What an OTL is, and why you should care

DonH50

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Jun 23, 2010
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Just a note to thank Ralph for this thread, cool stuff!

Disclaimer: I owned and/or worked on a number of tube amps through the years, including Futterman (to me sound was "ok" and reliability, well, good for a college kid working as a tech ;) ), and did build a couple of OTL amplifiers based upon a push-pull approach with feedback to get a reasonable (to me) output impedance and a servo circuit to control DC on the outputs. The latter, along with a matching preamp, was a differential circuit so complicated as all get-out. It measured and drove speakers better than any of the other ARC or CJ amps I had around but I never went further with it. Sold the prototypes, but the main feedback I got was "too SS" -- it was much cleaner than the tube amps of the day but that was not what people wanted to hear. I'd love to hear an Atma-Sphere some day.

IME tubes (non-OTL amps) on ESLs are gorgeous in the midrange but have trouble controlling the bass and get harsh in the highs as the impedance drops out of the "good" region of the output transformer. The latter is also a problem for many SS amps IME but most did better than tubes -- if they could handle the low impedance!

FWIWFM - Don
 

kach22i

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Apr 21, 2010
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Yes. They are using an outboard device called the ZERO (see www.zeroimpedance.com).
ML speakers are built with a low impedance in an attempt to make them more compatible with solid state amps. The problem is that ESLs in general are incompatible with solid state unless a bit of current feedback is employed in the amp. This is because the impedance curve of an ESL is not also a map of its efficiency, unlike a box speaker.

From one of the reviews in that link:

http://www.6moons.com/audioreviews/zero/zero.html
Ultimately, speaker manufacturers should install impedance-matching autoformers in their crossovers if design parameters enforce the use of 4-ohm drivers and even lower impedance sags. Such idealism of course is not for sale yet. Paul Speltz originally envisioned the Zeros as a solution for OTL amps.

I guess the question that first popped into my head is answered, and that is: could a impedance-matching autoformers be part of a speaker, or should it be part of the amp.
 

Gregadd

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Apr 20, 2010
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My dream amp would be a direct heated triode otl. I always viewed as wildly expensive and impractical given the dominance of low efficiency box speakers.
 

Atmasphere

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My dream amp would be a direct heated triode otl. I always viewed as wildly expensive and impractical given the dominance of low efficiency box speakers.
We built one of those back in the 1990s using four 6300bs (a beefy 300b with a graphite plate). It made about 7 watts. Essentially, DHTs aren't practical as there aren't any with a low enough plate resistance. But since we were able to compare using this rather impractical amp, it didn't offer anything that the regular OTLs didn't. I'm really not convince the DHTs offer anything special other than they are conveniently sized power triodes. IOW an indirectly heated triode would sound just as good if it otherwise had the same parameters as a DHT. But you can't find very many such triodes so that OTL we made was one of the very few ways to make any sort of comparison.
 

Gregadd

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Ralph I did hear your amp CAF driving the Cllassic horns. Very neutral
 

Gregadd

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Ralph
Since we have your attention what are the advantages of point to point wiring over a gold trace circuit board.
 

Mrmb

Well-Known Member
Jun 30, 2014
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IME tubes (non-OTL amps) on ESLs are gorgeous in the midrange but have trouble controlling the bass and get harsh in the highs as the impedance drops out of the "good" region of the output transformer. The latter is also a problem for many SS amps IME but most did better than tubes -- if they could handle the low impedance!

I've driven 3-different pairs of S/L (SoundLab) ESL's with tubes (M1's/M1PX's & Majestic 845's). With my S/L M1's, I changed to tubes upon the audition of CAT JL-1 monoblocks (100w each). The CAT's replaced a Mark Levinson 336 stereo amp (350w/channel). The CAT's did everything better than the Levinson and Parasound Halo JC1's that I simultaneously auditioned against the CAT's. I can't over emphasize enough, the positive differences that the CAT's made! In fact, the first thing that struck me when I inserted the CAT's, was how much better the BASS was and it was damn good with the Levinson and Parasounds, so much so, that I was gobsmaked with what the CAT's were doing and the control they had over the S/L's!

A few years later, I upgraded the CAT JL1's with CAT JL-3's -- which doubled the driver tube count from 16 to 32. But the in-house wound transformer remained at a weight of 55lb.(each amp); which I believe is one of the aspects, that makes the CAT's so special! The CAT JL-3's stated output is 150w/ea. Power-wise, these 190lb beasts (each) are amazingly underrated. As Ken Stevens of CAT has stated -- they will drive anything. My experience with SoundLab's suggests that may well be the case.

Later, I switched to Atma-sphere MA-1 Mk.3.3's. I've happily owned the MA-1's for a couple years now. Before purchasing them, I compared them to the CAT's, Pass Labs, and a Mark Levinson. Again, there was no comparison! The MA-1 OTL's trounced the solid state amps. From my experience with SoundLab's, the two tube amp manufacturer's that I've owned, have categorically and without question, outperformed the ss amps that I've tried and owned. Tube BASS in all cases was not only better, but it was in another league. And of course, mid-range and treble were also as good if not better; otherwise, I wouldn't have switched to tubes for my S/L's. Whether these facts are logical from a technical perspective or not, my decades of in-home, real-world experiences have proven otherwise.
 

DonH50

Member Sponsor & WBF Technical Expert
Jun 23, 2010
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I've driven 3-different pairs of S/L (SoundLab) ESL's with tubes (M1's/M1PX's & Majestic 845's). With my S/L M1's, I changed to tubes upon the audition of CAT JL-1 monoblocks (100w each). The CAT's replaced a Mark Levinson 336 stereo amp (350w/channel). The CAT's did everything better than the Levinson and Parasound Halo JC1's that I simultaneously auditioned against the CAT's. I can't over emphasize enough, the positive differences that the CAT's made! In fact, the first thing that struck me when I inserted the CAT's, was how much better the BASS was and it was damn good with the Levinson and Parasounds, so much so, that I was gobsmaked with what the CAT's were doing and the control they had over the S/L's!

A few years later, I upgraded the CAT JL1's with CAT JL-3's -- which doubled the driver tube count from 16 to 32. But the in-house wound transformer remained at a weight of 55lb.(each amp); which I believe is one of the aspects, that makes the CAT's so special! The CAT JL-3's stated output is 150w/ea. Power-wise, these 190lb beasts (each) are amazingly underrated. As Ken Stevens of CAT has stated -- they will drive anything. My experience with SoundLab's suggests that may well be the case.

Later, I switched to Atma-sphere MA-1 Mk.3.3's. I've happily owned the MA-1's for a couple years now. Before purchasing them, I compared them to the CAT's, Pass Labs, and a Mark Levinson. Again, there was no comparison! The MA-1 OTL's trounced the solid state amps. From my experience with SoundLab's, the two tube amp manufacturer's that I've owned, have categorically and without question, outperformed the ss amps that I've tried and owned. Tube BASS in all cases was not only better, but it was in another league. And of course, mid-range and treble were also as good if not better; otherwise, I wouldn't have switched to tubes for my S/L's. Whether these facts are logical from a technical perspective or not, my decades of in-home, real-world experiences have proven otherwise.

Took you over a year to respond?

Personal preference will always override technical aspects of your (anyone's) system. That said, among ESLs, SoundLabs are among the easiest to drive and thus best-sounding with tubes IME. Again just in my experience (also over decades), tube bass tends to sound "richer" or "fuller" due to added distortion but not as "tight" as SS, but I have not heard a modern tube amp on modern SoundLabs. And I completely agree that the output transformer of most tube amps is one of the overlooked components that distinguish "good" from "great".

I'd love to hear your Atma-Sphere setup; by all accounts they sound fantastic! Ralph has really hit the mark with modern OTL designs (I dealt with Futterman now and then and Atma-Sphere is a huge step forward).
 

Mrmb

Well-Known Member
Jun 30, 2014
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Took you over a year to respond?

Yeah DonH50, I saw this a year ago and made it a point to wait a year before responding. :rolleyes:

As a forum "sponser" and "technical expert" I'm surprised you hadn't noticed, that the beauty of forums is their use as a compendium of information; that like this post, has relevance far longer than the last posting date....
 
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DonH50

Member Sponsor & WBF Technical Expert
Jun 23, 2010
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520
Monument, CO
Yeah DonH50, I saw this a year ago and made it a point to wait a year before responding. :rolleyes:

As a forum "sponser" and "technical expert" I'm surprised you hadn't noticed, that the beauty of forums is their use as a compendium of information; that like this post, has relevance far longer than the last posting date....

Not mine, apparently, but no worries. I don't actually check WBF that often. Enjoy your system, in the end that's all that really matters.

I wish there were more Soundlab dealers around to make it easier to hear them live.
 

Mrmb

Well-Known Member
Jun 30, 2014
13
2
208
Hi Don, I concur about enjoying the system and music in general; which most all of us are here to discuss and share.

SoundLab ELS's are a big investment both in cost and size. However, once experienced, it's difficult to forego full-range electrostatic bass and the panel's coherence and line source radiating pattern. Plus, like Ralph Karsten, Dr. West is a talented designer, offering outstanding customer support and innovative products....-Mike
 

microstrip

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May 30, 2010
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I've driven 3-different pairs of S/L (SoundLab) ESL's with tubes (M1's/M1PX's & Majestic 845's). With my S/L M1's, I changed to tubes upon the audition of CAT JL-1 monoblocks (100w each). The CAT's replaced a Mark Levinson 336 stereo amp (350w/channel). The CAT's did everything better than the Levinson and Parasound Halo JC1's that I simultaneously auditioned against the CAT's. I can't over emphasize enough, the positive differences that the CAT's made! In fact, the first thing that struck me when I inserted the CAT's, was how much better the BASS was and it was damn good with the Levinson and Parasounds, so much so, that I was gobsmaked with what the CAT's were doing and the control they had over the S/L's!

A few years later, I upgraded the CAT JL1's with CAT JL-3's -- which doubled the driver tube count from 16 to 32. But the in-house wound transformer remained at a weight of 55lb.(each amp); which I believe is one of the aspects, that makes the CAT's so special! The CAT JL-3's stated output is 150w/ea. Power-wise, these 190lb beasts (each) are amazingly underrated. As Ken Stevens of CAT has stated -- they will drive anything. My experience with SoundLab's suggests that may well be the case.

Later, I switched to Atma-sphere MA-1 Mk.3.3's. I've happily owned the MA-1's for a couple years now. Before purchasing them, I compared them to the CAT's, Pass Labs, and a Mark Levinson. Again, there was no comparison! The MA-1 OTL's trounced the solid state amps. From my experience with SoundLab's, the two tube amp manufacturer's that I've owned, have categorically and without question, outperformed the ss amps that I've tried and owned. Tube BASS in all cases was not only better, but it was in another league. And of course, mid-range and treble were also as good if not better; otherwise, I wouldn't have switched to tubes for my S/L's. Whether these facts are logical from a technical perspective or not, my decades of in-home, real-world experiences have proven otherwise.

Great to learn about your experiences with the big Soundlab's. However we should remember that choice of amplifiers for Soundlab's depend on the version of the backplates (crossover + HV). We have at less four generations - with EI treble transformer TT) , very tube friendly, toroid TT low impedance (they preferred solid state) , toroid TT with a small mod (less tube enemy ;) ) and fortunately now the Toroid II TT that is very compatible with tubes. My A1Px have now the Toroid II and it is a real improvement in all senses.
 

Atmasphere

[Industry Expert]
May 4, 2010
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The first and second versions of the toroid backplate had a design bug based on a simulation, which turned out to be incorrect. Hence the toroid two, and why the first to versions were so hard to drive! There was a resistor in them that in order to survive, was rated at 200 watts. So a lot of amplifier power was being converted to heat. For those backplates, simply removing that resistor fixed most of the issues. Sound Lab has the resistor in place to prevent low frequency saturation but I don't know anyone that did that mod and reported that problem. But the Toroid two backplate is based on the actual impedance curve of the speaker and everything works.

With the prior backplates, the MA-2 was just about the only game in town to make that speaker really play. But with the change, you can easily do with a set of MA-1s, and the speaker sounds better too. In most rooms a set of MA-2s is nearly unlimited power on that speaker now.

IMO/IME with that change the Sound Lab took its place in the firmament of the world's best speakers. I think its one of the top 5 best made anywhere at any price- it does not require a crazy amount of power, its full bandwidth, very fast, very revealing and very neutral/low distortion.
 

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