Cartridges & Needles

A magnetic cartridge is a device used for the playback of gramophone records on a turntable or phonograph. more...

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It converts mechanical vibrational energy from a stylus riding in a spiral record groove into an electrical signal that is subsequently amplified and then converted back to sound by a loudspeaker system.

History

The first electric pick-ups were developed in about 1925. They used a piezo-electric crystal of quartz, stimulated by a stylus made of sapphire or diamond. The magnetic cartridge is presently the most common form of sound pickup used and came into use in the 1950s, following the introduction of magnetic cutter heads around 1945 for mastering records. Compared to the earlier systems, the magnetic cartridge gives improved playback fidelity, and reduces record wear by tracking the groove with lighter pressure, but with lower output voltage.

Types

There are two types of magnetic cartridge, moving magnet (MM) and moving coil (MC) (originally called dynamic). Both operate on the same physics principle, that of electromagnetic induction. The moving magnet type is by far the most common and more robust of the two, though audiophiles often claim that the moving coil system yields higher fidelity sound.

In either type, the stylus itself, usually of diamond, is mounted on a tiny metal strut called a cantilever, which is suspended using a collar of highly compliant plastic. This gives the stylus the freedom to move in any direction. On the other end of the cantilever is mounted a tiny permanent magnet (moving magnet type) or a set of tiny wound coils (moving coil type). The magnet is close to a set of fixed pick-up coils, or the moving coils are held within a magnetic field generated by fixed permanent magnets. In either case, the movement of the stylus as it tracks the grooves of a record causes a fluctuating magnetic field which causes a small electrical current to be induced in the coils. This current closely follows the sound waveform cut into the record, and may be transmitted by wires to an electronic amplifier where it is processed and amplified in order to drive a loudspeaker. Depending upon the amplifier design, a phono-preamp may be necessary.

In most designs, the stylus itself is detachable from the rest of the cartridge so it can easily be replaced. There are two primary types of cartridge mounts. The older type is attached using small screws to a headshell which then plugs into the tonearm, while the other is a standardized "P-mount" cartridge that plugs directly into the tonearm. Some mass market turntables use a proprietary integrated cartridge which is not upgradable.

Stereo reproduction

One reason that magnetic cartridges superseded the crystal pick-up was the relative ease with which it could be made to reproduce stereo recordings, which were introduced in 1958. In a stereo recording, the two channels are arranged to drive the record cutter head at an angle of 45° to the vertical, effectively encoding each channel in the left and right V-shaped walls of the record groove. This system worked well, since it provided full compatibility with a monaural pick-up, so stereo records could be played on older mono equipment. To reproduce the stereo signal, the cartridge simply arranges pairs of coils at 45° to complement the cutting process. With careful design, the coils can be shielded from each other electrically and mechanically such that stereo separation is maximised.

Read more at Wikipedia.org