Handbook for Sound Engineers

Analog Disc Playback 1021

adjusted to a normal listening level. When the record
with the quiet groove is placed on the turntable, a slight
hiss can be heard when putting your ear to the loud-
speakers. When the record with the quiet groove is
placed on the turntable and the stylus is placed into the
groove, listening to the increase in noise will show the
extent to which the turntable transmits the building
rumble. If the power to the turntable is turned on, the
noise contributed by the motor drive can be measured.
During this test, slightly tapping the base of the turn-
table can determine if the shock mounting is adequate
and whether or not loud music will add coloration to the
signal being reproduced. In summary, what is required
from the good turntable is that it reproduces only what
is recorded on the disc and is insensitive to all other
sources of vibration.

27.4.2 Turntable Design in the 21st Century

One of the most important features of turntable design
is the ability to keep noise and rumble created by
motors and bearings from being picked up by the car-
tridge stylus. Many inexpensive turntables have a direct
drive between the motor and the platter and inexpensive
bearings, allowing motor noise and vibration to be
transmitted to the platter and then to the cartridge stylus.
Remember, it doesn’t make any difference to the signal
whether it comes from the stylus moving versus the disc
or the disc moving versus the stylus.
VPI turntables, Fig. 27-6 use inverted bearings
instead of conventional bearings. In this design the
bearing assembly is in the platter rather than in a
bearing well below the platter. The spindle and ball are
attached to the chassis and the bearing well is inverted
and placed in the platter itself. With this design the
drive belt pulls through the center of the bearing
assembly rather than many inches away from the center
of the assembly, reducing teeter-totter effects to near
zero for better stability.
All motor assemblies are completely separated from
the turntable platter and tonearm, so there is no mechan-
ical connection between the motor and the chassis
except through the belt. This gives much lower noise
levels due to isolation from the source of noise.
The VPI HR-X turntable uses a dual motor flywheel
assembly to drive the platter. Two synchronous motors,
driven by a perfect sine-wave ac power supply, drive a
14 lb flywheel spinning at 300 rpm, which in turn drives
the platter. In this configuration the platter is driven by a
non-electromotive source as opposed to other tables that
are driven by the motor or combination of motors.
Running the platter with no motor or multiple motors

produces a velvety black background and perfect speed

stability.

27.5 Tonearms

Tonearms can be classified into two categories: pivoted

and tangential tracking, Fig. 27-7A and 27-7B.

Contemporary tonearms are designed to cope with a

variety of problems. However, rarely can one find a

tonearm with nearly perfect geometry and correct

design to establish correct performance. Most tonearms

have built in antiskating devices, adjustable counter-

weights to accommodate a variety of cartridge weights

and tracking forces, vertical height adjustment to set the

tonearm parallel to the record, and a variety of features

to facilitate installation and operation of the device. All

tonearms are at best a compromise. Very few tonearms

are dynamically balanced, and most rely on dynamic

unbalance to produce vertical tracking force. The

dynamically balanced tonearm is the tonearm that is

capable of playing a record with the turntable tipped at

almost any angle without changing the tracking force

and tracking ability.

Tonearm Geometry. Tonearms are designed to retrace

the modulation of the groove in the same way as it was

recorded. Design of the tonearm takes into consider-

ation the diameter of the records or the turntable, and

the distance between the center of the platter and the

Figure 27-6. High-quality noiseless turntable. Courtesy VPI

Industries, Inc.