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Reverse engineering a classic audio product?
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cvanc:
OK audio types, here's something you don't see every day...

Attached is the schematic for the Beveridge Electrostatic loudspeaker model 2SW-1.  Back in the day it was argued to be one of the finest speakers ever made.  Let's figure out how it works.

It has a built-in hybrid amplifier that uses tubes for the output stage.  The configuration is novel, allowing for transformerless drive to the transducer panels.

I would love to discuss how the output stage works.  Especially how does it bias the driver transistor pair, and how does the output node of the tubes stabilize at a quiescent state halfway between +HV and ground?  I am not visualizing it...

Thanks!
duak:
R1 & R3 look like feedback resistors to the driver stage.  A higher cathode voltage on V201 & V203 leads to more base current into Q108 & Q109 then Q110 & Q111 turning them on harder, pulling the grid voltages of V202 & V204 up causing their plate voltages to fall and bringing the cathode voltages back to nominal.  The actual voltage is a function of the voltage of the -60V rail and the values of the various resistors in the feedback circuit.

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