The Rational Design of Phonograph Pickups
Design problems associated with signal transduction and with proper groove tracking can usefully be considered separately. This paper deals with the latter and presents a rational basis for specifying all the important mechanical parameters of a pickup and its tone arm. The only available driving force is that derived from the bearing weight and this is arbitrarily allocated equally among the three independent and uncorrected driving-force demands imposed by tone-arm accelleration, stylus acceleration, and low-frequency tracking. These demands can be assessed by reference to standard record dimensions and the limiting characteristics of typical recorded material. Design inequalities can then be deduced which specify limiting values for the equivalent tone-arm mass, the stylus compliances, and the equivalent stylus mass in terms of the bearing weight. In turn, these parameters locate the frequencies of tone-arm resonance and of free resonance of the stylus system. The location of the stylus-groove resonances and of the cutoff frequencies for translation loss and scanning loss involve the foregoing parameters and also the stylus radius, the elastic modulus of the record material and the groove velocity. The effect on pickup performance of different placements of these critical frequencies is discussed, and attention is directed toward the possibility of achieving superior performance by arranging these frequencies in a certain pattern. The discussion concludes with a critical comparison of these design criteria with the specifications of commercially available pickups.
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