Handbook for Sound Engineers

1030 Chapter 27

tance of the semiconductors, about 800: changes
slightly, causing a varying dc voltage across the output.
This dc signal is ac coupled to the preamplifiers in the
power supply, providing an output voltage of 0.4 V for
each side. The cartridge employs mechanical equaliza-
tion that, in combination with the RC equalizer at the
output of each preamplifier, results in an RIAA repro-
ducing characteristic.

Piezoelectricity. Piezoelectricity is pressure electricity.
The voltage generated by the crystals in piezoelectric
cartridges is proportional to the amplitude of the stylus
displacement. The output voltage of the average piezo-
electric pickup is considerably higher than for other
type pickups. Piezoelectric pickups are treated electri-
cally as a capacitive-reactance device since the imped-
ance rises with a decrease of frequency. Simple RC
networks are used with this type of pickup to obtain a
frequency response corresponding to the standard RIAA
reproducing characteristic. Records recorded using a
constant-amplitude characteristic may be reproduced
without equalization.
In the ceramic stereophonic pickup, Fig. 27-23, the
moving system consists of two piezoelectric crystal
slabs of lead-zirconium titanate or similar material. This
particular material offers good mechanical and electrical

properties with high sensitivity and high capacitance.

The ends of the slabs are mounted rigidly in a mounting

block, and the front end is connected by a yoke made of

injected molded plastic. This coupling is critical

because the electrical performance and the mechanical

impedance seen at the stylus point by the record groove

depends on it. The coupling system is defined as that

portion of the mechanism that lies between the stylus tip

and the ceramic slabs.

The stylus bar is made from heat-treated, thin-walled

aluminum alloy tubing, with one end flattened to hold

the stylus at the desired angle. The other end of the

stylus bar is held in place by the stylus mounting block.

The coupling yoke is connected at a point about

midway on the stylus bar. This point is chosen because

it affords the most desirable electrical performance and

substantially reduces the mechanical impedance of the

yoke and ceramic elements as seen by the stylus tip.

Better designs have four output terminals, two for

each channel, to ensure the complete isolation of one

side from the other. Damping in the form of a viscous

material is used to control the frequency characteristics.

These pickups are of the constant-amplitude type with

the output voltage 10 mV for a peak velocity of 5 cm/s.

Ceramic pickups are not affected by magnetic or elec-

trostatic fields.

RC equalizer networks for both crystal and ceramic

pickups are shown in Fig. 27-24. The networks are

connected between the output of the piezoelectric

pickup and the input of the preamplifier. The character-

istics of these networks are such that they correspond to

the standard RIAA reproducing curve. Using a pickup

with a compliance of 15 × 10–6cm/dyn or greater, the

response can be within ±2 dB.

The internal impedance of the average crystal pickup

is approximately 100 k: , with a capacitance of 0.001 to

0.0015μF.

Figure 27-22. Stereophonic semiconductor pickup.

A. Beam construction of a semiconductor pickup.

B. Cartridge construction of a semiconductor

stereophonic pickup.

0.008"

Encapsulated semiconductor

0.008 × 0.005 × 0.005

element

Strap

0.020"

Force

0.020" Hinge

Epoxy

Output

gold

plated surfaces

0.001"

Mounting pad

Coupling yoke

Low mass stylus

0.090"

Needle damping pad

Elements

Leads

0.050"

Figure 27-23. Simplified drawing showing the construction

of a ceramic stereophonic pickup.

Mounting block

Ceramic elements

Stylus Coupling yokeStylus bar

Stylus mounting

block