Analog Disc Playback 1027the proper height above the record surface, all the while
moving the tonearm across the surface of the record.
The phonograph cartridge is an electromechanical
device designed to track or follow the excursions of the
record groove and to convert this motion, with the help
of a tracking mechanism-stylus assembly, into elec-
trical signals.
Cartridges are classified by the principle by which
they convert mechanical motion into the electric current
or signal, electrodynamic and piezoelectric. There are
also pickups designed to operate using strain gauges,
variable capacity, and light as sensors.Electrodynamic-Type Cartridges. Electrodynamic-
type cartridges are subdivided into three categories:
moving magnet, moving coil, and induced magnet or
moving-iron type. The electrodynamic principle consists
of using a magnetic field that, when it intersects the coil
windings, generates electric current. The construction of
the cartridge classifies the type. If the magnet is
attached to the stylus tube or cantilever and the coils are
stationary, it is called a moving-magnet cartridge. If the
magnet is made stationary and the coils move in the
magnetic field, it is a moving-coil cartridge; and if the
magnet and the coils are made stationary and there is a
slug of soft magnetic iron moving in place of a magnet
while being magnetized by the stationary magnet, it is
called a moving-iron or induced-magnet cartridge.Variable-Reluctance Cartridges. Since the introduc-
tion of the original variable-reluctance pickup, Fig.
27-18, many different versions of its design have
appeared. The magnetic structure consists of two pole
pieces A, with a small permanent magnet B between
them. The coil C is mounted with a soft rubber insert D.
The stylus, which is also the armature, is held in the
exact center of the magnetic structure by the rubber
insert. When the stylus is actuated, its movement causes
a voltage to be generated in the coil. Because of its
construction, the frequency response extends beyond
the normal audio-frequency band. Output voltage is on
the order of 100 mV at 1 kHz, with an output imped-
ance of 500:. The recommended stylus pressure is
15–20 g. The stylus weighs 31 mg and is removable.
Although the recommended pressure is 15–20 g, the
pressure could be as low as 7 g. The frequency response
is ±2 dB, 20 Hz–20 kHz.Moving-Coil Cartridges. Modern moving-coil car-
tridges are represented by a variety of designs. All of
them have coils that move, but not all of them are entitled
to be called moving-coil type. Designs that depend for
their functioning on the motion of the soft iron core
rather than on the motion of the coil itself should not be
classified as a pure moving-coil device. There the motion
of the coil is coincidental. Fig. 27-19 shows the cross
sections of moving-coil stylus assemblies as they move
during the playing of the record. The magnetic flux is
directed by the iron core or armature of the coil. If the
coil is made stationary and the core is vibrated, the signal
will still be generated. This fact prevents it from being
classified as a pure moving-coil device.
The advantage of this design is extremely low output
impedance, making the cartridge insensitive to capaci-
tive loading and allowing the use of very long cables
without altering the frequency response of the device.
On the negative side, the output of the cartridge is very
low, measuring in the tenths of a millivolt requiring an
extra 20–30 dB amplification to bring the electrical
signal to the required level as referenced to an estab-
lished sensitivity of 1 mV/1 cm/s of recorded velocity.
A step-up transformer or an extra stage of amplification
usually introduces additional noise and the effect on
capacitive loading. Other drawbacks of such design are
the weight of the cartridge and the need to use a heavier
tracking force.
One of the debatable points about MC cartridges is
the sound they produce. MC cartridges have a very fast
response to transients because of the very low induc-
tance and the impedance of the coils and the very rigid
cantilever, which has to be strong in order to move a
relatively heavy coil assembly. Another factor in this
type of design is the construction of the coil assembly,
which may have a number of turns in the coil unsup-
ported and free to vibrate, producing random signals at
higher frequencies. Also lead-in and lead-out wires may
not be secured properly and can vibrate in the magnetic
field producing random coloration of the signal. LeadFigure 27-18. Variable-reluctance magnetic pickup.DABCAE