Arm systems for turntables

Introduction

Basic arm design has changed relatively little. S-Type tonearms can be found on even the 1925 Victor Orthophonic phonograph. Originally, even though the tonearm was light for earlier electric pickups, 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 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. Most use a counterweight to offset the weight of the arm. A calibrated dial on the weight provides for quick change of stylus pressure. Stylus pressures of 1 to 2 grams are currently the standard.

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Pivoted arm

Tonearms are prone to two types of tracking errors that can affect the sound. As the tonearm tracks the groove, the stylus drags tangent to the disc surface and resistance along the arm combines to create a horizontal skating force towards the center of the disc. Modern arms provide an antiskating mechanism, using springs, hanging weights or magnets, to offset this force, so as to make the net horizontal force near zero. The second error occurs as the arm sweeps in an arc across the disc, causing the angle between the cartridge head and groove to change slightly. A change in angle, albeit small, may have an audible detrimental effect by creating a differential force on the groove walls. Making the arm longer to reduce this angle is a partial solution, but less than ideal, because the arm would need to be of infinite length to reduce angular errors to zero. Some arms (such as the Garrard "Zero" series) have been manufactured with a parallelogram arrangement which pivots the cartridge head on the arm to maintain a constant angle.

Tangential arm

If the arm is not pivoted, but instead travels horizontally along a radius of the disc, there is no skating force and no cartridge angle error. The arm is driven along a linear track using an electronic servomechanism to position it properly. Bang & Olufsen developed the first practical system with its model Beogram 4000 in 1972. Early Edison phonographs had utilized similar 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. It is nearly ideal, as the stylus replicates the motion of the recording lathe when the master recording was cut.