A   Direct-Drive Amplifier   for   Electrostatic Loudspeakers

By George Sifakis

A quick word about what an ESL Direct-Drive Amplifier is: An electrostatic speaker needs high voltage to operate, therefore a transformer is utilized to raise the low-voltage high-current amplifier output to a high-voltage low-current input that  the electrostatic speaker needs. Exactly the opposite happens at the output transformer of a tube amplifier.  Therefore, driving an electrostatic speaker by a tube amplifier, one has practically two transformers operating “back-to-back”, offering no advantage, instead degrading sound quality. The logical thing then if you’re building your own equipment is to skip the transformers altogether and drive the electrostatic panels directly from the anodes of the output tubes. This is exactly what this amplifier is doing.

The design goals were:

• All tube circuit. Because we think tubes sound better. It replaces the previous amplifier that used 8068 regulator tubes in the output, but dated IC technology in the input.
• High voltage plus high current capability in order to be able to charge those huge electrostatic panels. The 838 output tube more than fills the bill.
• Modern (or is it old?) design principles such as no negative feedback, single-ended (it’s really more like four single-ended amplifiers instead of two push-pull ones), directly heated triodes, DC coupling, oil caps, chokes, tube rectification and general simplicity and purity.
• Good sound of course.
• Good looks, which for me is lots of tubes and military/professional style.

But perhaps the number one goal was to have fun building and use as many parts as possible from the junk box. I doubt there’s a single new part in there, or for that matter any part manufactured after I was born, and I’m not young. The heart of  the power supply is a “boatanchor”  saved from the melting pot at the last minute, the PP-685 military power supply.

Lots and lots of design decisions were based on the parts on hand. Many more were “borrowed” in one form or another from the “Sound Practices” and “boatanchors” gangs, a big “thank you” to them all. This is the circuit that resulted and the underchassis wiring. The power supply deals with some serious voltages and some more conventional. Two more “inside” views of the power supply, one bird and one worm (the power supply sits upside-down). This is the  targeted electrostatic speaker homebrew of course, and here is the rest of the system either homebrew or heavily modified.

It’s been a lot of fun building this one and a lot of learning, too. This amplifier is worth just looking at those big tubes lighted up. But the sound is also excellent, much more substantial than the old one’s. I really prefer building to buying, plus it keeps me from going broke. I hope you enjoyed looking at those pictures, maybe got some ideas too, of what to do with that overfilling junk box. Comments and questions, contact me.