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Ironless Motor Assemblies and STEALLUS

smowry

smowry

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STEALLUS is an acronym that I used to designate an alternative audio transducer topology. STEALLUS was the result of the combination of stealth and phallus. The roots of my acronym was edited out of my Voice Coil magazine published discussion in 2007 by Ed Dell. Pearl HiFi still has a copy of the two-part discussion posted at their website.
https://pearl-hifi.com/06_Lit_Archive/14_Books_Tech_Papers/Mowry_Steve/Steallus_Motor_Design.pdf

The term stealth relates to the air core moving coil topology, while phallus relates to the geometry of STEALLUS. Like too many other audio transducer concepts, there are US Patents that claim the invention of ironless motor transducers. I participated in a study at GENELEC where it was demonstrated there there is direct relation between the Inductance of a low frequency transducer and distortion (non-linearity). And frankly, it is much worse than even what the study implied. Fortunately, GENELEC had Dr. Klippel's system identification system. Just for the record, it was I that lobbied for Dr. Klippel when I interviewed him to send a Distortion Analyzer system to Voice Coil magazine such that his marketing would be enhanced and the community could gain the knowledge from Large Signal parameter acquisition. https://pearl-hifi.com/06_Lit_Archive/14_Books_Tech_Papers/Mowry_Steve/Prof_Klippel_Interview.pdf

Well, Dr. Klippel sent a system to my main competitor who did and still does the system identification for Voice Coil and yes, "no good deed goes unpunished". I had hoped he would send a DA to me and I could support Voice Coil. He did subsequently send a dated system to me for professional courtesy but its use was limited. He did not include the positing sensing laser. Without the laser, the zero voice coil position cannot be identified.

Below is a typical Le(x) curve of an iron core transducer without any shorting rings.

1742957181895.jpeg

It can be observed that when the voice coil is displaced into the motor assembly, Inductance is increased due to the iron core. However, as the voice coil moves out of the motor assembly. the amount of iron within the voice coil ID is reduced and thus Inductance is reduced. This is an example of a dynamic inductor and a source of distortion and bandwidth limitation.

Things are even worse due the the self-inductance of the voice coil related to electrical current, i (A). Below is an example of a large signal parameter Le(i) plot of a motor assembly that appears to have a shorting ring(s) in that the curve is quasi-symmetric with a bit of -DC offset. Dr. Klippel calls this DC offset Xdc.
1742959489979.png

Having said all this, I thought that ironless motor assemblies and STEALLUS would be a worthy topic for discussion.
 
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Hey Steve, did you ever build any of these woofers? I'd be interested to see more measurements.

Seems a bit unfair to expect Klippel to send you $40K of measurements hardware for free (regardless of your marketing contributions in voicecoil etc.)

The KEF woofers have a lot more in common with a Dynaudio and Morel "double magnet" woofers that have been around since the 80's. Yes, the outer cup spaced away from the coil for the obvious reasons but this is a simple change/improvement and seems unrelated to your articles.

Wouldn't you say that the Borresen drivers are closer to what you're doing (with no steel return path)?
 
I had funding to develop the STEALLUS topology from Charlie Hansen from Ayre Acoustics. https://ayre.com/charles-hansen-1956-2017/
Unfortunately, Charlie became ill before we could complete the project and subsequently passed away in 2017. We got to the prototype stage and my funding ran out. I had some measurements from more than 10 years ago but they are gone now. After STEALLUS my focus shifted to coaxials. https://pearl-hifi.com/06_Lit_Archive/14_Books_Tech_Papers/Mowry_Steve/COAXIAL.pdf

Note that I first created the STEALLUS concept about 20 years ago! So doing. no, I retired about 10 years ago, did work on but in the past tense.

Yes, Borresen looks like pretty much what I discussed and proposed in 2005-2007. I was not familiar with the Borresen and was not aware of their ironless motor implementation until your post. I searched the US Patent database but I could not find anything. That could be because there was no invention, rather just an update to the public domain. Michael Borresen uses copper and silver element in his motor assembly. Is he making an induction heater? My material of choice for the magnet spacer would be a chunk of polyimide, an insulator with the permeability of air. My original STEALLUS concept used non-magnetic stainless steel. A highly conductive magnet spacer will make the voice coil drag through the gap. Take an aluminum pipe and and hold it perpendicular to the floor. Then drop a small magnet into the top of the pipe and it will float through the pipe and acceleration will be much less than G. Obviously if the pipe were iron, the magnet will not fall; it will stick. Then if the pipe were made of an insulator, the magnet would fall with an acceleration of G, 9.8 m/s^2.

Do you have or know of a Borresen Patent and/or Patent Application?
 
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smowry said:
Does anyone out there own Borresen's? The drivers look very interesting but everything that I can find is subjective, really super subjective.
Click to expand...
One of their speakers was reviewed on Erin's Audio Corner. Erin's conclusion was extremely... polite, but the data speaks for itself. It's pretty bad, and the distortion was nothing special. The nearfield data from the drivers doesn't show anything good either.
 
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smowry said:
Thanks but your link seems dead.

Maybe Erin didn't have the right power cord.
Click to expand...
Whoops, bad copy/paste job on my part. Fixed.

Yeah, those damn dirty electrons will get you every time.
 
Anyway, if I saw Borresen woofers on eBay, I would most likely bid on them. Most if not all of Erin's comments and measurements are system related. One really needs to see the large signal parameters, Bl(x), Le(x), Kms(x), and Le(i) to evaluate the transducers. Very High Performance transducers really need active crossovers. Borresen takes their ironless drivers and puts an inductor(s) in the signal path, huh! My recommendation to system engineers is to avoid reactive elements whenever possible. I am quite sure that many DIYer's could implement a killer system with well designed ironless drivers.
 
kyuu said:
One of their speakers was reviewed on Erin's Audio Corner. Erin's conclusion was extremely... polite, but the data speaks for itself. It's pretty bad, and the distortion was nothing special. The nearfield data from the drivers doesn't show anything good either.
Click to expand...
This X series motor isn't the iron free motor. It's an underhung with a neo magnet and copper sleeve on the pole. So, we're talking about different things.

Then again, I can't comment on the Borresen M series drivers because, other than hearing them at shows, I don't know specifics other than it's similar to what Steve had proposed all these years ago (except the large copper/silver rings).
 
smowry said:
I had funding to develop the STEALLUS topology from Charlie Hansen from Ayre Acoustics. https://ayre.com/charles-hansen-1956-2017/
Unfortunately, Charlie became ill before we could complete the project and subsequently passed away in 2017. We got to the prototype stage and my funding ran out. I had some measurements from more than 10 years ago but they are gone now. After STEALLUS my focus shifted to coaxials. https://pearl-hifi.com/06_Lit_Archive/14_Books_Tech_Papers/Mowry_Steve/COAXIAL.pdf

Note that I first created the STEALLUS concept about 20 years ago! So doing. no, I retired about 10 years ago, did work on but in the past tense.

Yes, Borresen looks like pretty much what I discussed and proposed in 2005-2007. I was not familiar with the Borresen and was not aware of their ironless motor implementation until your post. I searched the US Patent database but I could not find anything. That could be because there was no invention, rather just an update to the public domain. Michael Borresen uses copper and silver element in his motor assembly. Is he making an induction heater? My material of choice for the magnet spacer would be a chunk of polyimide, an insulator with the permeability of air. My original STEALLUS concept used non-magnetic stainless steel. A highly conductive magnet spacer will make the voice coil drag through the gap. Take an aluminum pipe and and hold it perpendicular to the floor. Then drop a small magnet into the top of the pipe and it will float through the pipe and acceleration will be much less than G. Obviously if the pipe were iron, the magnet will not fall; it will stick. Then if the pipe were made of an insulator, the magnet would fall with an acceleration of G, 9.8 m/s^2.

Do you have or know of a Borresen Patent and/or Patent Application?
Click to expand...
Congratulations on retiring! I'm not sure if I'll get to retire in a tropical paradise. Let me know if there are any loudspeaker design gigs in Thailand that I should be aware of ;)

I don't know of any patents of theirs on their motor but the former international sales guy from my work (Travis Townes) now works there, so I might ask him. He is a sales guy and very non-technical so I'm not sure how much info I'll get.
 
I just purchased 2 x LS50 Meta (red) drivers on eBay from Hong Kong. I will receive them next month. I also purchased 2 x Radian 6 in. ribbon coaxials on eBay recently for another project.

I moved to Thailand from Malaysia in January of 2000 to work for P.Audio and I never moved back to the US. I have lived in Thailand for 25 years! I love it here in Phuket.
 
smowry said:
I just purchased 2 x LS50 Meta (red) drivers on eBay from Hong Kong. I will receive them next month. I also purchased 2 x Radian 6 in. ribbon coaxials on eBay recently for another project.

I moved to Thailand from Malaysia in January of 2000 to work for P.Audio and I never moved back to the US. I have lived in Thailand for 25 years! I love it here in Phuket.
Click to expand...

I've had much the same interests. I bought one of those Radian drivers 5 years ago to take look if I wanted to do a circular planar, rather than rectangular.

They did a good job on these tweeters but the woofer and pole piece geometry on the smaller coaxials isn't good (IMO). There's a narrow conical section through the pole piece and a lot of diffraction.

The rather deep front magnet causes a peaking LP filter around 10 kHz. on these but they are still interesting. The tangential corrugation of the diaphragm and acoustic loading of the coaxial driver mounting allows them to play quite low for a planar driver this area.
 

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I also bought a TangBand 6 in. coaxial on eBay from the US but I sent it back. The impedance curve was real nasty around 100 Hz. Fortunately, it sustained cosmetic damage in transit. So I got a full refund including shipping just yesterday. eBay paid the return shipping to the US too.

1743024959492.jpeg


My Radians are made in the Philippines.
 
Yes, Pablo and Dwight Tobiano at Dai Ichi bought Radian and they ended up closing their manufacturing in California after a while.

Thankfully, they kept the quality up and are using the unique aluminum diaphragm recipe from the older radian compression drivers etc.
 
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