This question is perennial. I guess I should explain.
The electrostatic loudspeaker was artfully designed specifically to destroy push pull Class AB solid state amplifiers. There is no other way to put it.
The argument goes this way.
The PP SS amplifier, both Class AB and Class A, has one mortal enemy. Amp kryptonite is called cross conduction (CC). CC puts one rail in direct communication with the other - we can forget about the speaker here - via the output transistors. A horrific current flows, aided and abetted by the largish capacitance at the rails, and the output devices lay down their lives in the electronic equivalent of a massive auto burnout.
So, what causes CC?
Oscillation; uncontrolled switching of the output devices precipitated by phase shifts across the global feedback network. Negative feedback must remain NFB, and when it strays into the PFB region, the amp oscillates in short order.
And what causes PFB?
Capacitance in the output stage, amongst other causes, typically around 2uF. Capacitors change the relationship between current and voltage; phase shift is the result.
The phase shift induced by high capacitance at the load transmutes negative to positive feedback - voila, oscillation, CC, and solid state death!
An ES speaker has planar electrodes stretched quite close together. Of itself, this is perhaps not too much of a problem since the resulting capacitance is only of the order of picofarads. However there is a problem; ESs must be driven with very large AC voltages and for this we need a transformer with a high step up ratio.
Typical transformers used in the Quad have a step up ratio of 100 times. This means (by simple transformer physics) that impedance, and specifically capacitance, is magnified by 100 times squared, viz 10,000. This is the problem........
If we take the 200pF interelectrode capacitance of a Quad (these are guestimates; don't have the spec sheets in front of me) and multiply this by the requisite 10,000 to get the 'reflected' capacitance at the low impedance primary, we finish up with the 2uF specified earlier.
Now, the AKSA will cope with 2uF at amplitudes where the inevitable ringing does not precipitate oscillation. It will handle both the ESL57 and the ESL63 AT LOW VOLUME. However, BE WARNED: Turn up the wick, and destroy the amplifier!!!
OK, why the hell can't the AKSA handle full power in an ES? Good question.
The reasons have to do with careful dimensioning of the feedback system which promote sonics, but take the amp just inside the Bode-Nyquist feedback criteria. The AKSA is much more stable than most SS PP amplifiers, but even it has limitations, so be warned: the ESL is the proverbial wooden stake/silver bullet for the AKSA, just as it is for other SS PP amps.
If you want to drive ESLs there are only a couple of choices. One is tube amps, which, with low feedback circuits, are assuredly good with these loads. The second is zero feedback mosfet SE designs, a la Nelson Pass, which drive them beautifully. Some years ago I designed a 28W single ended tube/mosfet hybrid called the Glass Harmony which is eminently suited to driving ESLs, and in fact four years ago I installed two pairs of these monoblocks with ESL63s and a Scandinavian active subwoofer system. They have never given a moment's trouble, and the owner is as captivated with the sound now as he was when he first heard them. They go remarkably loud, too.
As far as PP SS technology is concerned, the only SS PP amp which can drive an ESL with complete safety must necessarily be agricultural in design, with enormous lengths taken to prevent oscillation. The 200W Levinson is such an animal, and not a bad amplifier. But these measures ALWAYS compromise the sonics, and in truth the best amps for ES speakers are zero global negative feedback amplifiers - a VERY tall order for the designer, I can tell you......
Moral - wait until I have a SE tube/mosfet hybrid in the commercial market. It won't be cheap, but expensive, exotic speakers demand unusual amplifiers.
I hope this explains the issues. It's complicated stuff.
Cheers,
Hugh
