Handbook for Sound Engineers

1028 Chapter 27

dressing, coil impregnation, and gluing techniques
control the purity of the sound produced by this design.
Step-up transformers require winding ratios of 1:10
or more. The transformer’s high-impedance secondary
winding is reflected back into the primary, and any
loading of the secondary in excess of the specified value
affects the signal output level and electrical damping of
the coils. Theoretically, shorted coils produce maximum
damping, while an unterminated winding of the trans-
former’s secondary will emphasize electrical resonances

and unchecked mechanical motion. It is important to

locate the step-up transformer near the preamplifier

input to minimize the capacitive load of the shielded

wires between the transformer and the input stage of the

amplifier. Because the levels handled by this input

transformer are extremely low, good transformer

shielding is necessary.

In lieu of the step-up transformer, a prepreamplifier

may be used. Additional preamplification, obtained from

active gain circuits, requires super low-noise circuits in

order to preserve an acceptable SNR. There have been

many such pre-preamplifiers designed using the most

exotic devices and circuits, operating with batteries or

special ac power supplies with maximum filtering and

voltage regulation and using magnetic shielding.

Moving-Magnet Cartridges. The most popular high-

performance stereo cartridges are the moving- magnet

type. Moving-magnet cartridges offer one of the most

sensible ways to design the stereo cartridge with a

replaceable stylus. This cartridge has low dynamic tip

mass, high compliance, and fairly high output. By using

the most powerful rare earth magnets and using the

most modern manufacturing methods, the frequency

response is extended from almost direct current to well

past the threshold of hearing, Fig. 27-20.

Induced-Magnet Cartridges. An example of an

induced-magnet or variable-reluctance pickup is manu-

factured by Bang and Olufsen of Denmark. It consists

of a small armature in the form of a cross, made of

Mumetal, which swings between four pole pins, Fig.

27-21A. A stylus bar constructed of aluminum tubing

0.002 inch (0.05 mm) thick is attached to the Mumetal

armature cross at one end. The stylus is secured to the

other end of the tube. Four pole pins with four coils are

placed at each end of the cross. With a 45° motion to the

right, a reverse voltage induction takes place. Such

action permits the coils to be connected push-pull, thus

reducing harmonic distortion induced by the nonlinear-

ity of the magnetic field. In addition, the coils provide

an effective hum-bucking circuit.

Crosstalk between the left and right channels is mini-

mized, since such components are bucked out. Modu-

lating one channel 45°, the cross arms on the orthogonal

channel rotate without changing the spacing; therefore,

there is no induced voltage in this channel, assuming the

positioning of the unit, with respect to the groove, is

correct.

A cross-sectional view of the magnetic circuit is

shown in Fig. 27-21B and is similar to the magnetic

structure of a loudspeaker employing a center magnet.

Thus, a closed magnetic circuit, which prevents leakage

Figure 27-19. Bottom view of the moving-coil cartridge
generator assembly.

B. Cantilever deflected to the right.

C. Cantilever deflected to the left.

Cantilever

Flux N1 to S

A. Cantilever in the neutral position.

Flux N2 to S

Armature

Coil

N1 N2

S