Belt-drive turntable

There are three main types of phonograph turntable drives being manufactured today: the belt-drive, idler-wheel and direct-drive systems; the names are based upon the type of coupling used between the platter of the turntable and the motor. In a belt-drive turntable the motor is located off-center from the platter, either underneath it or entirely outside of it, and is connected to the platter or counter-platter by a drive belt made from elastomeric material.

The design of the belt-drive turntable allows the use of a less expensive motor than the direct-drive turntable. Also, the elastomeric belt absorbs motor vibrations which would otherwise be picked up by the stylus.

Rotational stability is a key goal for turntable design that produces quality output.

Many belt-drive turntables with multiple speeds have mechanical devices or rely on manual effort to move the belt between different-sized pulleys on the motor shaft. Due to difficulties in designing multiple-speed synchronous motors whose speed can be controlled electronically, such turntables frequently have DC servomotors. The disadvantage of DC servomotors is that they rotate in steps rather than continuously – the resulting "cogging" can add noise during playback. Helical armature motors can be used to overcome this. Problems with belt instability and deterioration in the past have largely been solved by use of modern elastic polymers.

Since the 1970s, DC motors have declined in popularity; high end audiophile turntable designs use the synchronous motor, which are smoother running and lower noise. Due to the existence of instabilities of mains electricity, manufacturers might try to ensure pitch stability by generating its own sinusoidal waveform to power their AC motor. Other methods at achieving stability include using optical sensors on the platter which feed back to the electronics; or use of fly wheel assemblies. Some manufacturers use mass to deal with the problem. Heavy platters, which have higher inertia, are thus less susceptible to minute speed variations. However, mass stores energy that may be difficult to control. Heavier platter also increases wear on the bearing.

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