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   A third electrostatic headphone  amplifier was built using a dual Op-Amp (NE5532, I think) for the differential driver stage and a 6SN7 for the output. The Op-Amp automatically sets the bias and the phase splitting and gain of the amplifier. While the sound from this amplifier was superior to the hybrid amplifier from Stax (there was a shoot out at DB Audio in Berkeley Ca.), it did not sound as good as the all tube alternative. One problem that arose was that the output capacitors would sing. In other words, the output capacitors behaved as small electrostatic speakers. The solution was found in the use of spirally wound capacitors instead of stacked film and foil capacitors.

Electrostatic Bias Issues
  In order to achieve reasonable efficiency from an electrostatic diaphragm, a high voltage polarizing voltage must be used. Too high a value will burn out the electrostatic film and attract dust. Too little a value will decrease the efficiency and make for a soft sound. Of course, sometimes a soft sound is desirable, so desirable in fact that I included a 1 meg potentiometer in series with a 1 meg resistor to allow decreasing the bias voltage when softness is needed. The voltage specified by Stax is about 540 volts for the Lambda Pros.
   There will be more to come I am sure.


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