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Turntable Design and Technology discussed

This short look at turntable design and technology highlights the way record decks have evolved over the years to the point today. If you would like to add to this discussion please send your contribution to me by clicking here

Turntable construction

Inexpensive record players typically used a flanged steel stamping for the turntable structure. A rubber disc would be secured to the top of the stamping to provide traction for the record, as well as a small amount of vibration isolation. The spindle bearing usually consisted of a bronze bushing. The flange on the stamping provided a convenient place to drive the turntable by means of an idler wheel (see below). While light and cheap to manufacture, these mechanisms had low inertia, making motor speed instabilities more pronounced.

For the serious listener, turntables made from heavy aluminum castings were offered. Typically, they were machined on a lathe and balanced, operating with negligible vibration. Like the stamped steel turntables, they, too, were topped with rubber. Due to the increased mass, they usually employed ball bearings or roller bearings in the spindle for low friction. While some used idler wheel drive, most were intended for belt or direct drive. The high mass and inertia of such turntables helped keep the speed constant, even if the motor exhibited cogging effects.

Turntable drive systems

Most turntables employ either an idler-wheel drive, belt drive or direct drive system to rotate the turntable platter:

Idler-wheel drive system

Earlier turntable design used a rubberized idler-wheel drive system. However, wear and decomposition of the wheel, as well as the direct mechanical coupling to a vibrating motor, introduced low-frequency noise ("rumble") and speed variations ("wow and flutter") into the sound. These systems generally used a synchronous motor which ran at a speed synchronized to the frequency of the AC power supply. Portable record players typically used an inexpensive shaded-pole motor. At the end of the motor shaft there was a stepped driving capstan; to obtain different speeds, the rubber idler wheel was moved to contact different steps of this capstan. The idler was pinched against the bottom or inside edge of the platter to drive it.

Until the 1980s, the idler-wheel drive was the most common on turntable design, except for higher-end audiophile models. However, even some higher-end turntables, such as the Garrard "Zero" series and Dual turntables, used idler-wheel drive.

Belt drive system

Turntable design lead to belt drives which brought improved motor and platter isolation compared to idler-wheel designs. Motor noise heard as low-frequency rumble was much reduced. 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. For electronic speed control, it is difficult to design multiple-speed synchronous motors; consequently, DC servomotors with electronics providing speed control have gained favor. On the most sophisticated designs, optical sensors on the platter are used to ensure the speed of the platter remains stable.

Matsushita's Technics division introduced the first commercially successful direct drive platter, model SP10, in 1969. The Technics SL-1200 turntable, introduced in 1972 and still in production as of 2006 (in its MK2 and MK5 guises), is one of the most successful direct drive turntables ever produced. Its rapid acceleration up to speed, high torque, electric braking system and reliability made it a favourite with radio stations and disc jockeys across the world.

Many platters have a continuous series of strobe markings machined or printed around their edge to provide optical pulses to these speed-control systems. Viewing these markings in artificial light at mains frequency produces a stroboscopic effect, which can be used by the operator to verify rotational speed. 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. Problems with belt instability and deterioration have largely been solved by use of modern elastic polymers.

Direct drive system

In the late 1960s turntable design lead to direct drive turntables drive the platter directly without utilizing intermediate wheels, belts, or gears as part of a drive train. The platter functions as a motor armature. This requires good engineering, with advanced electronics for acceleration and speed control

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