Portable Electrostatic Headphone Amp

I’ll be going home for winter break, and don’t want to carry my big electrostatic amplifier, so I decided to make a Class-D portable amplifier.

Here are a few schematics… Eagle is too much of a pain to make nice schematics, so I’m not going to post a full schematic at this point.

Output stage:

Input stage:

Aside from the actual amplification stage, I need to make 300V rails and and a 350V bias rail (configurable for up to 580V) to charge the diaphragm in the headphones. To do this I’m using the LTC3803 flyback controller. This is a current-mode control chip with an external compensation network in a convenient SOT-23-6 package. I wanted to test this out before ordering the full board, so I built a prototype converter for the 300V rails (if that works, the DC bias flyback will almost surely work).

Tests show it working fairly well, although it draws quite a bit of no-load power. I attached a BJT as the load so I could vary the output current to plot efficiency vs. output, which resulted in the following data:

I fit the equation

which represents a constant controller current (including core losses with the output load of the feedback resistors). This fit results in a controller power draw of 1.25 0.05W and an efficiency of 0.78 0.04. This is a bit high on the controller power draw, which I may be able to get a bit lower by not using the shunt regulator in the controller. Efficiency could be better, but is quite tolerable given that the amplifier really won’t draw much of any current on the 300V rails.