Handbook for Sound Engineers

Analog Disc Playback 1023

Offsetting the tonearm introduces the skating force
that pulls the tonearm toward the center of the record. In
tonearms without the offset angle the skating force is
zero at one point and increases as the tonearm moves
away from this position. The zero tangent error point in
this tonearm coincides with the zero skating force posi-
tion, point A in Fig. 27-8.
Theoretically, the pivoted tonearm without the offset
angle and without any tangent error has to be infinitely
long. The tonearm designed by the Rabinoff brothers
revived the principle of tangential tracking used by
Edison and found application in many turntable
systems. In this system the tonearm motion has been
achieved using servomechanisms and utilizing various
types of arm position sensors. These tangential tracking
turntables practically eliminated the tracking error and
are quite popular with many hi-fi enthusiasts. There are
also drawbacks to this design. Usually, such tonearms
cannot be moved as fast as pivoted counterparts, and
this may become a handicap in operations when speed
of positioning the tonearm is of essence. The advantage
of tangential tonearms is that they are shorter, lighter,
and can be made more rigid to prevent many tonearm
resonances found in some inferior pivoted tonearms.
But the mechanical complexity of tangential tracking
tonearms requires the use of modern technology
including special integrated circuits and sensors.

Effective Tonearm Length. Fig. 27-11 defines the
turntable platter and spindle location in relationship to
the effective tonearm length, which is the distance
between the stylus tip and the tonearm pivot.

Modern tonearms have a built-in stop preventing
them from moving farther than the locking groove so
only three dimensions are of importance: effective

tonearm length, vertical pivot-to-spindle distance, and

the offset angle.

The accuracy of the cartridge tracking and mounting

depends on the effective length of the tonearm. If the

effective length of the tonearm is 7.87 inches and it is

properly mounted (7.04 inches away from the turntable

spindle), the cartridge will track to within +2¼° and

–1½°, providing the cartridge is mounted at an offset

angle of 27.8°. If the tonearm is longer, the lateral

tracking error gets smaller so that the tonearm with the

effective length of 10 inches will have a maximum

tracking error of less than 1° at the smaller disc radius

and a 1.7° error at the maximum radius.

Since the linear speed of the outer grooves is higher

and the wavelengths are longer, tracking angle errors

have lesser effect on the signal quality. Therefore,

tracking errors should be minimized at the inner

grooves for consistent quality of playback signal at all

radii.

Skating Force. Skating force is a force that can upset

the best aligned tonearm and cause considerable track-

ing error. The skating force is the result of tonearm

geometry and the friction between the stylus and the

record groove. Because of the offset angle and the over-

hang, one vector of this force pulls the stylus in a direc-

tion away from the pivot point of the tonearm and the

second vector pulls the tonearm toward the center of the

turntable, Fig. 27-12A. If this skating force is not com-

pensated for, the stylus will be deflected toward the out-

side of the disc at the angle much greater than the error

angle encountered in tracking the groove at different

radii, Fig. 27-12B.

Figure 27-10. Geometry of a modern tonearm.

Lateral tracking

angle error

Tangent of

groove

Music

begin

Stylus tip

Music end

Front to back axis

Edge of record

Figure 27-11. Relationship of the lateral components of a

tonearm.

Edge of

record 6"

Tone arm

pivot center

Optimal null

radius (2.6")

Maximum groove radius 5.75"

Stylus tip

Offset

angle

Maximum

groove

radius

5.75" Spindlecenter

Optimal null

radius 4.76"

Pivot to spindle distance

Tone arm effective length