First Model of Servo-Arm for Phonograph Pickups : Jacob Rabinow

First Model of Servo-Arm for Phonograph Pickups
ca. 1954

LP records are cut by a machine with a stylus that moves along a straight line. The cutting stylus is a sharp chisel that lifts a thin thread as it moves along the radius of the record. This thread is taken away by a vacuum suction device, leaving a flat surface and a V-shaped groove. Yet until Rabinow’s invention, the arm of the record player reproducing the sound did not move in straight line. It swung in an arc. A stylus moving in such an arc could not be tangent to the groove over its entire path. When the playing stylus did not move in the same path as the cutting stylus, it produced distortion. As other aspects of sound recording and reproduction technology became more sophisticated, this distortion problem was amplified.

Instead of making a long, swinging arm, Rabinow made a pivoting short arm mounted on a small carriage. The carriage rode on a straight track parallel to the motion he wanted to impart to the record, that is, parallel to the record’s radius. Between the arm and the carriage, he located a sensing device, which measured the angle between them. The carriage started by standing still, and as the stylus began to move in toward the center of the record, the angle dropped below 90°. Then the sensor activated a motor, which moved the carriage to catch up with the stylus.