There are three main types of phonograph turntable drives still 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 an elastomeric belt.
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 aspect for a turntable to produce quality output. Problems with belt instability and deterioration in the past have largely been solved by use of modern elastic polymers.
Many belt-drive turntables having multiple speeds used a simple mechanical system to change speeds, using a mechanism to move the belt between different-sized pulleys on the motor shaft. It is difficult to design multiple-speed synchronous motors whose speed can be controlled electronically. Consequently, turntables with electronics providing speed control are nowadays most frequently used with DC servomotors. DC servomotors rotate in steps rather than continuously. This is referred to as 'cogging', and can add noise during playback. Helical armature motors can be used to overcome this. On some designs, the manufacturer prefers to ensure pitch stability by generating its own sinusoidal waveform to power the motor. Others attempt to achieve this by using optical sensors on the platter which feed back to the electronics to ensure the speed of the platter remains stable.
Some turntables, such as the newer versions of the Rega Planar series, use a fixed plinth with a low vibration motor and bearing attached to the same flat surface, usually constructed of wood, metal or acrylic, without suspension. Others, such as the Linn Sondek LP12, have a suspension.
The principle behind the "suspended sub-chassis" design is that the turntable is an inert platform that allows the stylus to track the surface of the record accurately whilst being protected from external vibrations. The platter, sub-chassis, armboard and tonearm mechanically form a closed loop, and sit on top of dampers (usually three springs) which isolate the sub-assembly from its top plate and base. The motor, mounted (directly or indirectly) on the top plate, drives the turntable platter via a belt. The tonearm is usually sold separately, allowing for buyer choice and upgrades.
Bearings, mounted on the sub-chassis, generally use an oil film to lubricate between a metal ball-bearing connected to the platter by a spindle and the thrust-plate of the bearing's housing. More esoteric designs use an air bearing, where the spindle is supported by a high pressure flow of air.
Over time the drive belt can wear or lose elasticity, and begin to slip, causing variations in the platter speed. In addition, belt-drive turntables have much lower torque; the belt can also slip off the motor and/or platter spindle, and are thus not suitable for turntablism. DJs who scratch or mix generally prefer to use direct-drive turntables.
- Versa Dynamics 2.0 LP player, J. Gordon Holt, Page 7, Stereophile, Vol.10 No.8 December 1987