Electrostatics and tubes

The advantage ESLs have over other kinds of speakers is the fact that the Modus Operandi  of operation is powered. In a conventional speaker the voice coil moves when current is put through it in response to the magnetic field that is present. That magnetic field sags a bit when this happens,resulting in compression. In addition the voice heats up slightly with power, resulting in something called thermal compression.

ESLs don't have this problem so are one of the more dynamic speaker technologies out there. The real issue is driving them because they don't adhere to the Voltage rules which most speakers do. In that rule where the speaker is 'Voltage driven', its expected that the amp can behave as a Voltage source, which is to say it can put out the same Voltage regardless of the speaker load impedance.

ESLs have an impedance curve isn't based on a driver in a box. So there's no box resonance although there typically is an impedance peak in the bass region like many box speakers. But unlike box speakers they need to have power in that peak; in a box speaker this impedance peak is a resonance (a region of higher efficiency) so you want to throttle back the power so as to not play too much bass. If the amp behaves as a Voltage source then this happens.

So ESLs work better with amps that do not behave as a Voltage source. That's one of the reasons they can often work better with tubes. But ideally the tube amp should make constant power with respect to load and to do that the amp has to be either zero feedback or have equal amounts of Voltage and current feedback (which is rare; I don't think any such amps are in production today).  

Obviously this makes finding the ideal amp for an ESL a bit tricky. But once you have the right combination they can be pretty hard to beat at any price.

To add to Ralph’s (atmasphere) expert commentary, probably the most transparent music reproduction I have ever heard was a pair of QUAD ESL’s driven by a pair of his M-60 OTL amps.

Roger Modjeski was also a huge fan of the QUADs, and used that loudspeaker as one of the loads in the development of his Music Reference RM-10 power amp. Roger then took the ESL/tube pairing to it’s ultimate realization: he developed an amplifier in which the tubes directly drive the ESL stators, bypassing the input transformers of the QUAD. He made only a very few pairs of that amp before Cancer ended his life in 2019. I can only imagine how transparent that Music Reference/Direct-Drive ESL pairing must sound.

Let's clear up something here. Roger's direct drive amplifiers were made for either the Acoustat ESLs (non-hybrids) or his own ESLs. None were made for the Quad ESLs (the voltage was too high). The RM-10 was manufactured during the time Roger owned Vandersteen and Quad ESL speakers so those are the speakers he tested the amp on. By his own account he was surprised how well the RM-10 behaved on the Quad ESL. The key was that the output of the RM-10 was 35V which was the maximum tolerated output of the Quad ESLs so the amp wouldn't overdrive the bass panels. By his own admission Roger acknowledged in my presence while listening to the RM-10 on my Quad ESLs that while it was a nice amp for the speaker, and pretty flat, the bass was a bit lean.

Thanks for the clarification @clio09. I believe you have corrected me on that matter before .

I attended the talk Roger gave at Brooks Berdan Ltd., in which he introduced the RM-10. At that time I had a two pair of QUADS, mounted in a frame of my own design. At that talk I asked Roger if he recommended a single RM-9 or a pair of RM-10’s (one on each speaker pair) to drive the four ESL’s, and he told me a single RM-9. He said he did so because of the RM-9’s greater headroom capabilities.

At his next visit to Brooks’ shop, he came over to me and said he had been thinking about my question, and had changed his mind: he now recommended one RM-10 for each ESL pair. By that time I had returned to a single pair of QUADS, so the matter was moot.

I also owned a pair of the QUAD II tube amps made for the ESL’s, and it was easily outclassed by the RM-10. Bill (@whart), you really should get yourself a Music Reference RM-10 Mk.2 for your QUADs!

 The Link shows a variety of info relevant to the Quad ESL Development.

At the end in the Red Highlight, there is a reference to EAR having a Amp Design to be coupled as a Direct Drive. I have heard the Public Demo' of these in the 90's and from that day on, ended a search for a Speaker. The experience is one of the most indelible for being extremely impressive, I have had during my long term interest in using Audio Equipment 

I am a original 57 user and this evolved into using a Stacked Pair of rebuilt 57 models using One Thing Audio replacement Treble Panels.

I also as a result of the impression made by EAR, had Monoblocks Power Amps, Designed / Commission Built using 845 Tubes and Hand Wound Transformers to drive the 57's. 

I am also very Familiar with Quad ESL's owned by other, being 57's and stacked 57's,  the 63 model from different era periods of production, through to the 2912.

The 57's in a Good Condition !!, hold their own in all Quad ESL Company.

Early production 63, built as the first 200 Models has been used in demo's against 57's ,a later model  63 and 2912's, where the early production 63, has stood out as attractive in use as the 57 and 2912, this is a very difficult find, leaving the early and later Quad Models as the easier options to acquire.

Later model 63's in my experience of the ones demo'd, are enjoyable when encountered in use, but when used as part of a demo' comparison to earlier / later Quad ESL Models, the later model 63's I am familiar with are shown to be wanting more of an improved end sound to compete.

The 57's and 2912's are used in different systems regularly listened to. As systems, these have also been used to add other types of audio devices to for demo' purposes.

Whether the 57's and 2912's are being used with a different Source, Analogue Digital, Pre Amp Design as SS or Valve, Power Amp's as a SS or Valve. The 57's 2912's are always able to offer an impressive end sound, as well as easily be able to produce a sonic difference to help evaluate a change being made with a Systems devices.

https://www.quad-hifi.info/public/esl57%5B3015%5D.pdf