Article · Published 2026-10-18

Demograf Audio Electrostatic Headphone Amplifiers

Electrostatic headphones ask a great deal of an amplifier. Their diaphragm is a thin film held at a high, fixed bias voltage between two perforated stators. The amplifier drives those stators directly with an audio signal of several hundred volts, and the headphone presents a largely capacitive load rather than a resistive one. High voltage, a capacitive load and the need to swing quickly are why electrostatic amplifiers form a family of their own.

Most current electrostatic headphones, including STAX models, use a bias of about 580 volts, known as Pro bias; older STAX designs use a lower Normal bias of about 230 volts. An amplifier must supply the right bias for the socket it offers. Its output stage must also deliver large voltage swings into that capacitive load without running short of current at high frequencies, where the load’s impedance falls.

There are two broad ways to do this. A direct-drive amplifier runs its output tubes or transistors from a high-voltage supply and connects them straight to the stators. A transformer-coupled design steps up the output of a more conventional amplifier. Both approaches have their advocates. What matters in practice is headroom on loud passages, low noise in quiet ones and complete stability with the headphone connected.

At Demograf Audio, Dmitriy Gubchenko designs and voices the circuits, and each unit is assembled, tuned and measured by Dmitriy and his team. Our electrostatic amplifiers are tube designs.

The fullest account of this work is the archive entry on Rubedo, a push-pull electrostatic amplifier on vintage AL4 tubes, with Tango transformers and an external tube-rectified power supply. It gathers a long-time collector’s impressions, first published in 2014, of Rubedo and a later design on tantalum-anode tubes. He set them against the Sennheiser HEV90, the HeadAmp Blue Hawaii SE, the Woo Audio WES, a KGSS and STAX’s own amplifiers, using the STAX SR-009, SR-007 Mk2 and SR-Omega and the Sennheiser HE90. As with every owner report we publish, it is one listener’s subjective view in his own system, not a measurement or a claim of ours. You will find it in our archive under Rubedo, at /archive/rubedo/.

Our current Tantal, a Class A amplifier on tantalum-anode 3C24 triodes, also has a built-in STAX-type output.

If you are choosing an electrostatic amplifier, start with the practical points: the bias and socket your headphones need, and the space and ventilation a tube amplifier requires. Then listen at realistic levels to music with wide dynamics, such as a full orchestra or a solo piano played hard. If you can, listen with your own headphones, because different electrostatic models respond to amplification in different ways.

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