Equipment
Turntable : drive systems
Overview:
Most turntable designs use a either a belt drive or direct
drive system. Earlier designs also used a rubberized idler
wheel drive system. However, non-linear wear and decomposition
of the wheel introduced noise and speed variations into the
desired audio. These systems generally used a synchronous motor
which ran at a speed synchronized to the AC frequency of the
power supply. Different speeds were obtained by bringing differing
diameter wheels into position against the bottom or inside
edge of the platter.
Belt drive:
Belt drives brought improved motor and platter isolation
compared to idler wheel designs. Motor noise heard as low frequency
rumble was much reduced. It is difficult to design multiple
speed synchronous motors, consequently DC servomotors with
electronics providing speed control, have gained favour. On
the most sophisticated designs, optical sensors on the platter
are used to ensure the speed of the platter remains stable.
Many platters have a continuous series of strobe markings machined
around their edge to provide these pulses. A strobe effect
can be observed 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. Modern
high fidelity applications favor the use of belt-driven systems,
as these isolate the revolving platter from motor-induced vibrations.
Problems with material instability and deterioration have largely
been solved by use of modern elastic polymers.
Direct drive:
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. Matsushita's Technics division introduced
the first commercially successful direct drive platter, model
SP10, in 1969.
3345, the Vinyl Records Home.
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