Handbook for Sound Engineers

Analog Disc Playback 1029

of the magnetic field, is provided and being nonmag-
netic, it cannot be attracted to the steel turntable plate. It
also provides an effective shield for the coils. The stylus
bar pivots on a nylon thread, bonded to a plastic
support. The armature cross bears on a resilient disc,
Fig. 27-21C, which controls compliance and supplies
damping for the moving system. The rotational point of
the system is at the junction of the armature cross and
the nylon thread support. The output voltage is 7 mV
for each channel for a 5 cm/s cut. The stylus has an
angle of 15° at 2 g of tracking force and may be oper-
ated at a pressure of 1–3 g. Compliance is
15 × 10–6cm/dyn for both directions of motion.
Frequency response is 20 Hz–20 kHz ±2.5 dB.

Semiconductor Pickup Cartridge. A semiconductor
pickup cartridge operates on the principle of the strain
gauge. The pickup mechanism employs two small,
highly doped silicon semiconductor elements 0.008 inch
× 0.005 inch whose resistance varies as a function of the
stylus deflection, Fig. 27-22. The elements are mounted
on laminated beams of lightweight epoxy with
gold-plated surfaces. A notch in the beam under the

assembly acts as a hinge for stress concentration. In this

structure, two beams are used, each driven by an elastic

yoke, coupled to the stylus. Aside from the compliance

of the yoke and mounting pads, a mechanical advan-

tage of over 40:1 can be attained in the beam and stylus

lever. This mechanical transformer provides high com-

pliance and reduces the mass of the elements reflected

to the stylus. The stylus is elliptical in shape and set at

an angle of 15°.

Since the semiconductor elements are sensitive

modulating devices and not generators as in the conven-

tional pickup, very little energy is required for their

operation. The compliance at 1 kHz is approximately

25 × 10–6cm/dyn and the frequency response is from

20 Hz–50 kHz. A power supply, two single-stage

preamplifiers, and one inverter stage are required. As

the elements are deflected by the stylus action, the resis-

Figure 27-20. Basic principle of the moving-magnet pickup.

Mu-metal shield

Isolated

para-toroidal

coils

Laminated

coil core

with unitized

pole pieces

Vector-aligned

dual magnets

0.12-mm square shank nude-mounted

linear contact natural diamond stylus

0.3-mm diameter

beryllium cantilever

Radial damping ring

at cantilever fulcrum

Individual

compliance

adjustment

screw

Figure 27-21. Induced-magnet cartridge construction.

A. Simplified drawing of coils and cross armature.

C. Stylus mounting with damping control.

B. Magnetic circuit.

Magnet

1

2

Pole pin

Cross armature

Stylus bar

3

4

Record

Coils

Coils

Magnet

N

Cross

Record

S

S

N

Damping

material

Nylon

Stylus

Cross bar

Stylus