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Paul McGowan gets asked about rotary subs.

Paul McGowan of PS Audio has for years posted frequent (daily?) videos on YouTube in which he answers questions sent in by people from the world over. I just watched one in which he answers a question sent by a guy in India, inquiring as to why rotary subs are not more popular. Paul gets around to completely answering that question, but before doing so says this:

 

"The Rotary Sub was invented by a guy named Bruce Thigpen, and Bruce is a VERY (Paul’s emphasis, not mine) creative inventor who used to have a company---maybe he still does---called Eminent Technologies (sic. It’s actually named Eminent Technology). And Eminent Technologies, they made some GREAT (again, Paul’s emphasis) loudspeakers. They were---if I remember right---they were planar, or electrostatic---I think they were planars, they weren’t electrostatics, but they were REALLY (Paul again) good. And I don’t know what ever happened to that, but I DO know that Bruce figured out a way to make a subwoofer that could go well below what normal subwoofers do."

 

But this post is not about the Eminent Technology TRW-17 Rotary Subwoofer (there aren’t rotary "subwoofers", there is only one Rotary Subwoofer, the product of ET alone), it is about Eminent Technology itself. I mean geez, if Paul McGowan doesn’t know if Eminent Technology is still making planar loudspeakers, just how low IS the visibility of the company?!

To set the record straight: though Paul differentiates between a "planar" and an "electrostatic", while not all planars are electrostatics, all electrostatics are planars. I routinely see Magnepans referred to as planars (by Steve Guttenberg, for instance), which they of course are. But so are electrostatics. When Paul and Steve say planar, they are speaking of planar-magnetic loudspeakers. Both Magnepan and Eminent Technology make them.

 

The Eminent Technolgy LFT-8 planar-magnetic loudspeaker was introduced in 1989/90, and remains in production today. It has gone though a few revisions over the past thirty-three years: in 2007 an improved woofer replaced the original, with a change to it’s nomenclature: the LFT-8a. In 2015 an improved tweeter replaced the original, the new model designation being LFT-8b.

The LFT-8b remains available, and there is also a new version of the LFT-8: the 8c. The 8c consists of the same planar-magnetic panel as the 8b (which contains the midrange---180Hz up to 10kHz---and tweeter---10kHz and above---drivers), but with the monopole woofer of the 8b (for frequencies 180Hz and below) replace with a "gradient" dipole woofer (still a sealed enclosure, but with a 6.5" rear woofer added to the 8" in the front), which simply bolts on in place of the monopole woofer enclosure. Also included with the 8c is a power amp for the woofers, and DSP for the low-pass x/o filters for the woofers, time-alignment of the panels with the woofers, and equalization.

The LFT-8b retails for $3200, the 8c $4500, shipping in the U.S.A. included.

 

Magnepans are commonly discussed and owned (I own a pair), but the Eminent Technology LFT-8 remains virtually unknown (I also own a pair of the LFT-8b). Why is that? It has received rave reviews (REG in TAS, cudos from VPI’s Harry Weisfeld---who characterized the midrange of the LFT-8b as "the best I have ever heard", a number of reviews in the UK hi-fi mags), yet remains virtually unknown to the vast majority of audiophiles. I know ET has few dealers and does no advertising, but still.....

128x128bdp24
sandpat
41 posts
 

I have a pair of the LFT-8bs. When I look at them I see terrible build quality. Stuff is poorly welded together. The frame isn’t sturdy (you can move it with your hands). The legs are screwed directly into the woofer box without pilot holes or any housing for the screw to go into. The ribbons look like they’re wrinkly all over (my Apogees, also known for poor build quality back in the day, are a thousand times more solidly built). I also eventually stuffed a piece of Kleenex behind one section of ribbon because I heard it rattling back there. It worked!

But holy hell do these ETs sound great. I kid you not when I say they rival the best sounding speakers out there at audio shows. I don’t get it, but Thigpen is a master magician.

I plan to build a new walnut outer trim for them since the ones that came with the speaker are poorly put together.

I wouldn’t trade these for a pair of Wilson’s right now. Mainly because I know I’d lose something with the sound.

I do have them paired with an old 400 watts/ch Krell amp. Thigpen recommends no more than 200 watts per channel and I tried that but this 400 watt Krell allows these speakers to absolutely sing.

The ribbons are magic. The bass is tighter than anything decent I’ve heard at audio shows. Yet the woofers and the box they come in look like they were made from old pizza boxes. Okay, maybe exaggerating a bit on the poor build quality.

I was at one point considering buying 3-way YG Acoustics speakers, but these sound so absolutely perfect that I don’t want to jinx it.

I don’t even have them spiked into the carpet- at this point I’m afraid to touch them or experiment with them in some other position or even upgrade them to the new woofers because I can’t risk losing this beautiful experience.

 

 

 

petaluman
148 posts
 

I think electrostats never caught on for a couple of reasons:

They had a tendency to arc if over-driven.  I believe this has been largely solved. 

They also have to be plugged in.  Neither of these are problems with Magneplanars. Although there are SQ differences between the 2 designs, there's enough similarities that Maggies won the marketplace and electrostats got an even smaller wedge of the market.  Quads remain the most famous of the electrostats, but my guess is that Martin Logan has the most units sold.

 

FWIW, the low note on a concert grand piano is ~28 hz, with extended range pianos going down to 16, and at least 1 pipe organ to 8 hz!

bdp24's avatar
bdp24 OP
9,788 posts
 

@sandpat:

1- To get rid of the wrinkles you adjust the LFT driver tensioning mechanism.

2- I take it you don’t have the speakers mounted on the stands made by Sound Anchors specifically for the LFT-8b? Get a pair, they’re pretty cheap!

3- Grant Mye has made his stands for the LFT-8. It includes support arms which extend from their base all the way to the top of the LFT panels. That provides increased structural stiffness and improved sound quality. By the way, the MDF frames of Maggies are even less stiff than the LFT panel of the ET’s. That’s a penalty you pay with a planar loudspeaker.

sandpat
41 posts
 

@bdp24 thanks! I’ve read that in the user manual but I’ve just been afraid to mess with it. Really I need some time to adjust and listen, rinse and repeat. But they sound so wonderful even in their flawed state!

 

I do have the sound anchor stands- these are the large heavy thick metal legs that Thigpen recommends for his speakers, correct? I have them but they were a pain to screw into the woofer box- felt like I was probably causing some sort of new resonance due to the screws sticking out inside the box. My biggest worry though is that I’ll lose a nail and fracture my little toe on them. One of the speakers sits right in the path of my listening room door and I’ve accidentally caught my pinkie toe on it multiple times. The pain is awful. I’ve been on the search for some sort of rubbery material I can place over them to prevent losing a toe someday.

On the subjection of tensioning the ribbons on the panels- do you have any literature on how to go about it the right way? I don’t want to just be loosening/tightening hex screws without knowing what I need to be adjusting first.

 

Thanks for the tips! I can’t imagine these speakers sounding any better but there it is… something for me to chew on.

sandpat
41 posts
 

I think I found the tensioning adjustment service manual:

 

 

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