What Is an Electrostatic Headphone? STAX, Warwick, and How They Differ From Planar and Dynamic

An electrostatic headphone drives a whisper-thin diaphragm suspended in an electric field between two charged plates, instead of pushing it with a voice coil and a magnet. That diaphragm weighs almost nothing, so it starts and stops faster than a dynamic or planar driver, which is where electrostats get their reputation for near-holographic detail and transparency. The catch is that they need a constant high-voltage bias and a special amplifier (an “energizer”) to work at all. You’re buying a system, not a headphone you can plug into anything.

That’s the short version. Here’s how it actually works, and who builds these things.

How the driver works

Picture a diaphragm just a few microns thick, thinner than kitchen foil, coated with a conductive layer and held under a constant DC voltage (the “bias”). It sits in the gap between two perforated metal plates, called stators. The audio signal is applied to the stators as a swinging voltage. The charged diaphragm is pulled toward one plate and pushed from the other, and it moves the air.

Because the whole diaphragm is driven uniformly across its surface, rather than just where a voice coil is glued on, it behaves almost like an ideal piston. There’s very little mass to accelerate and very little to overshoot. STAX, which invented the format, has never called these “headphones.” It trademarks them as “earspeakers,” and once you’ve heard a good one the marketing starts to feel earned.

The tradeoff is efficiency. That electrostatic force is weak, so you need a lot of voltage to get meaningful output. Which brings us to the amp.

Why they need special amps (energizers)

A dynamic headphone wants current. An electrostatic headphone wants voltage, hundreds of volts of audio swing on the stators, plus a separate high-voltage bias supply to charge the diaphragm. No normal headphone amp produces anything close to that. Plug an electrostatic earspeaker into a standard 3.5mm jack and you get silence.

So electrostatics ship as closed systems. STAX earspeakers use a proprietary connector and require STAX’s own driver units, the SRM series energizers, which run from desktop boxes to the portable SRM-D10 II. Warwick Acoustics takes the same logic further and sells the headphone bundled with its energizer and DAC as one integrated unit. There’s no version of its APERIO that you buy à la carte and pair with whatever you own. The energizer isn’t an accessory here; it’s half the instrument.

This is also why cross-brand mixing is limited. Bias voltages and connector standards differ, so a STAX-bias earspeaker and a non-STAX energizer aren’t a safe match without an adapter designed for the job.

Electrostatic vs planar vs dynamic

Three different ways to move air, easiest to hardest to drive.

Dynamic is the classic design in nearly every headphone ever sold. A voice coil attached to a cone or dome sits in a magnetic gap, and current through the coil moves it. Cheap to build, efficient, plays loud off a phone. The diaphragm is comparatively heavy and driven from one point, so even the best of them can’t fully escape resonance and overshoot.

Planar magnetic stretches a thin film with a flat conductive trace printed across it, suspended between arrays of magnets. The whole membrane is driven, like an electrostat, so you get much of that low-mass evenness. But it’s driven by magnetic force and current rather than high voltage, so a normal (if beefy) headphone amp runs it. This is the sweet spot most boutique makers have settled on. Dan Clark Audio built its reputation on planars like the STEALTH and EXPANSE, and its Acoustic Metamaterial Tuning System (AMTS) waveguide.

Electrostatic is the extreme case: the lightest diaphragm, the fastest transient response, the most transparent top end, and the high-voltage energizing requirement that makes it a closed ecosystem. Planar sits between dynamic and electrostatic in both sound character and drive difficulty, which is exactly why it’s popular. You get a lot of the electrostatic character without the electrostatic tax.

One clarification is worth making, because the terms get blurred: a ribbon driver is not an electrostatic. It’s a distinct thing, and I’ll come back to it below.

Who makes them

The field is small on purpose, because this is hard engineering to get right.

STAX is the pioneer. Founded in Japan in 1938 by recording engineer Naotake Hayashi, it built the world’s first electrostatic headphone, the SR-1, in 1960, and has defined the category for more than 85 years. The current flagship is the SR-X9000 (introduced December 2022), with the SR-009S and the Lambda-series SR-L700 MK2 filling out the line. STAX went insolvent in 1995, was revived, and has been owned since 2011–2012 by China’s Edifier, which has kept the Fujimi, Saitama operation and the brand intact.

Warwick Acoustics is the notable exception, one of the only electrostatic makers whose driver is not descended from the STAX lineage. Spun out of the University of Warwick’s School of Engineering in 2002, it invented its own HPEL (High-Precision Electrostatic Laminate) transducer, a roughly 15-micron metalized film, from original research. Its consumer line first reached market under the Sonoma Acoustics name (the Sonoma Model One), later consolidated under Warwick, and now runs up to the cost-no-object APERIO system at around $24,000. The company is also pushing electrostatic transducers into automotive audio.

Dan Clark Audio is best known for planars, but it makes an electrostatic too, the CORINA flagship, which puts the San Diego shop in the rare position of a boutique that builds both.

Then there’s the ribbon caveat. RAAL-requisite, the US/Serbian outfit behind the SR1a and the circumaural CA-1a, often gets lumped in with electrostatics because both are exotic and both need unusual amplification. It’s a different technology. A ribbon driver is a corrugated strip of aluminum in a magnetic field, closer in principle to a planar than an electrostat, with no charged diaphragm and no high-voltage bias. RAAL made the world’s first full-range pure-ribbon headphone (debuted at RMAF in October 2018), handmade in Serbia by the same people who build RAAL’s ribbon tweeters. Excellent gear, just not electrostatic, which is worth knowing before you shop.

If you want to see how these makers line up side by side, who’s independent, who owns whom, and what each one actually builds, browse the full maker directory.