I suspected they wouldn't be hugely different. I've always liked the Betsy and BetsyK, and I've been afraid that improvements wouldn't be worth the cost. Thats why I've avoided doing any tweaky stuff to the speaker. It isn't as if I couldn't stick in a phase plug or mess with the motor.

While I was worried that the field coils wouldn't be much better, I was hoping for something revolutionary rather than evolutionary. They exceed my wildest expectations. They sound very, very different in a very, very good way.

Wild Burro Audio Labs - DIY Full Range Speakers

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