Using tube amp with electrostatic speakers.

I see now in the advert that the nominal impedance is 4 ohms, going down to 0.43 ohms at 20kHz, per Raul's statement too.  That's not a great situation for any sort of tube amplifier, but it could be made to work I guess.  Using Autoformers from Anti-Cables would help.

Unless Martin Logan  says otherwise, I would assume that they have not abandoned their stated policy of designing their speakers to accommodate solid-state amplification best of all. That really means the speakers are likely to have a low nominal impedance. You might check this out with Martin Logan. If I am correct, then you may be best off with a solid-state amplifier, but there certainly is no harm in trying those BAT amplifiers, if you have them on hand. If Raul is correct about the impedance at 20 kHz, that squeaky low impedance could be vexatious for any amplifier, solid-state or tube type. However, the saving grace is that very little amplifier power is needed at such very high frequencies.

I agree the fact that you or your customer(s) have tried it, and it sounds good, would trump my purely mathematical analysis.  And you are correct in noting that the formula I used does not take into account the third variable, which is the intrinsic change in speaker impedance vs frequency (although my off the cuff thought on that is that the actual -3db point with no resistor would be boosted above the calculated 4400Hz by a greater degree than is the calculated vs the actual -3db point with the 10 ohm resistor, because speaker Z is going down as frequency goes up, and the -3db point is inversely related to total Z).  

Something in my memory is telling me that both Will and I did try removing the resistor from the RC network in front of the old toroid, before proceeding on to Will's ultimate solution.  And the sound did improve when we did that, although not to the degree that we heard/hear with the replacement of the treble-only transformer with a full-range one and no R OR C in the circuit.  Anyway, I am happy that everyone else is happier, no matter how they got there.

Just to be clear-er, the Brilliance control is an "inexpensive" L-pad that in my opinion and that of many others colors the sound quite audibly.  Removing or bypassing it has absolutely no effect on the crossover point.  If one were to find that the treble is too bright with no Brilliance control in the circuit, one could add a high quality, high wattage, fixed value resistor in place of the L-pad.  The way to do that might be to set the speaker so you like the tonal balance with the L-pad in place.  Then measure the resistance across the L-pad.  Then replace the L-pad with that much resistance.  In my case, for my room, my ears, and my modifications, I am quite happy with no resistor at all.  If anything, I'd like to boost the treble a tiny bit, ideally.

Gary, That's a pretty accurate summary.  The notion to remove or bypass the brilliance control is actually older than the ideas to modify the backplate that came from Will and me.  

I remain puzzled that Sound Lab would upgrade to a larger toroid with presumably a better low frequency response and then continue to use the same values of R and C, thereby maintaining the same old crossover point (at round 440Hz for the values 10 ohms and 36uF).  But I don't actually recall what values SL used as OEM standard with the "old" original toroid, so it could be that the old values were actually something less than 10 ohms with something less than 36uF capacitance, which together or separately would give a higher crossover point.  Therefore, the values of 10 and 36 are "new" to Sound Lab.  As I now recall, the old values might have been 6-8 ohms (depending upon year of manufacture) and maybe 32 to 34uF.

Ralph, Are you referring to the resistor "R" in the RC network?  I totally agree that R is a big problem in the original design, but if you remove it entirely, that would make a dramatic change in the crossover point.  For the example given above where 10 ohms and 36uF values of R and C give you a X-over point of ~440 Hz, removing the R entirely would give you a high pass -3db point at ~4400 Hz.  This would leave a big gaping hole in the midrange, because the bass transformer does not work much above 2kHz, based on my earlier measurements of my speaker.  I am sure you are aware of all this, which is why I think I might be misunderstanding the nature of your recommendation.  You could remove R and preserve the x-over point, if you add more C, in this example.  I would suggest increasing the value of R so that less current flows through it and the crossover point is thereby lowered to a frequency more in the bass range.  This needs to be done with caution because of the limitations of the toroid itself, which we don't really know for sure (because SL is a bit secretive in this regard).  But I bet you could go to 20 ohms, giving a hi-pass at 220 Hz (assuming 36uF of capacitance).  That's actually what I thought SL was doing with their new toroid, lowering the hi-pass X-over point.

Jazzdude, Plitron do (or did) make a "full-range" ESL step-up toroid with a 1:75 ratio, and in theory this would work.  However, bear in mind that I cannot personally say I have tried it.  There are also other companies that make EI type transformers with a step-up in the needed range (1:75 up to 1:90).  The Plitron looks to be pretty robust, and it would be my choice, if I was starting from scratch, is all I meant to say.  If you go to some other brand and to an EI type, just be sure to pick a "big" substantial transformer that is rated for full-range use.  Some of the aftermarket ESL transformers look like toys and would not be up to the job.

So, if you try the Plitron, I wondered whether it could mount on the very same bracket used to mount the existing toroid.  If the diameter is too large for it to fit in that bracket, probably the bracket can be modified to accommodate a larger diameter toroid.  This is what I was thinking.

Thanks, Noromance.  I had no idea the cost would be so high.  In the photo (from 2015) they show a single pair of Quads. This year (and I think last year) EMIA ran a double pair of Quads. Believe me, with Dave's mods, there is no lack of bass response in the range where music lives.  The  speakers on demo the last three years of the show actually belong to a local aficionado who is a long time friend and customer of Dave's.  At that gentleman's house, he runs THREE pair of Quad 57s. I need to go hear that.  What's money, after all?

Yep. Quad 57s, KLH 9s, early Acoustats, Janszen tweeters.  They were all made with tube amplifiers in mind. For a real treat you might look into the Quad mods made by Dave Slagle, for driving them with tubes. Fantastic results.

Gary, Most of what I have done is to make the speakers sound optimal with my OTL tube amplifiers, which I genuinely believe are superior, and I did once borrow a pair of Innersound amplifiers for comparison. Since you already own a tube amplifier as well as your Innersounds, you might want to try bi-amplification, with the tube amps running the bass transformer and the Innersounds running the treble.  You would have to make allowances for balancing the output, by attenuating either the Innersounds or the tube amp, depending upon what you hear.

On the other hand, with the 20 ohm resistance and the same 36uF capacitance, you may like your tube amplifier, full range, best.

4 of those 2.5-ohm "coffin"-type resistors?  They are probably wired in series/parallel, but do you know the net resistance of the bundle?  Could be 10 ohms or could be 2.5 ohms.  Neither value is gonna make your OTL any happier than mine was before I chucked the crossover. (Really, give all credit to Will; all I did was follow his lead because I was so unhappy with the sound I was getting from my 845PXs.)  You are definitely better off with 12-14 ohms than 10 ohms.  However, I am a little surprised at the values; they are identical to what I was using with my "old" toroid (10 ohms/36uF), before the big modification (removing the crossover entirely).  The -3db point of the high pass filter is inversely related to the product of R times C, where C is in Farads and R in ohms.  For a 10-ohm resistance with 36uF, I calculate a crossover point of 440 Hz.  I would have thought they could use a much lower crossover point with the new toroid, but I bear in mind that Dr. West is very conservative in his design so as to favor reliability.  From my experience making measurements, the intrinsic impedance of the speaker is much higher than 10 ohms at all frequencies up to somewhere between 2kHz and 5kHz. At 5kHz, it's about 8 ohms for my 845PXs.  (Bear in mind, measurements were made with NO crossover, driving the SL factory bass transformer and the full-range Australian transformer in parallel with each other, directly with an audio frequency generator.) So, as you can imagine, when you interpose the RC network, at low and mid frequencies, much more of the energy is going through the resistor than through the speaker (R is smaller in value than speaker Z at those frequencies, so R dominates). This may be why some guys on SLOG discovered long ago that very high wattage resistors seemed to sound better.  There is almost no limit to the wattage ratings they tried, more than 100W, even.  Before my epiphany, I was using an enormous 10-ohm resistor hand-made by one of the British contributors to SLOG.  It could tolerate at least 500W. (One benefit of bi-amplification is that only the treble amplifier then "see"s that R in parallel with its output; the bass amplifier drives the bass transformer only through the inductor.  Thus paradoxically, your SS amplifier might work best on the treble (because it can handle the power demand and the low impedance created by the R, with your tube amp on the bass).

If you go up to 20 ohms, the crossover point would be ~220 Hz. (The beauty of the equation is that once you know the values for one set of parameters, you can calculate in your head what will happen if you change R or C.  If 10 ohms and 36uF give you 440Hz, then 20 ohms and 36uF give you 220Hz, etc.)  I would have thought that the new toroid would be quite happy at around 300 Hz, not too far from where you are at if you are using 14 ohms now.

There is no difference between bypassing or removing the brilliance control and setting it to zero. But I preferred to remove it; it's a cheap part that muddies up the signal path. You will hear an improvement. No, the speaker is not too bright without it, in my opinion.  The easiest thing to do is to re-solder the input and output wires to the brilliance control together on one binding post of the control.

As far as impedance variations of the speaker per se, the ESL can be thought of as a giant capacitor.  It has very high impedance at very low frequencies and then gradually falls off in a fairly linear manner to very low impedance at very high frequencies.  I wrote here earlier that in my opinion, very low impedance at 10kHz or 20kHz is not a real problem, because the energy requirements are very low at those frequencies.  What makes for problems is the circuitry that comes before the panel, e.g., that 2-ohm impedance dip at about 1kHz that came with the old toroid due to its crossover.  Nearly every company does something to make ESLs more friendly to SS amplifiers, which in turn makes it hard on tube amplifiers. 

Gary, Mea culpa.  I forgot that not only were there several different combinations of R and C used by SL over the years, but also at one point they did introduce an inductance in the hi-pass filter, which creates a 12db/octave or second-order slope.  I think that's the inductor you talk about.  I briefly experimented with that part (after purchasing a pair from SL), long before I made the major modification I described in my recent post.  For reasons I no longer can recall, but likely because it sounded bad, I ditched the inductor in the hi-pass filter and reverted to the simpler RC filter, which gives a 6db/octave slope.  I also forgot about the "brilliance" control, which is anything but brilliant.  Ditching the brilliance control was one of the first things I ever did to improve my speakers; I replaced it with nothing, no resistor even.

One must keep in mind that Dr. West is an engineer first and foremost.  Thus it seems he is more concerned with measurements than with how certain modifications affect perceived sound quality.  This means he does not like impedance peaks within the audio band, for example, but he seems less allergic to impedance dips.  At the same time, I regard him as a very fine person in all respects and someone who knows much more than I do about the technical aspects, of course. (Goes without saying, actually.) He communicated with me and Will, when we posted our findings with the AU transformer on SLOG.  Subsequently, he had the open-mindedness to replace the toroid he was using for treble with one that has a lower low end response, in recognition of what we were trying to "fix" in the first place.

For one example of his EE emphasis, Dr. West pointed out to me that my "solution", using two transformers in parallel where one is full range, results in a sharp impedance peak at 250 Hz. (I think he found this by computer modeling, not by actually measuring one of his speakers driven by the Australian 1:90 transformer that I use, because he never purchased one.)  When I made my measurements, I took impedance readings at 200 Hz and 500 Hz , thus completely missing the impedance peak.  (I used a log progression: 20, 50, 200, 500, etc) But the peak (assuming it is there in the real world) does absolutely no harm to the capacity of the Atma-sphere OTL to drive the speaker; there is certainly no discontinuity perceivable.

Just out of curiosity, what value of R does SL use with the latest toroid, if they use 36uF of capacitance?

Ralph,
Your information about the new Sound Lab treble transformer ("back panels") is consistent with what I have heard too, from other Sound Lab owners.  They all seem to like it, and they all sense the improvement in sound compared to the old set-up. Even the one or two guys who first tried "my" approach and who were not satisfied, are now happier with the new toroidal treble transformer.  No doubt that is the simplest thing to do, if one owns an older set of panels.  

Note to others that getting rid of the crossover entirely was not originally my idea; it was communicated to me by "Will" in Australia, via SLOG.  Will also helped me and others to obtain the Australian-made full-range 1:90 step-up that he used too to modify his 945PXs. I bought the last pair of the original run of those transformers, but the vendor had more of them made due to the demand from among SL owners. There may have been something odd or different about that second batch of production units, because I know of at least two guys who were not happy with them. We'll never know why, with certainty.  This is why I would recommend the Plitron 1:75 full range toroid, not the Australian transformer, to anyone who wants to try the mod I describe above.  I personally have never thought twice about adopting the new SL toroids to replace what I've got.  Anyone in the DC area is welcome to stop by my home for a listen.  

To be more clear, the Plitron toroid is a full-range step-up that one might choose to install if one wants to go with no treble crossover at all.  SL sells a toroid that requires a high pass filter in front of it; it's not designed to run full range.  But with the SL toroid, the hi-pass crossover point is down around 200Hz, which does dramatically improve the impedance curve compared to original and the efficiency of the speaker.

I am fairly sure that the A1s would look like my 845PX with respect to impedance, which is "very high" up to 100Hz (50 to 100 ohms), then around 4-5 ohms up to around 1kHz to 2kHz where there is a sharp notch dip to 2 ohms or so, then back up to more reasonable values, then back down at 5kHz and above, to 2 ohms or less at 20kHz.  This is from memory.  (I could REALLY hear an improvement in vocals when I got rid of that 2-ohm dip; see below.)

You can probably assume, if you don't want to or cannot measure, that the B1S will present a fairly high impedance to the amplifier, and in that respect it should be an easy load for any tube amp, but the question would remain how much amplifier power you need to make it sing.  You might ask the folks at Sound Lab.  

One option you didn't mention but which works quite well is to bi-amplify your A1s.  I've never done it, but you can experiment with that idea.  See if you can find the old "Sound Lab Owners Group" archives for some hints.

And finally, you might do what I did with my 845PXs: (1) Acquire a full-range ESL step-up transformer with a turns ratio of 1:75 up to 1:90.  Plitron makes a nice one, toroidal at 1:75, but there are several others. I used a EI type transformer made in Australia that is unfortunately no longer available.  (2) Remove the existing treble transformer in your A1s and also remove the RC network that effects a high pass filter feeding the treble transformer. That RC network is the problem; it creates the low impedance dip right around the crossover point, because of the value of the R that SL chose to use. Also, the R (typically 5 to 8 ohms in value depending on how old your speakers are) soaks up amplifier power, because it is in parallel with the panel at all frequencies. The intrinsic impedance of the panel is mostly way higher than 5 to 8 ohms, so most amplifier energy flows through that R.
(3) Install the full-range transformer in place of the existing treble transformer AND the RC network, so there is no longer an RC at all.  You can retain the inductor that feeds the bass transformer.  Make sure the two transformers are "in phase".  And then your A1s can be driven by your 60W tube amplifier all day long. The resulting impedance is going to be 20-30 ohms from about 100Hz to 5kHz, going down from there but fairly constant between 100 Hz and 2kHz. And the efficiency is ridiculously improved.  I doubt my speakers draw much more than 20W, steady state.

Or finally, talk to Roger West about upgrading your treble transformer to his latest device, which is more nearly full range. That allows for a lower crossover point and also gets rid of that midrange dip in impedance, according to them. With that option you keep the crossover (but change the values according to his instructions). Plus, this mod is factory sanctioned, unlike what I did.

It may seem that I am trying to avoid your actual question, but that is not the case, really.  First, most transformer-coupled tube amplifiers provide a choice of 4, 8, or 16 ohm taps, not 2, 4 and 8 ohms.  Second, the very low impedance at very high frequencies exhibited by an ESL is not so much of a problem for the amplifier, because very little energy is required at those high frequencies.  (For example, I made impedance vs frequency curves for my Sound Lab ESLs after eliminating the crossover components; impedance does not drop off significantly until 5kHz and goes to 2 ohms only above 10kHz. But every ESL will be different from every other one, in this respect.) I suggest that you try each pair of output taps for yourself and see what you prefer.  There is no right or wrong answer to your question, except to say that most amplifier energy will be required in the bass and midrange frequencies.

Nordicnorm, M-L candidly admits that they design their speakers to favor SS amplifiers.  (See also my post of 11-02-16.) This means that there is circuitry at the input that dramatically lowers the impedance seen by the amplifier, well below the native impedance of the speaker itself.  Thus your finding is not surprising. This is why I sold my last pair of M-L ESLs about 20 years ago and never looked back.  No way I would give up my OTL tube monoblocks just to suit one particular speaker.

I would only add one point to this discussion.  It is folly to talk about "tube amplifiers" and "ESL"s in general terms, because each and every tube amplifier and each and every ESL in the marketplace has its own idiosyncrasies of design that strongly influence the interaction between the two.  In my opinion, tubes and ESLs ought to be an audio match made in heaven.  (I have owned nothing but tube/ESL systems since the early 1970s.) However, in the era of solid state hegemony, many ESL companies have designed and marketed products that are tailored to work well with SS amplifiers, which usually has a corresponding negative effect on the capacity of the speaker to be driven by a tube amplifier, and/or there are designs with complex crossovers that impose impedance curves unfriendly to tube amplifiers.  Martin Logan seems to have adopted a pro-SS amplifier philosophy many decades ago.  Their original ESL, the CLS, had a nominal 16-ohm impedance and was a beautiful match with the Futterman amplifiers that I owned at that time.  They followed that up with the "CLS II", which I bought before auditioning them, just because I loved the original CLS so much.  The CLS II was a gross mismatch for an OTL tube amplifier like the Futterman, because it introduced circuitry that created an impedance as low as 2 ohms at critical midrange or upper midrange frequencies.  Zero autoformers did not exist at that time, and the mating of the CLS II with my Futtermans was a disastrous non-starter.  The moral of this story is that it behooves any buyer to educate himself as regards the technical aspects of the components he wishes to match together in one system.  There are no rules one way or the other.  

I am now using Atma-sphere amplifiers to drive Sound Labs 845PX speakers.  Frankly, this was not a perfect situation until I made some rather drastic modifications to the Sound Lab input circuit such that the speaker now presents an impedance that is 20 ohms or higher across the audio band up to about 5kHz, where it falls to about ~8 ohms and ends up at ~2 ohms at 20kHz.  (I know this, because I made the measurements myself.) As someone else pointed out, an ESL is in effect a giant capacitor, and there is no avoiding a drop in impedance at very high frequencies.  However, it is my opinion that so long as the impedance remains reasonable at least up to 10kHz, the low Z above that frequency is nothing to worry about; there is very little energy demand on the amplifier for frequencies above 10kHz, and the capacitative nature of the load should not bother a good tube design, whereas it can play havoc with SS amplifiers.  I might add that, thus modified, my 845PXs can be driven by much less than 50W of good tube power.  The ~100W Atma-sphere amplifiers are loafing even during loud passages.  

I think Al and I did allude to such issues.  It is not the high frequency information per se, unless you are talking about RF, which is never good for any system, but how the load (the speaker) reacts to those frequencies in a way that then feeds back on the amplifier.  Traditional SS amplifiers that use a lot of Negative FeedBack (NFB) tend to be driven into oscillation by the capacitative nature of an ESL as load.  Any modern design worth its salt should be Ok in this regard, probably including those made by FVA and RM, two of the oldest old timers in the business. (No disrespect intended; I suddenly find that I am an old-timer too.)

By the way, for most adult males beyond the age of 35 or so (which is almost certainly all of us here), even 14kHz is well beyond audible. Yet, I perceive that wide bandwidth is a "good thing" in electronics.

Dear Tim (Pryso), Let me help you with that.  Stewart is full of baloney if he insists on analogizing a magneplanar type speaker with an ESL.  They may both be "flat", but that's about where the similarity ends.  The principles of their operation are completely different.  Thus also their typical impedance characteristics and inherent efficiencies, respectively, are very different.  ESLs that need lots of solid state power are ESLs that were designed with SS amplification in mind and which typically contain crossover networks that grossly waste amplifier power and in the process also create nasty impedance dips at or near midrange and bass frequencies.  If he prefers SS amplifiers to tube amplifiers, that is another matter, and there he is entitled to his opinion.

Frankly, magnepans leave me totally cold. They are dead sounding; they fail to capture the natural decay of musical instruments. Only the later iterations that incorporate a ribbon tweeter are at all listenable, to me.  And that is only my opinion, of course.  The thread is about ESLs, however.

Dear Stewart, You wrote, "There's no analogizing......and I know they're different (that's why they're not both called electrostatic)"  Wrong!  Your magneplanars and even ribbon tweeters are electromagnetic, not electrostatic. In your speakers, the diaphragm is "wired"; the signal is conducted through wires glued to the diaphragm, which causes it to move back and forth in a magnetic field, much like any other traditional type dynamic speaker. In an ESL, a the diaphragm carries a very high voltage on its surface, and the signal is applied to the stators, stationary wires front and rear of the diaphragm, causing the diaphragm to move due to electrostatic forces. Therein lies the way in which ESLs are totally different.  Furthermore, ESLs, being electrostatic, do not produce a back EMF, as do electromagnetic speakers, so the whole idea of "damping factor", the reason some say that only SS amplifiers can provide "clean" ("tight") bass, goes out the window.  

Stewart,  I apologize for the tone of my remarks; I was feeling playful, and perhaps that came across as hostility, which was not my intent.  It seems impossible to get my point across to the assemblage, which is simply to re-state that one should make one's own decision on amplifier power and type based on the electrical characteristics of the speaker. Period.  There is no one correct answer that fits all situations. Of course, this goes for any type of speaker.  Sadly, most modern day ESLs are difficult loads for tube amplifiers, not because of the operating principle of an ESL per se but because of crossover design.  Martin-Logan is chief among the guilty.  But they really don't care; they assume you will buy the latest solid state behemoth to drive their speakers.  Try a Quad 57 or a KLH 9 sometime (both nominal 16-ohm loads), and you will know what I'm talking about.

And, since you brought up magnetic/planar speakers, others are now starting to discuss them as if there is any parallel with ESLs.  Apart from size and shape, there is not.  To run a magnepan with a tube amplifier, one should consider using the Zero autoformers.  I used to use Zeros with my Sound Labs, until I got some advice on how to "fix" their fundamental impedance characteristic.

Audioman,  ESLs (like Sound Labs) are not "controlled" by a high power amplifier in the same way that the misinformed might say that an electromagnetic speaker (like a Magneplanar) is controlled by high power.  Electromagnetic speakers, woofers in particular, generate a back EMF by virtue of the fact that there is spurious motion of the undamped diaphragm, to which a coil is attached, in the field of the stationary magnet structure.  A high power SS amplifier can ameliorate the problem, which manifests itself as a loss of bass definition, by virtue of its low output impedance.  Norman Crowhurst showed decades ago, however, that the output impedance of the amplifier does not need to be so infinitesimal as it often is for modern SS amplifiers, in order to achieve this goal. 

The reason some might like to use a high power SS (or tube) amplifier on a Sound Lab is because the crossover network wastes amplifier power at midrange frequencies.  However, like Ralph said, a good 60W to 100W tube amplifier is to my mind optimal. Sound Lab recently cured some of this problem by modifying the crossover.  ESLs, except at very high frequencies well above the range of hearing of most audiophiles, are NOT inherently current-loving speakers at all.  In fact, the impedance is inherently quite high across the midrange and the speaker could be said to need a voltage amplifier.  I am glad of this, because the amplifiers you named are wretched, with the possible exception of the Pass Labs.  Furthermore, the "big Krells" were notorious for upchucking when coupled to ESLs, because they were unstable into a capacitative load.

That's a well known question for Martin-Logan's design.  Most ESLs are not curvi-linear, however.  Most are truly planar.  The Sound Labs may appear to be curvilinear, but the apparent curve is composed of an array of planar facets, created by stretching the mylar over a frame.  Apart from the SLs, almost every other ESL is flat in fact and in outward appearance.  None of this pertains to the ease or lack thereof of driving ESLs with this or that amplifier.

Bag, The ORIGINAL ML CLS was a match made in heaven for a tube amplifier; it was a nominal 16-ohm load.  After I auditioned a pair in a store, I bought them immediately and loved them in my home system, although they were a bit bass shy.  I related this same story somewhere else earlier in this thread.  Very shortly after the CLS debut, ML could not leave well enough alone; they rapidly replaced the original with the "CLS II".  This is the speaker to which you refer when you speak of difficult to drive. Foolishly, I replaced my CLSs with CLS IIs without knowing much about the difference between them. Turns out, the II had a hellacious impedance dip at mid-frequencies, as you say, I think down to 2 ohms at something like 1-2 kHz. This was bad for any amplifier, and it sounded awful with my then Futterman OTL tube amplifiers.  ML got the message very quickly and replaced the II with the IIz, which was merely a bandaid on the problem they introduced when they went from the original CLS to the CLS II.  To my ears, the IIz bore no comparison to the original CLS in greatness. At that point in time (when the II was replaced by the IIz), I called the factory and asked them what was up.  They told me quite frankly that they did not care much that I could not drive the CLS II or even the IIz with my OTL tube amp; they were playing to the SS amplifier owners.  If you like the IIz with a tube amp, look around for a pair of original CLSs; they are Quad 57-like in their midrange transparency, only with more oomph.  Way better than any version of the CLS that came after them.

All flat-panel ESLs (and magneplanars too) will have issues with beaming, increasingly at treble frequencies.  I am not sure that the ML curve or the SL pseudo curve completely cures this problem.  If you love ESLs, you live with it.  I also don't think that the curvilinear design of the ML speakers is a "real" issue in causing audible distortion.  It's a theoretically real issue that a competing manufacturer, like Sanders, can use in advertising.  The original CLS was one of the lowest distortion speakers I have ever heard in my life.

I heard the Sanders amplifier (not his speakers) extensively driving my friend's large Acoustat speakers.  He and I were very underwhelmed, despite all the bloviating about voltage.  He sold the amplifier within a few months and replaced it with a Berning ZH270, a pair of them, in fact.

Arrrggghhh.  Audioman, please read the earlier posts in this thread.  You're preaching a certain kind of gospel put out by false industry prophets, but you're not speaking the truth.  Think for yourself and do some reading, and most of all know that (1) Magnepans and ribbons are entirely different electrically from ESLs and therefore should not be lumped with them, and (2) NO amplifier can "control" an ESL, because ESLs, not being electromagnetic in nature, do not produce a "back EMF".   Back EMF is the phenomenon best dealt with by an amplifier of any kind that has a low output impedance relative to the input impedance of the speaker at the frequencies in question.  This does not necessarily mean that only SS amplifiers need apply to "control" the bass response of an EM speaker; the output impedance need not be so infinitesimal as that uniquely afforded by an SS amplifier; tube amps with low output Z can do it as well or even better.  You might know this as damping or "damping factor".  Neither term applies at all to an ESL. 

If you want to argue that you prefer SS amplifiers to drive whatever planar speakers you own, that is fine.  Nobody's perfect.

Bag, You wrote, " Firstly, I never owned any versons before the IIz, and only heard a friends I many times as he sought to find an amp to drive them - Jadis, ARC, Krell, yada yada - he sold them. The Scintilla's are the only speaker that was harder to drive and that's my experience in my 45 years in the hobby. "

I hope you don't think I disagree with what you say.  The IIz was a failed attempt to correct the impedance problems with CLS II, and the "z" mod didn't do much good.  My only point was that the original CLS, before model II, was the best of the bunch sonically and was an easy load for any tube amp. No one would recommend a tube amp for the Scintilla, of course.

Well, we agree that the CLS IIz was best driven by an SS amplifier, and I added that this is because M-L were targeting such an audience and told me point blank that they were not interested in building a speaker for tube amplifier aficionados.