Handbook for Sound Engineers

1032 Chapter 27

because they are small, they follow the high-frequency
excursions of the groove more easily.

Stylus Characteristics. All playback styli are designed
to contact only the walls of the groove; therefore, the
stylus tip has to ride without touching the bottom of the
groove. Since the diamond gets slimmer as it wears
down, the tip gets closer and closer to the bottom of the
groove. When it starts touching it, noise increases
because debris has accumulated on the bottom of the
groove and is scooped up by the stylus. This is a clue to
change the stylus in order to reduce the noise and to pre-
serve the record from being destroyed by the sharp
edges of the worn diamond.
Currently, almost all styli manufactured are made out
of diamond. The quality and the price of the stylus
depends on whether it is made out of a solid piece of
diamond or a small chip bonded onto another material
that acts as an extension or pedestal for the diamond tip.
The technology of manufacturing diamonds has
advanced significantly so that chip bonding and
encasing can be favorably compared to solid or nude
diamond tips. In view of the fact that the area of contact
is only 0.2 millionths of a square inch (0.2 × 10^6 inch)
and as long as this area is made out of a diamond, the
overall performance of the stylus will not be affected.
All this is true providing that the mass of the bonded
stylus assembly is not higher than that of a conventional
diamond and not larger than the nude stone.
The vertical tracking force applied to the stylus is
divided between the two walls. Each wall is experi-
encing force that is equal to the total vertical force times
the cosine of 45° or 0.707, Fig. 27-26. For instance, if
the vertical tracking force (VTF) is 1 g, each groove
wall will experience a force of 0.7 g.

A very small area of contact exists between the
stylus tip and the groove so the pressure against the
groove wall can rise up to many thousands of pounds
per square inch. For instance, if each wall receives 0.7 g
of force applied through the contact area equal to two

ten millionths of an inch (0.2 × 10–6), the pressure is

7726 lb/inch. With such high pressures and force of

friction between the stylus and the vinyl, the outer skin

layer of the record material melts as the tip slides over

the plastic and then refreezes almost as fast as it melted.

Since the melting temperature of the vinyl is about

480°F, the same temperature exists in the contact area.

Stylus Cantilever. The stylus is attached to some type

of coupler or cantilever that connects it to the generat-

ing element of the cartridge, which could be a magnet, a

piece of iron, a coil, or a ceramic element. Because of a

very wide range of frequencies this stylus assembly has

to transmit, the construction material and shape of the

cantilever are very important. Theoretically, it has to be

very light and rigid. Over the century of existence of

mechanical sound recording, styli were made out of

cactus needles, whale bones, and all kinds of metal,

gems and stones, plastic, and wood. The final choice is

centered around an aluminum alloy thin-wall tube. It is

fairly strong, light, noncorrosive, nonmagnetic, electri-

cally conductive, and easy to manufacture.

The average diameter of the aluminum cantilever

tube is 0.03 inch (0.76 mm), and the length may vary

from ¼–½ inch (6–12 mm). A few exotic cartridges

have cantilevers made out of solid ruby or even

diamond and some from boron or beryllium copper

alloy. Although ruby and diamond are extremely rigid

materials, because of manufacturing difficulties and

high weight/length ratio, they are made very short. This,

in turn, brings the pivot point much closer to the stylus

tip that moves in a much smaller arc when reproducing

groove modulation. Since the grooves are modulated by

the cutting stylus that has its pivot quite a distance away

and is moving in an arc of much larger radius, the larger

the difference between the motions of the cutting and of

the playback styli, the larger the distortion.

On the other hand very long playback cantilevers are

unable to produce sufficient motion of the generating

element that results in a very low electrical output.

Compliance. The amount of force required to move the

playback stylus depends on several factors; the first is

the compliance of the stylus, and the second is mass.

Compliance of the cantilever or the stylus is the

ability of the stylus assembly to react to the groove

modulation. It is measured in cm/dyn or μm/mN

(metric) and gives the amount of stylus tip deflection

for the given force. Compliance is measured statically

and dynamically.

Static compliance is the amount of deflection of the

cantilever when a constant force is applied to the stylus

tip. Dynamic compliance is a measure of tip deflection

Figure 27-26. Stylus motion and forces acting upon it in a
stereo groove.

VTF

1.0 g

Modified

stylus direction

Plastic groove

indentation

0.7 g 0.7 g

Right channel

modulation