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The tonearm must minimize friction at its pivot point in order to move freely and smoothly across the record. Any resistance interferes with the stylus’s accuracy in tracing the groove and also transmits resonances from the pivot point, down the arm tube, and back into the cartridge. Tonearm bearings must be as friction free as humanly possible.
The bearings form a vital link too in the mechanical grounding loop because, if well designed, they will transmit low-frequency energy beyond the arm’s damping ability into the much larger mass of the table, where it can be dissipated. To do this, the bearings must be very rigid, permitting no slop or excess play. Such play could also cause audible rattling and chatter.
In fact, bearing rattle is provoked by the stylus vibrations traveling from the cartridge body down the arm tube, and also by acoustic feed back transmitted through either the arm tube or the plinth into the arm base. The vibrations of the rattling bearings then get transmitted back down the arm tube to the cartridge body and wiggle it, thus interfering with accurate music retrieval.
Once again, a tradeoff becomes necessary. Make the bearings tight enough to combat rattling, and the friction increases. Loosen them enough to reduce friction, and they rattle worse. Indeed, all hard bearings have a tendency to rattle more at higher frequencies, regardless of how tight they are. The bearings cannot be tightened sufficiently to move the rattling into the supersonic regions without simultaneously introducing excessive friction.
One solution to this conflict is an air bearing design such as the Eminent Technology arm uses. Correctly designed, it is friction free and nonrattling. Another inspired approach is offered by the Well Tempered Arm’s no-bearing design. The arm is suspended on a pair of crossed nylon monofilaments secured to a paddle immersed in silicone damping fluid, an arrangement that allows for high effective rigidity. As a friend explains it, this design produces an effect very similar to trying to swing a Ping-Pong paddle underwater quickly.
In conventional bearing design, this conflict between very tight coupling and friction-free movement is generally best resolved by extreme smoothness and hardness of the bearings, reducing friction while allowing rigidity. Naturally, such a fine-quality bearing ain’t cheap.