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34

CHASSIS

8WJR56,8WJR57,

AND

15WJR29

MAGNETIC

PHONO

CARTRIDGES

(See

Figure

24)

Since

we

are

introducing

magnetic

phono

cartridges

to

the

product

line,

it

would

be

wise

to

review

the

basics

of

magnetic

cartridges.

Although

all

magnetic

phono

cartridges

function

on

the

same

basic

magnetic

principle,

they

can

be

divided

into

three

general

types,

depending

on

the

mechanical

relationships

of

the

stylus/stylus

shank,

magnet

and

coils.

MOVING

ARMATURE

—

Movement

of

the

stylus

in

the

record

groove

causes

the

stylus

shank

(which

is

in

the

field

of

a

stationary

permanent

magnet)

to

move

in

the

center

of

the

stationary

coils.

This

varies

the

number

of

lines

of

magnetic

force

cutting

the

coil,

thereby

changing

the

electrical

output

of

the

coil.

MOVING

COIL

—

A

coil

of

very

fine

wire

is

wound

on

the

stylus

shank.

This

coil

moves

in

a

magnetic

field

created

by

a

stationary

permanent

magnet.

As

the

stylus

moves

in

the

record

grooves,

the

moving

coil

(on

the

stylus

shank)

will

cut

the

lines

of

force

of

the

magnetic

field,

inducing

a

voltage

in

the

coil.

MOVING

MAGNET

—

A

permanent

magnet

is

mounted

on

the

rear

portion

of

the

stylus

shank

subassembly.

As

the

stylus

moves

in

the

record

grooves,

motion

is

conveyed

to

the

mag



net,

causing

a

change

in

the

flux

in

the

stationary

coils,

result



ing

in

a

voltage

being

induced

in

the

coils.

This

design

generally

provides

a

lower

effective

mass

(of

the

tip

touching

the

record

groove)

compared

with

other

magnetic

types,

resulting

in

lower

wear

of

both

stylus

and

record.

Figure

24A

is

a

photograph

of

a

moving

magnet

magnetic

phono

cartridge

(Zenith

P/N

142-189)

manufactured

for

Zenith

by

Shure

Brothers.

In

the

illustration,

the

stylus

assembly

has

been

pulled

partly

out

of

the

body

of

the

cartridge

showing

the

two

major

assemblies

that

make

up

the

total

cartridge.

STYLUS

ASSEMBLY

(See

Figure

24B)

A

phantom

partial

view

of

the

magnetic

cartridge

stylus

assembly

appears

in

Figure

24B.

"A"

is

the

.6

mil

spherical

diamond

tip

which

is

first

mounted

to

stylus

shank

"B"

by

press

fit

and

then

additionally

secured

by

use

of

a

high

temper



ature

epoxy

cement.

This

method

helps

insure

permanent

axial

orientation

of

the

styli.

A

shank

of

a

special

heat

treated

aluminum

alloy

gives

the

necessary

rigidity

but

is

only

a

minimum

part

of

the

total

mass.

A

visco-elastic

suspension

block

"C"

can

be

considered

the

"heart"

of

the

bearing

in

which

the

stylus

shank

assembly

will

pivot.

A

high

compliance

bearing

offers

better

frequency

response

and

lower

distortion.

As

the

stylus

tip

moves

in

the

recQrd

grooves,

the

stylus

shank

follows

that

movement,

causing

magnet

"D"

to

move

between

Ws

Wils.

A

high

energy

Alnico

magnet

provides

an

extremely

strong

magnetic

field

for

its

small

size,

yet

contributes

less

than

20%

to

the

effective

mass

of

the

stylus

assembly.

Various

components

just

described

are

housed

in

carrier

"E"

which,

when

the

total

stylus

assembly

is

inserted

into

the

cartridge

body,

will

correctly

position

the

magnet

relative

to

the

coils

in

the

cartridge

body.

One

major

part

of

the

stylus

assembly