voltage swing at the highest frequency being reproduced must first be calculated:
Slew Rate = (2¶FVp) / 1,000,000.
In this case, where 20 kHz is highest frequency and 500 volts is the peak voltage swing,
Slew Rate = (6.28 x 20,000 x 500)/1,000,000
Slew Rate = 62.8 volts per microsecond.
Think of slew rate as a measure of the steepness of the waveform at its sharpest slope.
The next step is to measure the total capacitive load represented by the electrostatic headphone's stators and its cable. The Stax Lambda Pros come in at 130 pF stator to stator. Then this quantity is multiplied against the slew rate to reveal the needed amount of current to charge and discharge the capacitance at the desired frequency and voltage swing.
Current = SR x Capacitance.
In this case,
Current = 62.8 x 0.00013
Current = 8.16 mA.
This may seem like a fairly trivial amount of current, but do not forget the 250 volt voltage swing and the high voltage power supply, which when multiplied against the current equals a fair amount of power.
Once we have determined the peak current requirement, we can begin designing the amplifier.
Driving an intrinsically push-pull transducer with an SE amplifier has a perverse attraction, I must admit. This will require some thought, as the signal must be presented to the stators in a push-pull format. This could be accomplished by using an output transformer with a center-tapped secondary or by using a center-tapped choke as a load. The first configuration is obvious: a conventional push-pull output transformer is wired backwards so that the triode's plate works into the primary and the secondary feeds the stators, with the center tap going to ground.