Electrostatic or planar: what the amplifier must do

Electrostatic and planar magnetic headphones have one thing in common: both drive a thin, light diaphragm across its whole surface instead of pushing a cone from a voice coil at its centre. There the likeness ends. They work on different physical principles and present an amplifier with almost opposite loads. One needs a bias supply and hundreds of volts; the other needs current and power into a low impedance. This guide explains both principles, what each asks of an amplifier, and which Demograf Audio amplifiers are built for each.

Electrostatic vs planar headphones: what is the difference?

In short, an electrostatic headphone moves its diaphragm with an electric field, and a planar magnetic headphone moves it with a magnetic one. The electrostatic diaphragm carries a fixed electric charge and sits between two perforated plates that the amplifier drives with a high-voltage signal. The planar diaphragm carries a flat conductor, through which the amplifier’s current flows, between rows of permanent magnets. Both are sometimes called “planar” because the diaphragm is flat, but only the second is a planar magnetic design, and the amplifier requirements follow from that difference.

How an electrostatic headphone works

The diaphragm is a plastic film a few thousandths of a millimetre thick, coated with a conductive layer of very high resistance. A bias supply charges it to a high, fixed DC voltage through a large resistor, so the charge on the film stays almost constant while it moves. On either side, a short distance away, sits a perforated metal plate called a stator.

The amplifier drives the two stators in push-pull: as one swings positive, the other swings negative by the same amount. The electric field between the stators then pulls the charged film towards one and pushes it away from the other. Because the charge is held constant and the field between the plates is nearly uniform, the force on the film follows the signal voltage closely across its travel. This is why the push-pull electrostatic principle is commonly described as having very low inherent distortion, and why owners often describe the sound as fast, transparent and finely detailed.

The catch is that the force per volt is small. To move the film far enough for realistic levels, the stators need audio signals of several hundred volts at loud peaks, and the film needs a high bias. STAX, whose socket became the common standard, uses two bias levels: “Pro” bias of about 580 V on a 5-pin socket for its current models, and “Normal” bias of about 230 V on a 6-pin socket for older ones. Other makers have used their own connectors and voltages.

What electrostatic headphones ask of an amplifier

• A bias supply at the right voltage for the socket. The current it delivers is tiny, but the voltage must be stable and quiet.
• High-voltage, balanced drive. Two outputs in opposite phase, each swinging hundreds of volts, with headroom left for peaks.
• Current into a capacitive load. An electrostatic headphone behaves largely like a capacitor, so its impedance falls as frequency rises. The amplifier must deliver more current in the treble to keep the voltage swing intact on fast transients.
• Low noise and complete stability, because the transducer is at your ear and the load is reactive.

No ordinary headphone socket can do any of this, and no adaptor turns one into an electrostatic output. There are two established routes. A direct-drive amplifier, which STAX calls an energiser (or energizer), feeds the stators straight from output tubes or transistors running on a high-voltage supply. A transformer-coupled design steps up the output of a conventional power amplifier. Both have their followers; what decides in use is headroom on loud passages, low noise in quiet ones and stability with the headphone connected.

Safety: electrostatic outputs carry dangerous voltages. Use only the correct plug and socket, and never open an energiser or an amplifier with an electrostatic output yourself.

How a planar magnetic headphone works

A planar magnetic headphone uses the same electrodynamic principle as a moving-coil driver, unfolded flat. Its diaphragm carries a thin conductor laid out as a flat pattern across most of its surface, and rows of permanent magnets sit on one or both sides. When the amplifier’s current flows through the conductor, the magnetic field turns it into a force spread over the diaphragm, which moves as a whole.

There is no bias and no high voltage. The amplifier simply supplies a signal current, as it would to a dynamic headphone. The difference lies in the load. A planar magnetic headphone usually has a low impedance that is almost purely resistive and nearly flat across the audio band, and its sensitivity varies widely from model to model. Some planars reach loud levels from a portable player; others are among the hardest headphones of all to drive. The HiFiMAN HE-6 is the classic example, and owners often run it from loudspeaker amplifiers.

What planar headphones ask of an amplifier

• Current and power with headroom, so that bass and transients stay clean at realistic levels.
• A low output impedance for control. Because a planar’s impedance is flat, output impedance changes its tonal balance very little, but it still affects damping.
• Low noise and sensible gain for the more sensitive models, so that normal listening does not sit at the very start of the volume control.

That is why the answer to “which amplifier for planar headphones?” is often a loudspeaker amplifier with a headphone output. In a tube amplifier the output transformer turns the tubes’ high voltage into a low-impedance signal, which is what a power-hungry planar needs. Our guide to amplifiers for headphones and loudspeakers also covers dynamic headphones and the choice of headphone socket.

Electrostatic and planar headphones side by side

Electrostatic headphone amplifier: tube or solid state?

Both are built. Tubes are at home with high voltages, which is one reason many direct-drive electrostatic amplifiers use them in the output stage; transistor designs need devices rated for high voltage but avoid tube heat and tube replacement. Owners debate the two at length, and the same practical criteria apply to both: enough voltage swing for the headphones you own, low noise and complete stability. Our article on tube amplifiers for electrostatic headphones goes further into bias and drive, and the archive entry on Rubedo, a push-pull electrostatic amplifier on AL4 tubes, documents one of our earlier dedicated designs.

How Demograf Audio amplifiers serve each kind

Electrostatic: Tantal and the electrostatic option

Tantal runs two 3C24 triodes with tantalum anodes in Class A, 3 W per channel. It has a STAX-type output built in, for headphones such as the STAX SR-007 / SR-009 or the Sennheiser HE 90, beside a 6.3 mm socket, a 4-pin XLR socket and loudspeaker terminals. One chassis therefore drives dynamic, planar and high-voltage electrostatic headphones, as well as sensitive loudspeakers.

On our other tube models and on Bellerophon, an electrostatic output can be fitted to order; Tantal Senior, for example, lists a STAX-type output as an option. The output is agreed before the build starts, so tell us which electrostatic headphones you use. The amplifiers that can drive electrostatic headphones are gathered on the electrostatic headphone amplifiers page.

Planar: Helios MkII and Bellerophon

Helios MkII is a single-ended Class A integrated amplifier on G-811 triodes, 15 W per channel. Its headroom is aimed at hard-to-drive headphones such as the HiFiMAN HE-6 or Audeze LCD-3 as well as sensitive loudspeakers. It has a 6.3 mm and a 4-pin XLR headphone socket, with further connectors, including an electrostatic one, to order.

Bellerophon combines a DAC with a Class AB amplifier on Toshiba bipolar transistors, from 25 W per channel. It drives demanding planar headphones and full-size loudspeakers with the same ease, and it too can take an electrostatic output to order.

Can one amplifier drive both electrostatic and planar headphones?

Yes, if it has both kinds of output and the right kind of headroom behind each. An electrostatic output needs its own bias supply and high-voltage drive; a planar output needs current. Tantal has both as standard: its STAX-type output serves the electrostatics, and its output transformer feeds the 6.3 mm and 4-pin XLR sockets for dynamic and planar headphones. If your planar is one of the hardest to drive, Helios MkII or Bellerophon with the electrostatic output fitted to order gives more power on the planar side.

Which Demograf Audio piece fits

• Electrostatic headphones, STAX type or Sennheiser HE 90: Tantal with its built-in output, or another of our tube amplifiers or Bellerophon with an electrostatic output fitted to order.
• Power-hungry planars such as the HiFiMAN HE-6 or Audeze LCD-3: Helios MkII.
• Planar headphones and loudspeakers with a built-in DAC and no tubes: Bellerophon.
• Electrostatic, planar and dynamic headphones plus sensitive loudspeakers from one chassis: Tantal.

All our amplifiers with headphone outputs are listed on the headphone amplifiers page. Each is made to order and measured by Dmitriy Gubchenko in Kazakhstan, with its own measurement report. Tell us which headphones and loudspeakers you use, and we will agree the outputs before the build starts.