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Ideally, you would want the tonearm to have no mass so it would follow the gradual inward spiral of the record groove, moving without friction and without deflecting the stylus and cantilever out of correct alignment with the cartridge generator.
Yet if the cartridge and arm had no mass, the stylus would have no “anchor” to work against as the groove walls pushed it around. Instead, arm and cartridge body would faithfully match the stylus’s every wiggle without resistance, so that the three moved as a frozen unit. No work would be done, no signal would be generated, and there would be no music output. Proper music signals are produced only when the stylus moves relative to the fixed cartridge body. If the body wiggles also, then either no signals or wrong signals will be generated.
Therefore, the ideal pickup arm would have no mass at superlow frequencies to allow easy handling of LP warps or eccentricities and free movement across the record, but infinite mass caused by inertia, at audio frequencies so that the only movement was by the stylus.
Coming back to reality, however, records themselves are not ideal and invariably have flaws. Even seemingly flat LPs actually have tiny surface eccentricities, which the tonearm must be able to follow instantaneously. High mass here becomes a major liability because of its high inertia, which resists changes in movement. A low-mass arm, on the other hand, can respond quickly to record surface irregularities.
Here’s an example. An arm with too high mass will travel up the side of a warp, but may then overshoot the top instead of maintaining contact with the surface of the warp and following it down the other side. While a high-mass tonearm can be easily set in motion because its great mass has been counterbalanced on a precision bearing, once moving it’s hard to stop. As speed increases, so do inertia and resistance to quick changes of direction.
All records have mini-warps. A warp is like a ramp and, like Evel Knievel, the arm/cartridge comes flying along, shoots up the ramp, and may momentarily take to the air. If, for even a micro-moment, the stylus doesn’t exactly follow the groove modulation, you get a time distortion, which, as said earlier, is one of the most unmusical distortions to plague playback. Not accurately tracing the groove effectively creates two time domains, one of the cartridge/tonearm and another of the record, whereas the cartridge is supposed to march exactly to the time and tune of the groove.
A very low mass arm can precisely follow record irregularities, but cannot easily dissipate microresonances transmitted by the stylus up into the cartridge, headshell, and tonearm. In fact, with too little mass, the arm is easily driven into resonant behavior by the energy traveling up from the cartridge and record. Rather than serving as an energy drain, the arm will itself start to microvibrate and resonate, feeding vibrations back into the stylus. This, of course, interferes with the stylus’s ability to trace the groove accurately and muddies the sound. Another problem with low mass is that the arm, instead of providing a stable support and energy sink for the cartridge, can actually be pushed around by the cartridge, as described in the earlier discussion of the ideal tonearm. As the groove wall pushes the stylus, that motion is passed right along up into the arm. A high-mass (high-inertia) tonearm will most effectively provide an extremely stable, rigid cartridge support as well as being a good energy drain for the microvibrations of cartridge and headshell.
Balancing these two opposing needs of mass versus freedom from inertia is what keeps tonearm designers busily perfecting tonearms. Different designers choose slightly different tradeoffs, but an effective compromise is a medium-mass arm, one that is as light as possible for good tracking of LP irregularities, while still having sufficient mass to resist the stylus movement and to prevent the resonating cartridge body from causing the arm to vibrate. Most arms have a medium 10 to 13 grams of effective mass.
The arm’s effective mass is also very strongly influenced by the weight of the cartridge and headshell, since these sit out at the farthest end from the pivot point. The effect of tonearm mass increases with the square of its distance from the pivot point.
The mechanical-grounding-loop principle discussed above also helps to control resonances.