Electrodynamic loudspeaker

An electrodynamic loudspeaker or field coil loudspeaker is a dynamic loudspeaker in which the field is produced by an electromagnet rather than by a permanent magnet.

An electrodynamic loudspeaker therefore has two coils:

• The voice coil common to all dynamic loudspeakers, positioned in the air gap between the pole pieces, the motion of which moves the loudspeaker cone.
• A fixed field coil which together with its magnetic core replaces the permanent magnet of other dynamic loudspeakers.

The first electrodynamic loudspeakers were produced in the 1930s, to address the problem that strong permanent magnets of the time were extremely heavy. A compromise was therefore necessary between loudspeaker efficiency, which required the strongest possible magnet, and weight. The use of a strong but relatively light electromagnet solved this problem.

While now uncommon, electrodynamic loudspeakers were once common in top quality mantel radios and similar domestic audio applications, particularly in the post-war period of World War II. In these appliances the field coil of the loudspeaker was also used as the main or only filter choke in the high tension power supply to the valve anodes.

Advances in permanent magnet technology rendered the traditional electrodynamic loudspeaker obsolete. The type survives in some high-end hi-fi applications.

External links

• Definition at answers.com
• Replacing an Electrodynamic Loudspeaker with a Permanent Magnet Loudspeaker deals with the use of the field coil as a filter choke
• Shindo labs field coil loudspeakers