Handbook for Sound Engineers

Analog Disc Playback 1019

retracts away from the motor shaft to protect the rubber
ridge from forming a flat spot.
The advantage of the rim drive is that it provides
positive torque to the platter, and if the motor is strong
enough, it can bring the turntable to the desired speed
almost instantly. The mechanism is simple, and it is the
most reliable type of drive. Unfortunately it is also the
noisiest because of the positive coupling between the

motor and the platter idler or puck that transmits a

certain amount of the motor vibrations to the platter and

consequently to the record, as shown in Fig. 27-4C.

The third kind of turntable drive is the direct drive

where the motor drives the shaft of the platter directly.

There are also variations of the design. Some turntable

designs are very sophisticated, using the platter itself as

a rotor of the motor and drive is provided by the

self-contained, quartz-controlled oscillator. The motion

is extremely accurate and the speed of rotation may be

displayed on the digital display, which is part of the

control panel. There is also a weak point in this seem-

ingly perfect drive. Because of the slow speed at which

the turntable rotates, and because the motor has a finite

number of poles, there is a slight cogging action in the

platter motion, which may manifest itself with increased

loads. This handicap is only related to turntable platters

with fairly small mass and small moments of inertia. If

the platter is heavy, it will overcome this problem.

The performance of the turntable depends very little

on the type of drive used but more on the correct execu-

tion of the design by understanding the problems

involved. The ideal turntable should have the following

properties:

• It will start fast without hesitation.


• It will rotate with exact speed without variations.


• There will be no motor noises or vibrations heard
  while the system is in operation, they will not be
  transmitted to the platter.


• The turntable should be adequately shock mounted
  and isolated from the surface on which it sits to
  prevent the transmission of rumble and vibrations
  from the room. These loud sounds can actually shake
  the platter and the tonearm.


• The platter should be treated against ringing either by
  using a turntable mat with damping properties or by
  undercoating the platter.


• The turntable must be easy to maintain and to repair.

Not many turntables meet all these criteria; there-

fore, in order to know how to evaluate the unit, it is

important to know how they work.

Speed of Rotation. Before evaluating the entire system,

there are tests that can be performed on the turntable

alone. The first one is speed of rotation. There are many

ways of checking the speed of rotation, but the simplest

one is by using the stroboscopic disc.

A stroboscopic disc is a circular disc containing a

number of black-and-white bars, which are used for

checking the speed of turntables and other rotating

machines, Fig. 27-5. The disc is placed on the turntable,

Figure 27-4. Various types of drive mechanisms.

Motor

Motor

pulley

Turntable platter

Inner diameter of

turntable platter

Rubber

idler wheel

Spring

Idler wheel

lever

C. Lever system jams the idler wheel between

the motor pulley and the platter rim.

A. Single-motor belt-drive system.

B. Puck- or idler-driven turntable.

Rubber

idler wheel

Turntable

platter

Motor

pulley

Motor

Direction of spring

tension lever (not shown)

Inner diameter

of turntable platter

Idler pulled in this

direction