Equipment
Turntable : arm systems
Basic arm design has changed relatively little. S-Type tone-arms
can be found on even the 1925 Victor Orthophonic phonograph.
Originally, even though the tone-arm was light for earlier
electric pickup, the full weight rested on the record. Right
through to the crystal pickup, this was required to create
sufficient tracking force to follow the grooves adequately
with the relatively stiff styli. Naturally, record wear was
not given much consideration. With the advent of the better
technologies, including more powerful rare-earth magnetic cartridges,
far lighter tracking forces became possible, and a balanced
arm came into use. Many use a counterweight to offset the weight
of the arm. The addition of a calibrated dial on the weight,
provides for quick change of stylus pressure. Stylus pressure
of 1 to 2 grams are currently the standard.
Tracking errors:
Two types of tracking error incident to a standard arm can
affect the sound. As the tone arm tracks the groove, the stylus
drag tangent to the disc surface and resistance along the arm
the combine to create a horizontal skating force towards the
center of the disc. Modern arms provide a spring-loaded or
hanging weight bias which offsets this force, so as to leave
the net horizontal force near zero. The second error occurs
as the arm sweeps in an arc across a disc recording, causing
the angle between the cartridge head and groove direction to
change slightly. A change in angle, albeit small, may have
an audible effect by creating a differential force on the groove
walls. Making the arm longer so as as to reduce this angle
is one solution. Some arms have been manufactured with an auxillary
arm which pivots the cartridge head on the main arm to maintain
a constant angle.
If the arm is not pivoted at a fixed point, but travels horizontally
along a radius of the disc, there will be no skating force
and no cartridge angle error. The arm is driven along a linear
track using a servomechanism to position it properly. Matsushita's
Technics division developed one such practical system with
its model SL-10 turntable in 1979, ten years after the introduction
of the first direct-drive turntable. Early Edison phonographs
had utilized similar but spring powered drives to carry the
stylus across the record at a pre-determined rate. In practice
the linear tracking system is not widely used today due to
its complexity and attendant expense. However some of the most
sophisticated systems do employ this technique.
3345, the Vinyl Records Home.
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