Handbook for Sound Engineers

1018 Chapter 27

3180 μs time constant to flatten the low end of the
curve. In today’s modern amplifiers, the equalization is
accomplished by placing the network with proper time
constants into the negative feedback loop of the ampli-
fier, thereby achieving lower distortion, better SNR, and
improved signal-level-handling capability of the circuit.
Because the recording space on the record disc is
limited, records are cut with constant amplitude charac-
teristics of the signals in the upper half of the frequency
range. When reproduced by the pickup, these signals are
equalized to a constant velocity characteristic. In
playing back these preemphasized disc recordings,
different equalization has to be used for different types
of cartridges. For instance, dynamic cartridges, which
include moving-magnet, moving-iron, moving-coil, and
variable-reluctance pickups, are constant velocity
devices; therefore, they respond to the speed of the
stylus movement. The faster the stylus is deflected, the
higher the output voltage. Ceramic or crystal cartridges
are pressure-sensitive devices, and they respond to the
force applied to the stylus. They are called constant
amplitude devices, and when records with constant
velocity recording are played with ceramic cartridges,
no additional equalization is required. The combined
characteristics of both the recording and the cartridge
complement each other, returning the signals to their
original form. Only a minimal amount of signal
grooming may be necessary to compensate for the
effects of capacitive loading and nonlinearity of the
cartridge.

27.4 Turntables

To play a record, the turntable or device to rotate the
disc at the required speed is needed. This is the basic
requirement for all turntables.The construction and exe-
cution of the requirement may differ greatly between
the models and the designs of different manufacturers.
The history of evolution of the record drive mechanisms
takes us from the days of hand-cranked cylinder

machines, through the age of spring-wound phono-

graphs with mechanical governors for speed control,

and into the age of electrically driven machines with

electronic control. Today the accuracy of turntable

speed is measured in small fractions of 1% in deviation

from the desired speed.

27.4.1 Drive Systems

Turntables are driven by electric motors. The method by

which the power from the motor is transferred to the

turntable platter classifies the drive mechanism. The

turntable platters can be belt driven, puck or idler

driven, and driven direct.

The first category, the belt-driven type, encompasses

all models that have motors mounted to the side of the

platter with the belt stretched over the motor pulley and

outer rim of the platter, Fig. 27-4A. Some platter

designs have an additional internal rim to hide and to

protect the belt.

Many turntables have synchronous motors or motors

with some type of speed control mechanism, such as a

centrifugal switch that disconnects the power to the

motor when the speed exceeds the preset value. The

later types of motors are usually low-voltage,

battery-driven motors used in portable equipment. Also,

in portable turntables there is electrical feedback to

control the speed of the low-voltage motor.

Another version of the same idea uses a low-voltage

ac motor driven by a self-contained crystal-controlled

oscillator allowing variation of the speed of the platter

and achievement of great speed precision. The only

source of speed variation can come from belt slippage

or a defective belt. Belt-driven turntables are normally

the quietest turntables. The speed selection of the

belt-driven turntable can be accomplished either by

changing the speed of the motor or by having the

stepped pulley on the motor and by shifting the belt

from one pulley onto another.

The second type of turntable is a puck-driven or

idler-driven turntable, Fig. 27-4B. The coupling

between the platter and the motor shaft is achieved

through the intermediate idler wheel or puck, which has

the outer edge covered with neoprene rubber or polyure-

thane for positive drive and to isolate the motor vibra-

tion from the platter. The idler wheel rotates on the shaft

that is attached to a sliding bracket. When one side of

the idler pulley (or puck) is in contact with the inner

side of the rim of the platter and on the other side with

the motor shaft, the idler wheel will transmit the motor

rotation to the turntable platter. The mechanism is

designed so when the motor is turned off the idler wheel

Figure 27-3. DIN recording and playback characteristics.

20

10

0

10

20

Frequency—Hz

2 5 10 25 50 100 250 500 1K 2K 4K 8K 16K

Level—dB

30

7960 MsMs Ms 75 Ms