Amplifiers for headphones and loudspeakers, including electrostatic headphones

Many listeners keep two systems: loudspeakers for the evening and headphones for late hours. An amplifier that drives both saves a box, but the two loads ask for very different things. This guide explains what a headphone output needs for dynamic, planar magnetic and electrostatic headphones, and which Demograf Audio amplifiers are built to drive headphones and loudspeakers from one chassis.

Can one amplifier drive both headphones and loudspeakers?

Yes, if it is designed for it. Loudspeakers need watts into 4–8 Ω. Most headphones need milliwatts, a few need several watts, and electrostatic headphones need hundreds of volts. Three things decide whether a loudspeaker amplifier also makes a good headphone amplifier:

• Noise. A faint hum that disappears 3 m from a loudspeaker can be clearly audible with the transducer against your ear. The quieter the background, the more sensitive the headphones it can serve.
• Gain and volume range. With sensitive headphones, too much gain leaves only the first few degrees of the volume control for normal listening.
• Output impedance. It shapes how the amplifier controls the headphone, and it matters more for some designs than for others, as described below.

In a tube amplifier the headphone output is usually taken from the output transformer, which already turns the tube’s high voltage into a low-impedance signal. The transformer is the reason a single-ended tube amplifier can drive low-impedance headphones well, which a small tube headphone amplifier without one often cannot.

What do dynamic headphones need?

Dynamic headphones range from about 16 Ω to 600 Ω. High-impedance models such as the Sennheiser HD 800 need voltage swing rather than current; low-impedance models need current. Many dynamic headphones also have an impedance peak around their bass resonance. If the amplifier’s output impedance is high compared with the headphone’s, the frequency response follows that peak and the bass loosens. A common rule of thumb is an output impedance no higher than one-eighth of the headphone’s impedance.

What do planar magnetic headphones need?

A planar magnetic headphone drives a thin diaphragm carrying a conductive trace between rows of magnets. Its impedance is usually low and almost flat across the audio band, so output impedance changes its tonal balance very little. Many planars are not very sensitive, though, and need real current and power to reach clean peaks. The HiFiMAN HE-6 is the classic example: owners often run it from loudspeaker amplifiers. The demands of the Audeze LCD range are more moderate but still above those of most dynamic models.

This is why the answer to “which headphone amplifier for planar headphones?” is often a loudspeaker amplifier with a headphone output. It has the power reserve that planars need for dynamic music, and the headroom keeps bass and transients intact at realistic levels. Owners of the Audeze LCD-5 describe the effect of tube and solid-state amplification on that headphone in our LCD-5 owner impressions.

What do electrostatic headphones need?

An electrostatic headphone has no voice coil. A very thin diaphragm is held at a high bias voltage between two perforated stators, and the amplifier drives the stators directly with an audio signal of hundreds of volts. Current STAX models use Pro bias, about 580 V, on a 5-pin socket; older models used Normal bias, about 230 V, on a 6-pin socket. The load is mainly capacitive, so the amplifier must deliver large voltage swings and enough current at high frequencies, where the load’s impedance falls.

No ordinary headphone jack can do this. There are two established routes. A direct-drive amplifier, which STAX calls an energiser, feeds the stators from output tubes or transistors on a high-voltage supply. A transformer-coupled design steps up the output of a conventional power amplifier; STAX itself once made adaptor units that took their signal from a loudspeaker amplifier. Both approaches have their followers. What matters in use is headroom on loud passages, low noise in quiet ones and complete stability with the headphone connected. Our article on tube amplifiers for electrostatic headphones covers bias and drive in more detail.

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

Which Demograf Audio amplifiers drive both headphones and loudspeakers?

Ten models are specified for “Universal” use, loudspeakers and headphones. All of them have a 6.3 mm TRS and a 4-pin XLR headphone socket as standard; 2.5 mm, 3.5 mm, 4.4 mm Pentaconn and dual 3-pin XLR terminations are available on most of them.

Which headphone socket should you choose?

A 6.3 mm plug carries both channels on one shared return wire. A 4-pin XLR or 4.4 mm Pentaconn socket keeps each channel on its own pair of wires all the way to the headphone, which a balanced amplifier requires and which also keeps the channels’ returns apart in the cable. Most current headphones accept detachable cables, so the simplest course is to choose the socket that matches the cables you already own, or to order a cable to match the amplifier. Electrostatic headphones are the exception: they need their own socket with the correct bias, and no adaptor can turn an ordinary headphone output into one.

Tantal: a loudspeaker amplifier with a STAX-type output

Tantal runs two 3C24 triodes with tantalum anodes in Class A, 3 W per channel. Beside the 6.3 mm and 4-pin XLR sockets it carries a STAX-type output as standard, so one chassis drives dynamic, planar and electrostatic headphones, such as the STAX SR-007 and SR-009, as well as sensitive loudspeakers. For anyone searching for a STAX amplifier that also runs a pair of loudspeakers, this is the model built for that brief.

Rubedo: an electrostatic amplifier from the archive

Rubedo was a dedicated push-pull amplifier for electrostatic headphones on vintage AL4 tubes, with Tango transformers, Jensen capacitors and an external kenotron power supply. It is no longer in the catalogue, but the archive entry documents it inside and out, together with a collector’s review from 2014 that set it against the Sennheiser HEV90, the HeadAmp Blue Hawaii SE, the Woo Audio WES, a KGSS and STAX amplifiers. As with every owner review we publish, it is one listener’s view in his own system, not a measurement. A similar dedicated instrument can be discussed on request.

Tube or solid-state for headphones?

Both work, with different strengths. A single-ended tube amplifier adds mostly low-order harmonics, which many listeners hear as density and texture, and its output transformer gives it a firm grip on low-impedance planars. A solid-state design offers lower output impedance and noise and, in most of our range, more power, without tubes to warm up or replace. For electrostatic headphones the same holds: owners debate tube and solid-state energisers at length, and what decides in practice is headroom, noise and stability.

Which Demograf Audio piece fits

All the amplifiers with a headphone output are listed on one page: headphone amplifiers.

• Electrostatic headphones (STAX): Tantal with its built-in output, or any Universal model with the electrostatic output option.
• Power-hungry planars such as the HiFiMAN HE-6 or Audeze LCD-3: Helios MkII, designed with these headphones in mind, or Endymion and Ether for loudspeakers that also need more power.
• Low-impedance dynamic headphones and high-sensitivity loudspeakers: Atlas, or Solaria for faster bass on instrumental and electronic music.
• A complete system with a built-in DAC and no tubes: Bellerophon (Class AB) or Hades (Class D).

Every amplifier is made by Dmitriy Gubchenko. Headphone sockets and the electrostatic output are agreed before the build starts, so tell us which headphones and loudspeakers you use.