Using tube amp with electrostatic speakers.

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.

tomcy6:

Here is Dick Olsher on "Curvilinear" diaphragms from Sterophile:

"A truly curved diaphragm (à la MartinLogan) can have wide horizontal sound dispersion, but the problem is its limited linearity at low frequencies, where large excursions are required. When the diaphragm is pulled forward, it is stretched, increasing tension. When pulled back, its arc of curvature decreases, which causes it to lose tension. This difference in tension in the two halves of a push-pull cycle severely limits the linear excursion range where distortion will be low. What's a poor designer to do?"

Read more at http://www.stereophile.com/content/sound-lab-1-electrostatic-loudspeaker#KSGYY7hMRcYqtpsY.99

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.

@tomcy6 An "autoformer" describes the construction of the output transformer, while "output transformer" describes it's function.  In this case they describe the same part of the amplifier.

In an autoformer there is no secondary winding, and therefore, no galvanic isolation compared to a dual coil transformer.

Best,


Erik