What an OTL is, and why you should care

DonH50

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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
 
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kach22i

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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.
 
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Gregadd

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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

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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

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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

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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

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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.
 
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Mrmb

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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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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

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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.
 

Phantom-Audio

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Why is the industry not focused on 16 ohms or higher Speakers?
 

Atmasphere

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Why is the industry not focused on 16 ohms or higher Speakers?
IMO it should be but of course its a forlorn hope. Cheap power is available with amps that can easily double power into 4 ohms as opposed to 8, meaning that such an amp will make half its rated 8 ohm power into 16. IOW its a horsepower race. But in high end audio its usually more about getting the music to sound as real as possible, which is an entirely different endeavor.

What 4 ohm loudspeaker manufacturers are ignoring is that all amplifiers whether tube, solid state or class D will make greater distortion into lower impedances, and this distortion is audible because the ear assigns tonality to distortion. Put another way, regardless of the amplifier it will sound smoother and more detailed when driving a higher impedance- i.e. it will sound more neutral and more transparent.
 
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sbo6

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Why is the industry not focused on 16 ohms or higher Speakers?
Please help me explain why we should go back to 16 ohm when pretty much every amp post - Vietnam can drive 8 ohm if not 4 ohm speakers?
 

Atmasphere

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Please help me explain why we should go back to 16 ohm when pretty much every amp post - Vietnam can drive 8 ohm if not 4 ohm speakers?
The answer is just above:
What 4 ohm loudspeaker manufacturers are ignoring is that all amplifiers whether tube, solid state or class D will make greater distortion into lower impedances, and this distortion is audible because the ear assigns tonality to distortion. Put another way, regardless of the amplifier it will sound smoother and more detailed when driving a higher impedance- i.e. it will sound more neutral and more transparent.
Sound pressure is one thing, sound quality is another! The speaker can only reproduce information supplied to it by the amplifier. If the amplifier is making more distortion on account of the load impedance, so will the speaker. That all amps do this is easy to see in their specs. You might think it 'negligible' but if the distortion is made of higher ordered harmonics it will be audible as harshness and brightness, since that it the tonality assigned to the higher orders by the human ear.

The ear is keenly sensitive to the higher orders since it uses them to sense sound pressure- and the ear has over a 120dB range! We're more sensitive to the higher orders than almost anything else. BTW it is the presence of the higher orders in low levels that is responsible for the traditional brightness and harshness of solid state amps in the last 60 years: quite literally this is why tubes are still around so long after being declared obsolete back in the 1960s.
 

Gregadd

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Took you over a year to respond?

Personal preference will always override technical 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".
(portions omitted)
I suppose this could be considered "trolling. "OTOH I just can' t seem to get a clear answer.
"A personal preference...'richer' or fuller." Richer or fuller to what? The source? The real thing? Does that mean that ss. is leaner than the source or for that matter identicial?. If ss is not identicallto rl music then it distorts real music. Does that mean some have a preference for the poorer leaner sold state sound?
 

Atmasphere

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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".
Does that mean that ss. is leaner than the source or for that matter identicial?. If ss is not identicallto rl music then it distorts real music. Does that mean some have a preference for the poorer leaner sold state sound?
If I can shed some light on this:

Not all solid state amps are the same and neither are all tube amps.

We must also keep in mind that the Sound Labs are not easy for solid state amps to drive. This is because they are 30 ohms in the bass. For this reason a tube amp of 140 watts can keep up with a solid state amp rated at 600, for the simple reason that a 600 watt amp is going to make about 150 watts into that load.

Now regarding the 'sound' of amplifiers its really all about the distortion that amps make or do not. It is completely false to say that 'solid state amps are more neutral' when so many of them exhibit brightness and harshness as a coloration. This coloration is caused by distortion.

Now it can be solved if you have enough feedback, but to have enough feedback (in excess of 35dB) you have to jump through several hoops. First the amp has to have a phase margin that allows for the feedback without oscillation. The second thing you need is enough Gain Bandwidth Product; which is similar to gas in a car- when you run out of gas the car stops, when you run out of GBP there's no more feedback.

Because a good 99% of all solid state amps ever made have insufficient GBP they also have insufficient feedback. At 100Hz they have plenty and so measure well and play bass well. But at 1KHz and 10KHz the same amp into the same load has higher distortion. This is because feedback is decreasing with feedback due to insufficient GBP.

We've been hearing this problem for a long time.

IOW a desirable distortion characteristic is consistent distortion at all frequencies. The other desirable characteristic is that the lower orders (2nd and 3rd) are in sufficient amounts so that they can mask the higher orders. This allows the amplifier to have a smoother sound.

Now you can do this with zero feedback but you might get a fair amount of distortion. I'll make a further distinction here- there is a difference between a quadratic non-linearity and a cubic non-linearity. The former generates a 2nd order as the primary distortion product (SETs are a good example) while the latter makes a 3rd (at a much lower level). Of the two, the cubic is preferable (IMO/IME) because as the order of the harmonics is increased they fall off at a faster rate. This means that such an amp will sound smoother as it lacks the higher orders and the 3rd (which is treated by the ear the same way as it treats the 2nd) is masking them.

That is why we make zero feedback fully differential amps with no output transformer- we can get lower distortion with greater neutrality.

In a nutshell if you can identify either the 'tube' sound or the 'solid state' sound either way you're talking about distortion. The trick is to make the distortion inaudible by masking it with a lower order to which the ear is insensitive- and to that end keep the distortion otherwise as low as possible.

You can make a solid state amp sound nice and smooth- all you have to do is run enough feedback so it has the ability to clean up the distortion caused by the application of feedback itself (which happens thru a process called 'bifurcation'). You need at least 35dB to do that and frankly most designers don't bother and for many years (decades) the semiconductors needed to do that really didn't exist. They do now and have been around for a while but you still have to have the designer to recognize that and have the will to do something about it. One neat solution is to use a class D circuit and give it so much feedback that it exceeds its own phase margin and goes into oscillation. You then use the oscillation as the switching frequency. This type of class D amplifier is called 'self oscillating' and is capable of having enough feedback to get the job done. If you've been hearing about a class D project we've been working on its based on this principle. If the amp has the right distortion signature (and what we're finding is the signature is more important than the actual amount of distortion) then the amp can be quite neutral, smooth and detailed- literally the best of both worlds.
 

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