Eminent Technology LFT-8b in Harry's system


I recently signed up for the V.P.I. Industries newsletter, and today received my first such. In it, Harry Weisfeld reviews a Grado phono cartridge, but this post concerns one of the speakers he listed as being those he uses to listen to music and evaluate recordings through. All but one are traditional dynamic cones/domes in a box designs, only one being a planar/dipole. That planar is the Eminent Technology LFT-8b. I'm pretty sure Harry could, if he so chose, have instead as his sole planar a pair of $6000 Magneplanar MG 3.7i's, or even $14,000 20.7's. But nope, he instead chose the $2500 ET LFT-8b, imo the greatest value in a loudspeaker on the market. I compared it to the 1.7i, and the difference was dramatic.
128x128bdp24

Nice Lew! I'm a fan of the B139 (Dave Wilson is too, using a pair in each WAMM, for mid-bass), and I have a pair of ESS TranStatic I's that use it in a quarter-wave (I think it's called) transmissionline. The  midrange is covered by a KEF B110 5" Bextrene cone, the highs by a trio of those great RTR ESL tweeters, the same ones Bob Fulton used in his Model J. I had a pair of those in '74, for about six months.

I'm dying to hear Modjeski's current Direct-Drive ESL speakers, powered by tube amps of his with no output transformers---the amps driving the ESL's directly (hence their name ;-).

First of all, what a coinkidink!  Back in around 1970, I was being paid very little as an intern in a hospital, and I craved a pair of IMF Monitor loudspeakers.  As a financial compromise, I bought a pair of IMF Studios, instead (from Lyric Hi-Fi in Manhattan).  The Monitor used a KEF B139 in a Bailey-type TL, a KEF B110 midrange, and a KEF tweeter.  Not entirely satisfied with the Studios, I eventually decided to try to build my own pair of Monitors from scratch.  I had a patient who agreed to let me use his table saw, and I acquired two 4X8 sheets of HDF, about 1.2 inches thick, and lots of clamps, glue, screws, etc. I bought the B139s and B110s easily enough, and then I found a guy in California willing to sell me RTR ESL tweeters (the blue rectangular ones), as many as I wanted, with a power supply to drive them.  I bought eight, with the intention of using four per speaker.  Thus I built a TL speaker with the B139s and B110s in entirely separate TLs. (The woofer cabinet is nearly a carbon copy of the Bailey design, a la the IMF Monitor.)  And the four RTR ESL tweeters were arranged in a linear array along one side of the woofer enclosure.  A few months later, I met an EE who helped me design the crossover and in fact wound me some inductors.  This was a darn good speaker that blew the pants off a pair of Magneplanar Tympani 1Us, when they came on the market.  I moved from this speaker to full range ESLs shortly thereafter ( 2 pairs of KLH 9s), and I sold the home-made speakers to my cousin, who used them for at least 20 years.  When he finally wanted to be rid of them, I bought them back, cut off the parts of the cabinets that supported the B110s and the tweeters, and stored the TL woofer cabinets in my basement for another 20 years.  Then I bought the Bev 2SWs and finally saw a use for the B139s in the TL. (I had kept a pair of NOS B139s, all that time.) I still use one of those B110s as a test load when I work on my amplifiers.  The B139 won't take much amplifier power, but I think that in a TL, it is one of the lowest distortion woofers ever made.

A tube amplifier with no output transformer is called an OTL.  OTLs can drive any speaker, other than an ESL, with a high enough input impedance.  What the Beveridge, the Modjeski, and other ESL "direct-drive" amplifiers do in addition is to drive an ESL with no step-up transformer at the ESL end of the chain.  So, no transformer in the signal path, at all. To do this, direct-drive amplifiers have to develop thousands of DC volts at their outputs; the audio signal rides on the high DC voltage, which simultaneously biases the ESL.  (The Bev speakers are a special case of an ESL where the diaphragm is low impedance; classic ESLs have a very high impedance on their diaphragms.) DD amps are OTL, but not all OTL amps can DD, is my point.

I know what an OTL is, ya big silly ;-). I had a pair of Atma-Sphere M60’s for years, which I bought for driving my old Quads. One cool thing about them is you can take out half the output tubes, to create a 30 watt version of the amp, perfect for the easily-overdriven Quads.

You’re right, an ESL speaker with a direct-drive tube amp has not only no output transformer in the amp, but no step-up transformer in the speaker, eliminating two sources of distortion. I know Modjeski’s current ESL speaker/D-D amp product is designed and built that way, as I believe the Beveridge was. I think it was to accomplish that that Mr. M was hired by Mr. B. I think they were both in the Santa Barbara area at that time, a beautiful city on the California/Pacific Ocean coast, just north of Los Angeles. Modjeski relocated to the San Francisco bay area a couple of years ago, where he runs Music Reference and teaches a course in amplifier design at a local adult education school. Wish I still lived in San Jose!

When I bought the TranStatics in ’81 (for $400, out of The Recycler, a weekly S. California paper), one of them had a generic oval woofer instead of a B139. I called ESS in Sacramento, and they had one, and only one, left. I paid all of 39 bucks for it! The B139 ESS used in the speaker wasn’t the standard version---not having the four "ears" through which mounting bolts pass---just a perfect oval. I first heard the speaker in ’71, but being a starving musician couldn’t come close to buying a pair. They retailed for $1200 in ’71, the Infinity Servo-Static $2000. Ten years later I was happy to find the TranStatics, and could now buy them. Still have 'em, but man are they heavy---140 lbs. apiece

bdp24, Sorry for the pedantic discourse on "OTL".  You would be surprised at the number of audiophiles who do not know the differences in meanings of the terms: OTL, balanced, push-pull, single-ended, and direct-drive.  My main point was, as you know, that your Atma-sphere amplifiers could not directly drive the screen of an ESL, but they are OTL.  However, once in a while, Ralph Karsten makes noises about building a direct-drive amplifier; I am sure he could do it.  I use a pair of MA240s to drive my Sound Lab speakers, in my "main" system.  Joe Curcio, I think, also claims to have a tube-based input circuit mod for the original Bev amplifiers.  The originals, like mine, use solid state input circuit, and yes yes yes, they are direct-drive.  (Bev speaker Models 1, 2, and 2SW are directly driven.)

No apology necessary Lew! To assume ignorance is not a bad idea ;-).

Modjeski’s new ESL is sold as an integrated speaker/direct-drive tube amp---no output transformer in the amp, no step-up transformer in the ESL. A stand-alone direct-drive amp is impossible, as a universal model/design can’t take into consideration the different nature/characteristics/needs of all the speakers it may be asked to drive. A direct-drive amp must be designed for a specific speaker, and for that speaker alone. I’ll bet the Beveridge was Roger’s first attempt at such a design, and I wonder how his new one differs from it. He did also design a "normal" OTL, the rights to which he sold to Counterpoint, who manufactured and marketed it as the SA-4.

Your KEF driver project was very advanced for a hobbiest, Lew. Transmissionlines are not easy to build, requiring a lot of knowledge of acoustical theory, speaker enclosure design, and cabinet building. I’m impressed!