The Magnetic Personality
With very few exceptions, quality high fidelity phono cartridges use a basic magnetic principle to convert the motion of the stylus in the groove into electrical energy. If an adjacent magnet and coil of wire move relative to each other, a small electrical signal is generated in the coil. There are three popular approaches to the use of this principle.
The Moving Magnet
The most widely used design concept locates a magnet at the end of the stylus cantilever where it will move when the stylus traces the groove. A coil, with pole pieces extending near the magnet, senses this motion and an electrical current is induced in the coil. This small signal is amplified converted into sound by the speaker.
The Moving Coil
The second approach simply trades the location and size of the two elements. A very small coil is located at the end of the cantilever so that it can move in the field of a relatively large fixed magnet. A similar, but generally smaller electrical current is generated in the coil. This signal is then amplified and converted into sound by the speaker.
The Moving "Iron"
A third variant uses a piece of ferrous metal like iron, attached to the stylus cantilever. It is located between the fixed magnet and fixed coil so that as it moves it varies the magnetic field and generates an electrical signal in the coil. As can be seen. all three designs use the same basic principle, differing only in the way the concept is applied.
It Takes Two
While our simple drawing shows just one magnet and one coil for clarity, in a real stereo cartridge there must be two complete systems to sense the two independent stereo signals contained on the two sides of the record groove. These two systems must fit within the confines of a tiny phono cartridge. And they must operate with minimum effect on the motion of the stylus as it traces the stereo groove. In the next column we'll discuss some specific approaches.
Good listening, and eventually is turned back into sound by the speaker.
Jon R. Kelly, President Cam' Audio-Technica U.S., Inc.
1221 Commerce Dr., Stow, OH 44224 audio-technica.
The World's Favorite Phono Cartridge
(Adapted from Audio magazine, Oct. 1984)
Also see:
audio-talk from audio-technica -- Number 11 in a Series (Jan. 1985)
audio-talk from audio-technica -- Number 9 in a Series (Nov. 1984)
audio-talk from audio-technica -- Number 7 in a Series (Sept. 1984)
audio-talk from audio-technica -- Number 4 in a Series (June 1984)
Audio-Technica AT15Sa cartridge (Feb. 1977)
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