South Street Service Company

How Turntables Work

The turntable is a flat, circular platform upon which the record is placed. An electric motor rotates the turntable at constant speed, usually 33 1/3, 45, or 78 revolutions per minute (rpm). The tonearm is a rod with a jewel-tipped stylus, or needle, at its free end. The tonearm either pivots to keep the stylus in the record groove, or is suspended by a mechanism that enables it to stay oriented in the same direction while it moves across the record. When the stylus moves along the undulating groove of the revolving disk, it vibrates, and the vibrations are converted into equivalent electrical impulses by the cartridge in the tonearm. These impulses are conducted by wire leads to an electronic amplifier and then to one or more loudspeakers. All modern phonograph systems had certain components in common: a turntable that rotated the record; a stylus that tracked a groove in the record; a pickup that converted the mechanical movements of the stylus into electrical impulses; an amplifier that intensified these electrical impulses; and a loudspeaker that converted the amplified signals back into sound.

Drive systems: Direct and Belt

The technology of the modern turntable is generally very simple, with either belt drive or direct drive dominating. Earlier designs also used an indirect drive system using rubber-tyred wheels, but this fell out of favor since these have poor rumble performance. These systems generally used a synchronous motor which ran at a fixed speed, locked to the frequency of the mains supply. Different speeds were engaged by bringing different diameter transfer wheels into play between the motor and the platter.

The belt drive system improved on this by creating greater isolation between the motor and the platter, so the transfer of noise (usually heard as low frequency rumble) was much reduced. Since it is difficult to make synchronous motors to give multiple speeds, the use of DC motors gained favor, with additional electronics for speed control. On the most sophisticated designs, feedback from the platter itself, in the form of a series of pulses, was used to ensure that the speed of the platter remained locked and absolutely stable. (Many platters have a continuous series of reflective markings machined around their edge to provide these pulses). A drawback of basic DC motors is that of cogging, where the motor tends to rotate in steps rather than smoothly. This effect can be transmitted to the platter and hence add unwanted noise to the playback. Special motors such as those with helical armatures can be used to overcome this.

In a direct drive system, the motor drives the platter directly, without any intermediate wheels or belts. In fact, the platter itself can form part of the armature of the motor. This approach requires very careful and sophisticated design, with a considerable amount of electronics for starting and speed control. However, it has been made to work very successfully, and some of the most renowned designs use this approach.

How to tell If You should Replace Your Needle

For More Info Click Here