Field-coil loudspeakers explained: electromagnets, field supplies and the Supravox 215-EXC

Field-coil loudspeakers are the oldest kind of moving-coil loudspeaker, and they have never quite gone away. Small makers still build them, and their owners describe a quality they do not find elsewhere. This guide explains what a field coil is, how it differs from a permanent magnet, what the field supply does, what owners report, and what to consider before living with a pair.

What is a field-coil loudspeaker?

A moving-coil driver works by placing a voice coil in a narrow magnetic gap. Current from the amplifier flows through the coil, the magnetic field pushes it back and forth, and the cone attached to it moves the air. Something has to create that magnetic field. In almost every modern loudspeaker it is a permanent magnet: alnico, ferrite or neodymium.

In a field-coil driver it is an electromagnet. A large winding of copper wire around the pole piece is fed with direct current from a separate supply, and the magnetic circuit carries the resulting flux to the gap. Cone, voice coil and suspension are the same as in any other driver; only the source of the magnetism differs. The French term is haut-parleur à excitation, which is where the EXC in Supravox model names comes from.

A short history: from Western Electric to today

The moving-coil loudspeaker described by Chester Rice and Edward Kellogg in 1925 used an electromagnet, because the permanent magnets of the day were too weak to give a strong field in a small gap. Through the 1920s and 1930s cinema, radio and public-address loudspeakers were energised by field coils, including the Western Electric theatre systems built around the 555 compression driver. In many radios the field coil did double duty as the filter choke of the power supply.

Alnico magnets, from the 1930s onwards, and cheap ferrite after the war made the field supply unnecessary for mass production. Field coils survived among enthusiasts, particularly in Japan and France, and a handful of makers build new field-coil drivers today.

Field coil, alnico, ferrite or neodymium?

The question “field coil or alnico?” is therefore about more than sound. Alnico gives a strong, fixed field without a supply; a field coil gives a field the designer can set, and the owner can adjust, at the cost of a second box.

What do owners hear?

Owners of field-coil loudspeakers commonly describe speed, fine low-level detail and a sense of ease: music that starts and stops cleanly, with large dynamic contrasts and little grain. Many report that the sound changes with the field supply, and that a stronger field tightens the bass.

Part of this has a plain engineering explanation. A strong field coil can put a high flux density into the gap, and a strong motor gives a driver high sensitivity and firm control of its cone. Because the flux can be adjusted, the driver’s damping can be matched to its enclosure. Claims beyond that, for example that an electromagnet reacts differently from a permanent magnet to the voice coil’s own field, are debated and not settled. Strong motors also exist with permanent magnets, so the surest way to judge is to listen in your own room.

The field supply: why it matters

The field coil needs a steady direct current for as long as the music plays. Any ripple left on that current modulates the magnetic field and reaches the cone as hum, so the supply must be well filtered. Designs range from tube-rectified supplies with choke filtering to regulated solid-state supplies, including current-regulated ones that hold the field constant as the coil warms. Owners frequently report that the type and quality of the supply is audible, much as an amplifier’s power supply is.

Two practical details follow. A field coil is a large inductor, and interrupting its current suddenly produces a voltage spike, so supplies include protection and the switch-on and switch-off sequence given by the maker should be followed. And the supply draws power continuously while it is on, which is part of the heat budget of the room.

Advantages and drawbacks of field-coil loudspeakers

Advantages

• Motor strength set by the designer and adjustable by the owner.
• High sensitivity is common, which suits low-power tube amplifiers.
• No risk of demagnetisation, a known weakness of alnico.
• A presentation that many owners describe as fast, detailed and relaxed at the same time.

Drawbacks

• An extra supply chassis and mains cable for each loudspeaker or pair.
• Warm-up: copper resistance rises by about 0.4% per degree Celsius, so with a constant-voltage supply the field current falls slightly as the coil warms. Many owners switch the supply on well before listening.
• Weight and size: the motor and the supply are heavy.
• Hum if the supply is poorly filtered or the cables are badly routed.
• Heat, which matters in warm climates and small rooms.

How to set up a field-coil system

• Place the field supply where it can breathe, away from turntables, phono stages and signal cables, since its transformer and chokes radiate a magnetic field.
• Run the field cable apart from interconnects, and cross them at right angles where they must meet.
• Follow the maker’s switch-on sequence, which usually brings the field supply up before the amplifier, and give the coils time to reach working temperature before serious listening.
• If the supply allows the field to be adjusted, change it in small steps and listen to bass control and tonal balance at each one.

The Supravox 215-EXC in Perseus MkII

Perseus MkII is built around the Supravox 215-EXC mk2, a French field-coil full-range driver, so the whole range comes from one diaphragm without a crossover. Its sensitivity is 93–102 dB, which suits single-ended tube amplifiers as well as solid-state ones. The standard enclosure is aperiodic, an acoustic resistive matrix that makes room placement easier; open baffle, sealed box, transmission line and bass reflex are available on request. Other Supravox drivers from the 2000, EXC and Heritage series can be fitted, and for larger rooms the same cabinet is built as a three-way system with Fostex horn tweeters and a 10–18 inch woofer. Perseus is passive by default and can also be built with amplification inside.

In voicing, the treble is silky and detailed, the midrange carries voices and instruments with their real weight, and the stage has depth and air. Ask about the field supply when you commission a pair: its type, its size and where it will stand in the room.

Other field-coil options in the catalogue

• Zeus is a subwoofer with a 15-inch woofer on a field-coil motor wound by Demograf Audio, covering 10–300 Hz. Its field magnets are energised from the mains. The passive version is driven from your amplifier through its speaker terminals; the active version carries its own Class A or Class D amplifier. It is a natural partner for high-sensitivity full-range loudspeakers, which rarely reach below 40 Hz on their own.
• Orpheus, our three-way loudspeaker at 103 dB, can be ordered with its AER BD-1, BD-2 or BD-3 drivers fitted with custom Demograf Audio electromagnets.

Which Demograf Audio piece fits

For a field-coil full-range loudspeaker driven by a single-ended amplifier, start with Perseus MkII. For the highest sensitivity, consider Orpheus with electromagnets. To add the lowest octaves to either, add Zeus. On the amplifier side, Perseus suits Atlas on 300B triodes in a medium room, Solaria on 45 triodes in its more sensitive configurations, and Helios MkII on G-811 triodes where more headroom is needed. Our guide to power triodes for single-ended amplifiers explains how to match power to sensitivity, and getting the most from your loudspeakers covers placement.

Every loudspeaker is made by Dmitriy Gubchenko. Loudspeakers need no routine service; drivers, crossovers and terminals are repaired for as long as parts exist.