Half of a Williamson amplifier used to drive electrostatic headphones


    Amazing as it is to me now, at the time, I found the circuits unsatisfactory as they did not use any feedback. All the amplifiers I designed used feedback back then.
  The circuit that follows is the basis of the amplifier I built in 1986 and still use in a modified form today.  The amplifier uses two tubes per channel and a raw, unregulated +/- 330 volt power supply. The output is at DC level and directly applied to the stators. An unbalanced or balanced input signal can be used and two Differential amplifiers consisting of a 12AX7 and a 6SN7 provide sufficient gain (just barely)  to drive the headphones from a high output CD player.

  The first stage uses a 12AX7 and functions as a Long Tail phase inverter when presented with an unbalanced input signal and as a Differential amplifier when presented with a balanced input signal. A twist is found at the cathodes. Each cathode has its own small valued resistor, 500 ohms, that meets the other cathode resistor and the common resistor that leads to the negative power supply connection. Each cathode also finds a feedback resistor, 50k, that bridges the cathode and to one of the outputs. (I remember the feedback ratio being 6 dB.) With the feedback loops left open, the gain is increased and the sound becomes less constrained, but the power supply noise edges itself into threshold of perception. 

First  version of the all tube electrostatic headphone OTL amplifier


pg. 5


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