Amp for Martin logan CLS Originals

Celtic66 is dead on accurate. I owned CLSiiz's and used a vast variety of SS amps, namely Plinius SA250, BAT VK500, Pass Aleph 2 monos, but the best overall performance I experienced was after mating the CLS's to a pair of Manley Ref 450 mono's, that I of course acquired used. Those speakers came alive and sang with joy from power o' plenty!

The CLS 1 (the original) had a high impedance and was a match made in heaven for tubes, even OTLs.

The CLS 2 was an example of Martin-Logan's attempts to make ESLs compatible with transistors. It was very low impedance, 0.5 ohms at 20KHz. Almost any amplifier sounds better on it if using a set of ZEROs to help with the impedance.

The CLS Z and later models do show some moderation with regards to impedance, but still are built to favor transistors.

Its hard to make transistors sound right on a full-range ESL due to the impedance curve. There is a tendency to be weak in the bass and too bright in the highs. As a result many ML owners place the speaker fairly close to the rear wall to try to get back some of the bass, but it is often one-note bass due to the placement (ESLs need to be out in the room at least 5 feet).

But the CLS 1 (which, BTW, it sounds like ML still has parts on hand to build them) is an easy load for most amps and IMO the best of the lot; of course I have a bias :)

Atmasphere, you have the impedance thing backwards ;--) The original CLS, CLS I were .5 ohm. The CLS II/IIa were .6 and 1.0 ohm respectively; and the CLS IIz is 1.5 ohm

In all models, this "low impedance" occurred only above 15KHz. Raising it initially resulted from adding a second transformer to the electronics module, starting with the CLS II/IIa. The electronics module for the CLS IIz was a complete re-do and included a more sophisticated crossover network (filter network) and a 'signal sensing' circuit which turned off the high-voltage circuits when the speaker was idle -- a great idea for keeping dust from accumulating on the panels, but a disaster in terms of performance! It takes overnight for an electrostat to fully charge, or "form" -- it's just a big capacitor after all -- so most owners have probably never really heard what a IIz can sound like (unless you keep it playing music 24/7!) People with IIz's who know about this, defeat that function so the speaker is always charged up and ready to go. I never had a dust/smoke problem anyway; and Jim Power at ML said that a dry climate is the most important factor in panel longevity.

The later models were (sligntly ;--) easier on ss amps, but the changes to the electronics module were for the purpose of reducing the high end brightness and improving the upper mid-range. Even with the (1 ohm) impedance increase, you still had to buy an unnecessarily high-powered Krell, Threshold, Bryston, etc. to insure they wouldn't see the low impedance as a short! Tube amps by contrast, can handle shorts, but don't like open outputs (ie, nothing connected to the speaker terminals.)

I actually have a 1992 ML factory memo detailing most of these points, including a recommendation that the CLS be placed on stands.
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Nsgarch you know who atmasphere is right?

No. Why do you ask?