TURNTABLES: THE TONEARM: Introduction

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Getting good sound from a record requires that the stylus very accurately trace the groove modulations. Imagine what would happen if you tried to trace a picture while your hand was shaking. If, as the pencil traced the picture, your hand trembled, the tracing (transcription) would be distorted.

The same thing applies when transcribing a record. The tonearm is like your hand, the cartridge stylus like the pencil point. Musical accuracy depends on having an absolutely steady hand (tonearm) and stable surface (turntable). Unless the arm holds the cartridge rock-steady over the groove during the stylus’s half-mile encounter with each side of the record, the musical transcription will be distorted. The function of the tonearm is to follow the groove so the stylus can follow the modulations inscribed within the groove, and replicate as closely as possible the motion of the cutting lathe stylus.

THE MINUTE SCALE OF RECORDINGS

What makes the table system’s job so devilishly demanding is the truly minute scale of the music signal that must be released from the grooves. This signal must be magnified some 30,000 times from the groove to the speakers in order to be heard.

Visualize the fineness of a record groove, and then consider that it combines two distinct channels of information, each with completely different modulations. Some of the signal modulations in the groove are on the same order of size as a wavelength of light, which means the stylus has to “read” a signal as small as a millionth of an inch. Add in all the variation and complexities in the scale of the music itself, from crescendos to pianissimos, from piccolos to contrabassoons, and you can begin to see the stylus has quite a job.

The recorded audio bandwidth is the range of frequencies (i.e., rates of vibration) the human ear can hear, which extends from 20 Hz to 20,000 Hz. Hertz (Hz) is the same as cycles per second. You can hear a piccolo note whose fundamental vibrates as fast as 4,698.6 Hz and whose harmonics extend well up to 18,000 Hz and beyond, and a contrabassoon with a fundamental plunging as deep as 29 Hz. Your ear can also detect ranges in loudness of 60 dB, a ratio of 100,000 to 1. Not only is this tremendous range captured in the record groove, but then the stylus has to release it.

For the half mile or so of record groove per LP side, the stylus must precisely trace abrupt changes in the direction of the undulating groove, sometimes traveling at speeds several times the acceleration of gravity, without ever losing contact with either wall or blurring together the modulations.

Groove friction heats the stylus up to 350 degrees Fahrenheit and the groove vinyl momentarily liquefies each time the stylus passes over it. (This is why one should let a record rest for at least 30 minutes before replaying it, and preferably for 24 hours.)

Even though the cartridge tracking weight is commonly set at only about 1.5 grams, the entire weight is supported on the minute side edges of the stylus. As a result, the downforce applied to the groove on a per-square-inch basis is several TONS.

Combine these extreme conditions of weight, heat, speed, and need for exquisite maneuverability, then add in the scale of environmental vibrations that interfere with the stylus as it retrieves the music from the groove, and it’s extraordinary that ANY music (as opposed to noise) is heard through an audio system. The feat of retrieving all the music from the groove is analogous to an elephant trying to thread a needle.

To help one better grasp the magnitude of the difficulties in retrieving all the music from the record, the Boston Inch Scale (developed by E. B. Meyer and published in the Boston Audio Society’s magazine, The Speaker) converts signal and table measurements from their real-life micron scale into inches. A micron is a millionth of a meter, or one thousandth of a millimeter, which is equivalent to 0.0039 inch.

Using the inch scale, a stylus is 30 feet high, affixed to a cantilever 50 feet thick and 275 feet long, which extends from a cartridge body 2,000 feet long, sitting 80 feet above the record. The tonearm, 450 feet in diameter, crosses 1,300 feet above the record from its pivot point 4 miles away. On a typical line-contact stylus, the stylus down- force temporarily deforms the vinyl by as much as an inch (20 times the size of a violin harmonic), leaving a stylus footprint on the groove wall measuring 10 inches long and 4 inches wide.

A typical midrange signal demands that the stylus move 16 inches from peak to peak of the wave form. A deep bass note 10 dB louder requires the stylus to move 10 feet 6 inches whereas for a high-frequency harmonic at a very low sound level, the stylus must move only 0.68 inch. Even the simplest piece of music is likely to contain, at any one time, enormous numbers of frequencies at different levels.

(Incidentally, the same microsonic scale applies to compact discs. Though it is technologically feasible to make the pits smaller than they are now, and thus fit more information onto a single disc, the laser fine enough to read those smaller pits has yet to become commercially practical.)

Attention has commonly been focused too much on the cartridge, as the component that actually “collects” the music from the groove. But what allows the cartridge to do its job properly is the quality of the arm and table. If these two components do not meet certain standards, the cartridge will not perform up to its own quality. The best pencil in the world is of little benefit in a trembling hand on a shaky surface. You must think in terms of the table system—the table, arm, and cartridge form one component. This, and not the cartridge, is the real transducer.

To enable the cartridge to retrieve the most music from the record grooves, the tonearm must accomplish four tasks: (1) provide a rigid platform to support the cartridge over the groove, (2) conduct resonances away from the cartridge while introducing very few (ideally none) of its own, (3) move freely and smoothly across the record, and (4) provide sufficient adjustments so the cartridge can be accurately set up in correct geometry to the groove. (1) and (2) are both accomplished through a balance of mass and rigidity.

Next: High Mass, Low Mass, and Rigidity

Prev: Platter Mats and Record Clamps

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Updated: Friday, 2016-05-13 19:13 PST