Phonographic - Arm Systems

Arm Systems

The tone arm (or tonearm) holds the pickup cartridge over the groove, the stylus tracking the groove with the desired force to give the optimal compromise between good tracking and minimizing wear of the stylus and record groove. At its simplest, a tone arm is a pivoted lever, free to move in two axes (vertical and horizontal) with a counterbalance to maintain tracking pressure.

However, the requirements of high-fidelity reproduction place more demands upon the arm design. In a perfect world:

• The tone arm must track the groove without distorting the stylus assembly, so an ideal arm would have no mass, and frictionless bearings, requiring zero force to move it.
• The arm should not oscillate following a displacement, so it should either be both light and very stiff, or suitably damped.
• The arm must not resonate with vibrations induced by the stylus or from the turntable motor or plinth, so it must be heavy enough to be immune to those vibrations, or it must be damped to absorb them.
• The arm should keep the cartridge stylus tangent to the groove it's in as it moves across the record, with minimal variation in angle.

These demands are contradictory and impossible to realize (massless arms and zero-friction bearings do not exist in the real world), so tone arm designs require engineering compromises. Solutions vary, but all modern tonearms are at least relatively lightweight and stiff constructions, with precision, very low friction pivot bearings in both the vertical and horizontal axes. Most arms are made from some kind of alloy (the cheapest being aluminium), but some manufacturers use balsa wood, while others use carbon fiber or graphite. The latter materials favor a straight arm design; alloys properties lend themselves to S-type arms.

Prices vary largely: the well known and extremely popular high-end S-type SME-arm of the 1970-1980 era not only possessed a complicated design, but was also very costly. On the other hand, even some cheaper arms could be professional quality: The "All Balance" arm made by a now defunct Dutch company, "Acoustical" was only €30 . It was used in that period by all official radio stations in the Dutch Broadcast studio facilities of the NOS, as well as by the pirate radio station Veronica. Playing records from a boat in international waters, the arm had to withstand sudden ship movements. Anecdotes indicate this low cost arm was the only one capable of keeping the needle firmly in the groove during heavy storms at sea.

Basic arm design has changed relatively little. S-type tonearms can be found on even the early 1925 Victor Orthophonic Victrola. Though early electrical pickup tonearms were light, their full weight rested on the record. This created the tracking force required to transmit accurately to the crystal pickups of the day, with relatively stiff styli. Friction resulted, and record wear was high. As better technologies emerged (such as magnetic cartridges), far smaller tracking forces became possible, and the balanced arm came into use. Quality arms employ an adjustable counterweight to offset the mass of the arm and various cartridges and headshells. On this counterweight, a calibrated dial enables easy adjustment of stylus force. After perfectly balancing the arm, the dial itself is "zeroed"; the stylus force can then be dialed in by screwing the counterweight towards the fulcrum. (Sometimes a separate spring or smaller weight provides fine tuning.) Stylus forces of 10 to 20 mN (1 to 2 grams-force) are typical for modern consumer turntables, while forces of up to 50 mN (5 grams) are common for the tougher environmental demands of party deejaying or turntablism. Of special adjustment consideration, Stanton cartridges of the 681EE(E) series feature a small record brush ahead of the cartridge. The upforce of this brush, and its added drag require compensation of both tracking force (add 1 gram) and anti-skating adjustment values (see next paragraph for description).

Even on a perfectly flat LP, tonearms are prone to two types of tracking errors that affect the sound. As the tonearm tracks the groove, the stylus exerts a frictional force tangent to the arc of the groove, and since this force does not intersect the tone arm pivot, a clockwise rotational force (moment) occurs and a reaction skating force is exerted on the stylus by the record groove wall away from center of the disc. Modern arms provide an anti-skate mechanism, using springs, hanging weights, or magnets to produce an offsetting counter-clockwise force at the pivot, making the net lateral force on the groove walls 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, will have a detrimental effect (especially with stereo recordings) by creating different forces on the two groove walls, as well as a slight timing shift between left/right channels. Making the arm longer to reduce this angle is a partial solution, but less than ideal. A longer arm weighs more, and only an infinitely long arm would reduce the error to zero. Some designs (Burne-Jones, and Garrard "Zero" series) use dual arms in a parallelogram arrangement, pivoting the cartridge head to maintain a constant angle as it moves across the record. Unfortunately this "solution" creates more problems than it solves, compromising rigidity and creating sources of unwanted noise.