tube amps and electrostatics

Autoformers cost more than many speakers themselves. That's a factor to consider in the equation that can't be ignored.

Have you checked the price on Zero autoformers? Even in the finished boxes I cannot see how your assertion could possibly be true.

Mapman, I'm not sure I get the gist of your point in relationship to the chart.

I am over 50, and I know my ears like most older ears do not even hear much above 12khz or so like they used to when I was a young audio "stud".

So getting flat response that works becomes a lot easier with age. Roll off of highs with ES speakers may in fact be a non-issue for most of us old audio farts. WHose to say a lot of younger ears that listen to the same are sensitive enough for it to matter either.

I used to own, the Acoustat 3 powered by a Conrad Johnson MV75A-1 -75 watt tube amp, no problems, beautiful sound.
I ran stacked Quad 57's with an AtmaSphere 30 watt stereo amp, beautiful sound ,no problems, also drove the same stacked pair with a 1961 Bell tube integrated(20 watts)great sound no problems.
Used to like my CLS 11Z speakers, but never found a tube amp that drove them as well as my mono Classe DR 6 amps.
Had a pair of Quad 63, that I never tried with a tube amp, but they did like the old Metner mono block power amps.
Now I am back to tubes and stats but it's the Acoustat X with it's own servo OTL tube amps.
Best combination so far of pairing a stat with tubes, but that should be as expected when each is made for the other.

Quad switched from tube amps to the current dumping solid state amps for any number of reasons.
Perhaps because tubes were felt to be obsolete, or because the newer 63 needed- demanded- more stability from an amp, or just because they wanted to, perhaps spurred on by the need to update their line.

I think,there are no rules set in stone,let common sense prevail and don't push either type of amp beyond it's limits.

As the ear sensitivity chart at the link I referenced above shows, human hearing is far from flat over the 20-20kz frequency range normally associated with human hearing.

That means nothing that truly measures as flat will ever be heard as being flat. Far from it, in fact.

So technically flat or superior does NOT necessarily mean better sound that we hear as a result of being flat at the source.

So you can hear things that are flat as something otherwise or you might hear something that is not flat as being flat, or any shade of grey in between.

Fun stuff! I guess the wide range of things that might end up sounding good depending mostly on personal preference is what keeps Walmart from taking over the high end audio business.

I think I will go out on a limb and argue that the desirability of flat response at the source is a function of the variability of the music listened to. If its flat at the source, then a random sample that represents all music possible will overall sound better as a whole. If its not flat, results will be more biased towards certain types or patterns of music.

That's consistent with how I understand things and what I actually observe in practice in that I listen to and enjoy all kinds of music equally, though I know I could tweak things to sound better with certain kinds only if I wanted to.

I do that to some extent by running multiple systems in multiple rooms, each of which sound different, but my main (my reference) system in my main room is the one that I strive to perfect technically in the interest of best sound overall.

Sounds like one could create a technical reference quality system using the right tube amp with a particular set of ES speakers, but perhaps it will be more of a challenge to achieve reference quality in practice, as opposed to merely sounding really good.

Beside Roger Sander answer above in my last post which the non believers are conveniently blind to.
Here is and excerpt from the Stereo Times review of the difference when driving the Quad ELS2805 (while still hard to drive are easer than most ESL's) using tube then good solid state. Which also backs up everything Roger said in his White Paper and I've been saying and heard.

Stereo Times:
"The Quads are not as easy a load as one would be lead to believe by how good they sound with tube amplification. The Magtech further revealed the tube shortcomings (which I had already noticed) by expanding the peak performance range of the Quads. The “sweet region” grew to the limits of the speakers themselves. Meaning they suddenly didn’t have extended bass but what they did have was tighter and more refined. The top end was not more extended, just smoother and less brittle. The Magtech amplifier did not create new loudspeakers; it just let them be all that they could be."

Cheers George

"And although I can't formulate a meaningful technical explanation, and I suspect that a good explanation would be a complex one involving a multitude of factors, my perception over the years has been that there is a tendency for those qualities to often be compromised in systems that are designed and assembled in a manner that results in flat frequency response being a leading priority. "

I'll offer up this wonderful resource again here to help with that.

Audio Chart

Factor in the ear sensitivity curves to help account for why flat may be technically correct but not sound best.

Ralph & Bruce & Mapman, thanks very much for the nice words.

Frogman, great post! I agree completely. Richness, body, and dimensionality are terms I find myself using frequently, I believe in the same sense as you are using the terms fullness and image density. And although I can't formulate a meaningful technical explanation, and I suspect that a good explanation would be a complex one involving a multitude of factors, my perception over the years has been that there is a tendency for those qualities to often be compromised in systems that are designed and assembled in a manner that results in flat frequency response being a leading priority.

Best regards,
-- Al

Autoformers cost more than many speakers themselves. That's a factor to consider in the equation that can't be ignored.

I recieved an email from Rodger. (below in parentheses)

Roger Sanders ESL and amp guru is not one to get into heated debates on forums, this is why he has presented his white paper on the subject as simply as he knew how, so it can do the talking for those who can understand it.
So if this is wrong by anyone, a white paper should be done by the "one" who has the so called "knowledge" to oppose it. So he can also be then be also laid bare for all the audio community to view, like Roger has done with his white paper.

"Hi George,
I completely agree with your assessment of the tubes vs. transistors controversy among audiophiles.
Transistor amps are the only type that can produce linear frequency response and high output when driving ESLs. But getting audiophiles to understand and believe the technical reasons why this is so is difficult. You have your work cut out for you -- but I support you 100%.

I hope that posting my white paper will help some audiophiles gain a better understanding of amplifiers. I do not have the time to post on forums. And in any case, I am not interested in getting into arguments with audiophiles. So I will not be making any posts.
However, any interested individual may feel free to contact me directly if he has any questions."

Great listening,
-Roger"

Cheers George

Ralph and Al, the discussion begs the question of how can one know whether an ESL was voiced to be driven by a tube versus SS amp.

Bruce, altho' you've directed the question to Ralph & Al, just my 2 cents: since ESL behave like (giant) capacitors which have impedance inversely proportional to freq, wouldn't it always be true that ESLs are almost always voiced with tube amps (not saying that you cannot or should not use a BJT-based power amp if you want to)? Barring those like Sanders Sound Lab ESLs where the impedance is deliberately kept low so as to use a s.s. power amp.

Ralph and Al, the discussion begs the question of how can one know whether an ESL was voiced to be driven by a tube versus SS amp. Obviously Sound Lab ESLs were designed to be tube friendly, ergo Ralph's comment that his amps have been coupled with Sound Lab ESLs for years. Is it the same old answer -- call the manufacturer and ask?? Or try to determine the type of amp that was used by the manufacturer to display the ESLs at shows??

That's easy. Most of the ESLs intended to be driven by transistors tend to have very low maximum impedances. For example MLs tend to be 4 ohms in the bass. When you figure that the typical ESL varies by 10:1 in impedance from the bottom octave to the top, that means that the MLs should be about 0.4 ohms at 20KHz, and a number of them are.

However by use of a set of ZEROs you solve this problem, as I have mentioned before. Let's take the example of the ML and its 0.4 ohm impedance at 20KHz. The ZERO has taps that set up the amplifier load at 16 ohms, assuming that the speaker load is 4,3 or 2 ohms. If you are using the 4 ohm tap that 0.4 ohm load will look like 1.6 ohms. If the tube amp employs negative feedback it will have no trouble making this work with flat (linear) bandwidth. If the tube amp lacks feedback, you might have to use the 2 or 3 ohm tap, and it will still work quite well.

So really what it comes down to is whether the tube amp makes enough raw power that might be suitable to drive the speaker. IME, the Accoustat is a good example. Some versions of the Accoustat, as mentioned earlier in this thread are high impedance, meaning a little OTL like our M-60s can drive them fine across the entire band. Other Accoustats have that dreaded low impedance and require the use of the ZEROs. A good friend of mine had a set of Accoustats like that, and he used our M-60s with a set of the autoformers. The amps made plenty of power- and the setup was in a room 17' by 24', and we never clipped the amps.

The point here is that one must not confuse efficiency with the impedance. The ZERO is a problem-solver here- it allows almost any tube amp to effectively drive almost any ESL, provided the amp makes enough power in the first place.

Rodman99999, Thanks, I think I see what you mean.

" The argument that there is too much variability in the sound of live to establish a benchmark is nonsense; there is much more that is consistent than there is that is different. "

Agree with Frogman on this. All live music has certain things in common. Patterns emerge and can be recognized with experience.

Also with the commonly accepted mindset that documented or measured technical specs and parameters are insufficient to tell the whole story regarding how well overall any playback system delivers the illusion of being real to individuals.

Its the twilight zone of home audio, that which cannot be explained fully based on scientifically established or otherwise known facts, that helps keep things interesting in that one never knows exactly what one will encounter until one encounters it. And each case will be just enough different most likely to still matter.

Just remember that the twilight zone is a prime feeding ground for charlatans and other purveyors of (intentional) disinformation as well. None of those in this discussion though, I would say. Misinformation (unintentional) is more prevalent as well.

Al, I don't mind; and, thanks. I think your comment "Which of those two kinds of amplifier/speaker interactions results in the most correct acoustic output from the speaker is a separate question, however." is particularly valuable and relates to another issue that you bring up, perspective.

As I said previously I tend to let those more technically astute explain these interactions, but after a lot of years in this hobby I am convinced that part of the reason that some of us "are at loggerheads" (besides ego and stubbornness) is that there are still aspects of sound and music and the perception of those that the technical doesn't fully explain. I would like to offer some thoughts about this from my perspective.

In my experience, and almost without exception, systems assembled with the stated goal of "linearity" don't sound the way that live music sounds. More times than not the result is sound that is tipped up in the highs and lean through the midrange compared to the sound of acoustic instruments. The argument that there is too much variability in the sound of live to establish a benchmark is nonsense; there is much more that is consistent than there is that is different. Most of music takes place in the midrange and most audiophile systems sound too lean in this range without enough fullness and image density particularly in the lower midrange. I can't remember how often I have heard comments about a system lacking bass when there really was no true bass content in the music and what the listener was missing was the appropriate fullness in the lower midrange and, perhaps, upper bass that gives music much of its power. "Linear" highs will often result in upper partial information that is not well integrated with the fundamental frequency and add excessive "presence" to the highs.

"Brightness" is an interesting descriptive term in that it is often confused with "texture". I have heard many systems (or recordings) that are described as "bright" which I would describe as harsh or grainy but definitely not bright. In fact I have heard systems that are dark and harsh (usually ss based) and others that are bright and overly smooth (usually tube based). Incorrect texture is what I hear oftentimes with ss amps driving electrostats and not necessarily excessive brightness (even when the sound is too lean, giving the illusion of brightness), and that is the main reason that tubes driving electrostats usually sound more natural to me: the fullness and image density of the midrange and the proper integration of harmonics with their fundamentals creating a texture and clarity that is much closer to the sound of live acoustic instruments even if, in absolute terms, the sound does not measure as "linear"; whatever that is.

George, I happen to enjoy reading many of your posts in other threads. But I feel compelled to politely suggest that Ralph Karsten (Atmasphere) and Al (Almarg) are among the most respected A'gon members. Both are EE techies. And Ralph is the gent behind Atmasphere as in the "designer" and maybe the owner too.

In short, you're taking on some pretty heavy-duty guys. I suggest you back down and carefully re-read what Ralph and Al posted. I happened to think that what they wrote makes sense.

Ralph and Al, the discussion begs the question of how can one know whether an ESL was voiced to be driven by a tube versus SS amp. Obviously Sound Lab ESLs were designed to be tube friendly, ergo Ralph's comment that his amps have been coupled with Sound Lab ESLs for years. Is it the same old answer -- call the manufacturer and ask?? Or try to determine the type of amp that was used by the manufacturer to display the ESLs at shows??

Seems to me that what gets a bit dicey is the case where the ESL is coupled with a cone/dynamic speaker to cover LFs. Then we're back in the soup with "rock and roll" impedance curves.

Folks, let's try to remember this is a just a cool hobby and we're all friends.

Cheers,

Bruce

I'll stand on my position that technical specs never tell the
whole story, and any given reasonable combo of gear, even with
electrostats, can win on any given day. Too many other
factors come into play. no substitute for listening and
hearing. Tube sound lovers will probably still like the tube
amp sound for the usual reasons and vice versa. Neither will
likely hold all the cards always. Like usual. Good sound is
shades of grey usually, not black and white.

Ralph- Save your keystrokes.

Some cannot see the forest for the trees.

Martin Logan ESL Montise sterophile review, driven by Audioplax 80w tube monoblocks, also by ProLogue tube Premiums, and then by Simaudio Moon Evo 7's solid state.

Quote from the measurements by JA:
"The shape of the impedance trace will result in the Montis's top octaves shelving down when the speaker is driven by a tube amplifier having a high source impedance. This is why Robert Deutsch found that his Audiopax amplifier sounded too soft and lacking in definition."

Quote from the Robert Deutch review:
"The other tube amp I had on hand was the Audiopax Model 88 Mk.II. The Audiopax driving the Avantgarde Uno speakers is a "magical" combination: detailed and transparent to the source while minimizing the "electronic" artifacts of the reproduction process. The Audiopax-Montis marriage was not a happy one. Although the Model 88 Mk.II's rated output is 30Wpc—not that much less than the ProLogue Premium's 40Wpc—the Audiopax was dynamically on the subdued side even at moderate levels, and the sweetness and liquidity that had been so appealing with the Avantgardes now came across as too soft and lacking definition.
Next up was the Simaudio Moon Evolution W-7, a 150Wpc solid-state amp. The sound of the Montis driven by the Moon W-7 was vastly different from its sound with the PrimaLuna or the Audiopax. It now had dynamics in spades, evident as an ability to play much louder without strain, as well as more clearly present the ebb and flow of music at moderate levels. Bass was more extended and better controlled; the double-bass passages in Sylvia McNair's Sure Thing: The Jerome Kern Songbook (CD, Philips 442 129-2) were more distinct."

I have asked Roger Sanders to come to this disscusion he has cred both in ESL's and amplifiers to drive them with, so I hope he will respond. It's was posted here that he of course is biased towards his own S/S amps for esl's, but that was quickly edited out, why was that????

Cheers George

Here is what is real... the feedback used in transistor amps is what keeps them from destroying the speakers. That same feedback is also one of the things that makes a transistor amp bright. It can do that to tube amps too, but tubes can run with less or even zero feedback and have good HF bandwidth even on ESLs.

It appears that George and I are at loggerheads. I have in the past had difficulty taking much George has posted with anything other than a grain of salt, as often (as we see in the post above) the *math gets ignored.*

He is the only one I know of to challenge a technical comment made by Al.

If we can see an ESL efficiency curve that shows that the efficiency of the speaker drops with the impedance curve, I would be willing to concede he had a point. But no such curve exists. What information that does exist suggests that any ESL has the same efficiency at 10KHz that is does at 50 or 100Hz.

There is also personal experience. Something like 80% of our MA-2 production over the last 23 years has been for Sound Lab installations. During that time, we have sold a lot of amps to customers with Quads, Acoustats, Audiostatic and King ESLs. In that time its been really obvious when we encounter an ESL that is designed for transistors- the highs are muted as George suggests. The thing is, that does not happen with *all* ESLs, only some, and there is a fix for that- the ZEROs as I mentioned earlier.

Also in this case I was careful to use the word 'associated' in my comment that he quoted (which excuses it from being a generalization, instead it is a statement taken from the experiences of many people).

Its my opinion that George is grinding an axe.

"That, coupled with the normal brightness associated with solid state amps'

That is a massive overgeneralization, and should be totally ignored.

"its easy to see why- it is common that the amplifier will be trying to make nearly 10x more power at 20KHz than it might be at 50Hz!"

Doesn't mean it's giving 10x the spl at those frequencies, it means it's staying linear and a have a flat frequency response into those load/s and frequencies.
If that were the case nearly every Solid State amp with good current ability would blow ESL's sky high at high frequencies, get real!

Cheers George

Al, as most here already know, is one of the more knowledgeable contributors on this website. I feel really fortunate that we often agree :)

Al did indeed get it right, as usual. The problem you often run into when trying to use solid state with ESLs is not enough bass and too much treble. When you look at the impedance curve of nearly any ESL, its easy to see why- it is common that the amplifier will be trying to make nearly 10x more power at 20KHz than it might be at 50Hz!

The thing is, as pointed out earlier, the ESL has a low impedance at high frequencies due to its capacitive nature and *not* because it is less efficient at that or any other frequency. It is the latter fact that is often understood by many, (apparently) including Georgelofi; this is why his comments can be safely ignored.

Of course there is not nearly as much energy at 20KHz, but the result is easily heard as brightness. That, coupled with the normal brightness associated with solid state amps (due to odd ordered harmonic distortion) is why such amps do not usually make the best choice for ESLs- even those meant for transistors.

GEorge,

I think you are right about that in general, from what I have read.

Still, not all Class D amps are created equal. I think I have read of some cases where they have worked well with very low impedances in some cases. It would be a concern, but you never know for sure till one tries. I'm wondering if anyone has?

Mapman, most Class D's I know off and some I have tested are greatly troubled by loads that go under 3-4ohms, and ESL's can dip to below 1ohm.

Cheers George

Anyone ever heard a decent Icepower amp running newer Quad ESLs? That's something I would like to hear.

When I was auditioning for new speakers a few years back, the Quad ESL demo I got at a local dealer hit the bullseye for the sound I wanted, running of CJ amp gear, save for the punch in the bass not being up to snuff with the best I had heard. I tend to think as that as always the main problem with Quad ESLs at least from what I have read and heard with most any amp out there. The rest is more shades of grey amp to amp.

Increased emphasis for a SS amp versus tube in general sounds right to me. The FR with the SS may well be flatter as well, if not perfectly flat.

HEy but then there there is the old nasty odd order harmonics that muck with our ears due to NF issue with the SS amp for sure.

There may be no single right answer, at least on paper. Only our flawed ears know what they hear for sure always.

No he didn't he, used the word "increased emphasis" where he should have used the word "linear", as in flat in regard to frequency response.

Cheers George

I think Al got it right again, as usual. :-)

There's no question that when driving the many ESL's that have impedances descending from high values at low frequencies to very low values at upper treble frequencies nearly all solid state amps will provide an increased emphasis of the upper treble in comparison to nearly all tube amps (assuming the amps being compared are operated within the limits of their voltage, current, power, and thermal capabilities).

Which of those two kinds of amplifier/speaker interactions results in the most correct acoustic output from the speaker is a separate question, however.

Regards,
-- Al

Just the Quad ESL57 impedance curve, my ML's and Acousats are even worse when I measured their impedances.
http://www.quadesl.com/quad_main.html
and add to this the highly capacitive nature of ESL's, and you can see why Roger Sander says Solid State is better than Tube or OTL Tube for ESL's

The only tube amp which is great match and is not worried by these impedance drops is High Volatge direct drive tube amps (up to 5000v at the speaker terminals). For ESL's like the oringinal Acoustat X. But these are difficult and hard and make safe. As you can imagine.

Cheers George

12-04-13: Frogman
OK, George, you win; clearly, I don't know what live music sounds like.

Hopefully Frogman won't mind if I provide some perspective on his comment by stating what he is too modest to say, that he is an accomplished professional musician in the classical, jazz, and other genres.

Regards,
-- Al

And what you would have also heard was a more defined and extended treble because those quads and these (ML's Acustats and other els's) go below 1ohm in the treble and start that impedance dip at 10khz.
Tubes I aggree with these speakers still sound good but they are definately subdued in the treble compared to a GOOD s/s amp that is BJT output and that can do current into low impedances. Like you found with the ML2 which can almost keep doubling it's current down to 1ohm for each halving of impedance.

This is not entirely true. Quads and Soundlabs have a higher impedance at 20KHz. In the case of the ESL57 and ESL63, the 20KHz impedance is high enough that even our small M-60 (60-watt OTL) can drive the speaker easily with no loss of HF performance.

In the case of ML, later Quads and Sanders ESLs, because the manufacturer is wanting to open up their market to more transistor amps, the impedance is quite low at 20KHz (some MLs are only 0.5 ohm).

In cases like these, you do as Tomcy6 mentions- you get a set of ZEROs (http://www.zeroimpedance .com) which allow you to easily drive ML or whatever! We have several customers doing just that with our older MA-1s (12-tube version) that only make 100 watts/channel, and they have no problem whatsoever making high frequencies on the speaker, with proper response to 20KHz and beyond.

As a SoundLab dealer, I have had the opportunity to hear many of their full range electrostatic speakers driven by many different amps, both tube and SS. While out of the price range of many, I have found that Ayre MX-R SS amps do a superb job, every bit as good as the best tube amps I have heard.

I have never heard quad esls have a dynamic low end like is possible with good full range dynamic speakers. I always figured that to be their inherent achilles heel, as is the case with planars generally. Have not compared with a good beefy SS amp though. I would expect some difference there I suppose, the question would be how much and what happens to all the rest.

I have heard some very good results with bass dynamics using es headphones, like Stax, so I have to think such things on a larger speaker scale are possible, at least in some rooms with some amps.

I tend to like tube amplification best though I would say with my Stax electret (not es) phones, FWIW. Of course, those use the custom Stax transformer box to help get things right.

Maybe there is a similar transformer device possible for use with large es speakers to help address the electrical issues?

Did not realize es speakers had those electronic properties. Sounds like you almost need some kind of extended bi or multi-amping to really get best results, more so than with most any other kind of speaker perhaps? OR maybe a Stax-like transformer box designed to solvethe problem, if such a thing exists?

I read the key to driving ES speakers effectively is high voltage. That is an attribute of Class D Icepower amps I believe.

Has anyone tried a high voltage Class D amp with ES speakers? I'm thinking it might be a good match.

OK, George, you win; clearly, I don't know what live music sounds like.

****It's not what works for me, it's fact, and electronic maths made up of Ohm's and Kirchoffs Laws, there is no voodoo involved.****

I will go back to sticking pins into my tube sockets now.

Interesting (appropriate?) moniker, BTW ;-)

"Frogman: Georgelofi, thanks for the link and I am glad that you found what works for you."

Like I said, I even liked the sound of tubes into ESL's, but when you hear the extension and dynamic presence that's missing with tubes once you've heard a "good" solid state BJT (bi-polar not mosfet) output stage amp into them with lots of watts and current, you will think hard about what's missing, but then maybe you like that subdued rolled off top end.

It's not what works for me, it's fact, and electronic maths made up of Ohm's and Kirchoffs Laws, there is no voodoo involved.

Stated by one of the masters of ESL's design Roger Sanders that I quoted from the white paper I gave the link to. Here again in case you didn't fully read it.
http://sanderssoundsystems.com/technical-white-papers/172-tubes-vs-transistors

Cheers George

Georgelofi, thanks for the link and I am glad that you found what works for you. So did I, and it's tubes with electrostats. This is an old story in our hobby and after many years enjoying it I have learned that one of the few things that I can consistently trust to tell me what's "right" is my ears.

Ive heard quad esl of cj tube amp. Very nice match!

Frogman: You know I hope that most SS amps have a lower output impedance than most tube amps.

Here is a quote from Roger Sanders of ESL fame a white paper on Solid State v Tubes for ESL's.

"This impedance problem is relatively minor when dealing with conventional, magnetic speakers. But an electrostatic speaker is an entirely different animal. An ESL is a capacitor, not a resistor like a magnetic speaker. The impedance of a capacitor is inversely proportional to frequency. Therefore the impedance of an ESL typically varies from around 150 ohms in the midrange to about 1 ohm at 20 KHz.
A tube amp will be able to drive the high impedance frequency bandwidth (the midrange and lower highs) of an ESL with linear frequency response. However, at higher frequencies, the impedance of the ESL will drop below the impedance of the amplifier and the amp will then roll off the highs to some degree depending on the exact impedance mismatch and the frequencies involved.
I think you can now see why I prefer very high power, solid state amps without any protective circuitry for driving ESLs. This is because they can drive ESLs with linear frequency response, while tube amps roll off the highs."

You can read the whole White Paper here:
http://sanderssoundsystems.com/technical-white-papers/172-tubes-vs-transistors

Cheers George

Atmasphere,
I have a pair of ML Ethos driven by a Plinius SA 201 (class A/B) with an Aesthetix Calypso preamp. They sound great but I would love to get a more natural, more 3D sound. What tube amps would you recommend trying with a pair of zero autoformers?

I can't dispute your technical assertions, but I can tell you unequivocally that the electrostats driven by good tube amps that I have heard (my own Stax, Quads/Jadis, Quads/Quicksilver, Soundlab/CJ, and others) have ALL sounded more natural in the highs, and certainly not "deficient" in the highs than even the Quads/Levinson. Your definition of "defined and extended" may not necessarily be my definition of natural.

Frogman: "original Quads driven by Levinson ML2 sounded very good."

And what you would have also heard was a more defined and extended treble because those quads and these (ML's Acustats and other els's) go below 1ohm in the treble and start that impedance dip at 10khz.
Tubes I aggree with these speakers still sound good but they are definately subdued in the treble compared to a GOOD s/s amp that is BJT output and that can do current into low impedances. Like you found with the ML2 which can almost keep doubling it's current down to 1ohm for each halving of impedance.

Cheers George

I couldn't agree more. I base that strictly on what my ears tell me whenever
I hear electrostatics. What I hear is that the transparency of electrostats
lays bare ss amps' tendency to sound leaner and less dimensional than
tube amps with a resulting sound that can can be sterile and dimensionally
flat; as opposed to the dimensionality and image density that a good tube
amp offers. A couple of examples of this (besides my own) that come to
mind are Martin Logans run by Threshold amps at audio shows,
Quads/Spectral at Lyric, and most recently a friend's InnerSounds with the
InnerSound amplifier. On the very same InnerSounds, my Manley tube
monos sound timbrally closer to real with full and and dimensional images;
but, admittedly, not capable of as much volume or bass extension. In
fairness, original Quads driven by Levinson ML2 sounded very good.

Regardless, the ability of an amp to double power as impedance is cut in half is not an advantage on an ESL. This is because the speaker really wants to see constant power regardless of its impedance at a particular frequency, as the impedance curve is based on a capacitor rather than the resonance of a driver in a box.

Consequently tube amps have been the preferred choice for ESLs since the 1950s and remains that way today- if you really want to hear what the ESL does, you gotta have tubes, else you are leaving a lot of the speaker's capabilities on the table.

Rodman999999 You mis-read my statement on I vs R.
I said "that can almost double", the app word being "almost" you read what you wanted to see.
And here is the advertised specs of an Australian amp called and ME1500 and yes it can "almost" double all the way down to 1ohm 220-8 430-4 800-2 1500-1. And thats both channels driven at the same time!!

http://www.me-au.com/ME_1500_data_1.jpg

http://www.me-au.com/ME_1500_data_2.jpg

And even the smaller ME amps do the same amount of "almost" doubling. Show any Mosfet that can do these kind of figures, and like I said I'll show you a case for false advertising.

Cheers George

I am no fan of SS amps(except for driving woofers), but-
when someone makes blanket
statements, regarding MOS-FETs and ESLs; I have to take
exception.
Whatever their application; they still protect themselves
against thermal runaway, unlike BJTs, and can remain stable
in high-current situations. I
will side with those that have used them(past and present),
to great success,
in
their output circuits, ie: David Hafler's SS amps, Acoustat
TransNova TNT 200,
Van Alstine Synergy 450, Accuphase A-60, BAT VK-655SE,
Goldmund Mimesis8, Jeff Rowland M-9, to name a few(all Class
A or A/B). I've
not
heard any of those accused of being unstable, into any kind
of
real-world load. It's been my experience, as well as
others,
that MOS-FETs(in a correctly designed circuit) have no
problems driving ESLs. You mentioned PASS and I'll use the
following article, as my parting shot. Especially pay
attention to the portion entitled, 'Stretching Those
Muscles':
(http://dagogo.com/pass-labs-x350-5-amplifier-review)
Happy listening!

Rodman99999,
this debate on MOSFET vs BJT has been duked out before many times & I do NOT want to get into it again. So, I'm writing this with a bit of trepidation -
both your references for MOSFETs seem to deal with MOSFETs used in a high frequency switching application (this is the key here - switching application) where MOSFETs are deemed better power devices than BJTs simply because MOSFETs are all-majority carrier devices (unlike BJTs which are bipolar carrier devices. hence the word "Bipolar" in BJT - both charges exist in this device - majority carriers in the emitter & collector & minority carriers in the base). Today's class-D amplifiers use power MOSFETs in their final output switching stage & for good reason as this stage operates at 10X the max audio frequency & we want to minimize the losses in this stage. For traditional class-A & class-AB power amplifiers, the output stage is NOT being used as a switching output stage; it's being used a continuously-variable analog output stage with high current capacity. I.E. the output stage is a gain stage & not a switching stage. There's a big difference here. You can definitely use power MOSFETs in the output analog stage as we see with Pass Labs, Threshold & many other brands. Not saying it cannot be done....

MOSFETs make better high frequency switches because at high frequencies we worry about the AC losses (CV^2f). These are the losses that are directly proportional to frequency & the parasitic capacitances of the device. Recovery time from off-to-on & on-to-off is also a big consideration in switching application & in MOSFETs this time is less than in BJTs as one does not have to wait for the majority & minority carriers to move from & get back to home base 'coz in MOSFETs there are no minority carriers!
MOSFETs are self-limiting current output-wise (higher temp means higher resistance, which means lower output current. BTW, if you did not realize this - this is negative feedback! But this negative feedback is localized to within the device) & all that is very well.
Output current-wise I also believe that BJTs do a better job - for the same amount of drain (MOSFET) or collector (BJT) current, the gain of the semiconductor device is higher in the BJT than in the MOSFET. Current in a BJT is linear relationship to circuit parameters like Vbe, circuit resitances, etc. In a MOSFET, the drain current has a square-law relationship to gate-source & threshold voltages.
At the very heart of it, the MOSFET is a voltage device (yes, it does convert input voltage to drain current, hence, the transconductance) but, naturally, it is a Field Effect device. And, intrinsically, the BJT is a current device. Yes, applying voltages to the terminals does create high electric fields but the applied voltages are meant to create electron flow (current) in a BJT & not setup a source-drain channel (like in a MOSFET).
MOSFETs have become very good power devices off-lately but I believe that BJts do a better job of high current delivery in analog circuits.

MR G- I can't recall ever seeing anyone publish specs,
claiming to double output wattage, all the way to two ohms.
Nor did I claim that for the amps that I sold. re: Current
handling/MOS-FETs
vs BJTs: Maybe that first article wasn't clear enough, or too
technical. Here's something a bit easier to digest, perhaps:
(http://www.ixys.com/Documents/AppNotes/IXAN0061.pdf)

Sorry but Mosfets can't do current like BJT's can. They can do watts no problem though.
Show me published specs on a Mosfet amp that can almost double it's "tested" watts output all the way from 8 to 4 to 2ohms and I'll show you a case for legal action.

Cheers George

Mr G- You may want to realign your blanket statement, re:
MOS-FETs
and
current. MOS-FETs are innately self-protecting, regarding
thermal
runaway, unlike bi-polar transistors
(http://www.onsemi.com/pub_link/Collateral/AND8199-D.PDF).
I sold(mostly modded)
David
Hafler products, for years, which all used MOS-FETs as
output
devices. As a sales point: I used to demo the DH-500,
measuring the amperage into a 2 ohm dummy load. I had read
as
high as 48 amps, without damage or overheating(and thermal
protection activation). You could practically arc weld with
the suckers! I, as well as a couple other customers, owned
Acoustat Model III's, and had excellent results with the
Hafler amps, at realistic SPLs.

You forgot to mention Dhcod that they also dive down to below 2ohms as well. And don't go by the Stax published impedance graph, it's fudged quite a bit, we measured it and it's far from what they pictured. Add to that it's capacitive loads and you have a speaker that is easy to drive at some frequencies and a down right pig at other frequencies.
And a quad esl 57 is even worse. And other ESL's are just as bad if not worse again, look at the Martin Logan Montise crazy amp impedance load and add to that the nasty phase angle, amps with current need only to apply.

http://www.avsforum.com/t/1454033/amp-for-electrostatic-speakers post number 12

http://www.stereophile.com/content/martinlogan-montis-loudspeaker-measurements

For over 40 years I've only used ESL's
Quads, RTR's, ML, Acustat, Stax. Sure some of my big tube amps will sound good into them, but nothing drives them like a strong current bjt output stable solid state can. Direct drive esl's can also sound good but that's getting into the deadly territory

Cheers George

I copied this from a post from Kent of ESS. He's the expert on F81s. Has them. Sells them. Rebuilds them.

The Stax ELS F-81 is one of the most difficult loads one can find. From 40Hz-800Hz the impedance is well above 100 ohms. Around 180Hz the impedance peaks around 512 ohms! Amazing.

Solid state does not like this sort of load. A Zoebel network is highly recommended if using solid state. OTL tube amps are ideal but can be pricey. Transformer coupled tube amps will handle this kind of load.

Power output will depend on required SPL in the listening room. A smallish room with a nearfield set-up will yield SPL in the 90-92 range with a mere 60 watts. I use an ARC D-70, 65 watts, with good results except for clipping if I want to drive things. Or I have an ARC Ref300, 280 watts, that will drive them louder than I like without clipping issues. Well I can clip the Ref300 but then I am pushing the speaker close to failure, probably the transformer.

Any power amp will need to have a large, robust and stable power supply. An ARC VT130, 110 watts, would be ideal with these speakers and is within your budget. The AtmaSphere M-60 would be a great match if maybe just a little under powered.