著者

川西 哲夫

出版者

一般社団法人日本音響学会

雑誌

日本音響学会誌 (ISSN:03694232)

巻号頁・発行日

vol.34, no.6, pp.342-347, 1978-06-01

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A frequency characteristic of traditional pickups of moving magnet type (MM type) is generally flattened by suppressing a large mechanical resonance peak of the vibrating system with a low Q pickup body. Therefore, the upper limit of frequency range is restricted by the mechanical resonance frequency, and also the transient response characteristic is not always excellent. In this paper, a new pickup of electro-magnetic type having double resonances of electrical and mechanical systems is presented. The electrical resonance frequency of the pickup body is chosen about two times as high as the mechanical resonance frequency of the vibrating system, and a so it is required that the mechanical resonance peak is small and the electrical resonance peak is large. Thereby it is expected that the frequency range of the pickup is widened up to about two times as high as the mechanical resonance frequency. The vibrating system is approximated by a mechanical model shown in Fig. 1, and an electrically equivalent circuit of the pickup body is shown in Fig. 2. Three constants of the circuit are determined from Fig. 3 and Fig. 4. The validity of the equivalent circuit is comfirmed from Fig. 5. Using Fig. 1 and Fig. 2, analysis of the pickup is carried out by means of Laplace transform. The frequency characteristic and the step response can be respectively calculated from Eq. (12) and Eq. (17). Fig. 6 which is obtained from Eq. (12) shows the typical frequency and phase characteristics of several designed pickups. Fig. 7 shows with circle marks some conditions that the frequency deviation of the pickup is within 3dB up to 1. 8 times as high as the mechanical resonance frequency, and it is known that Q of the pickup body above 3 and the damping factor of the vibrating system above 0. 5 are the necessary conditions for the new pickup. Fig. 9 which is obtained from Eq. (17) shows three typical step responses of pickups of different type having same mechanical resonance frequency (see Fig. 8), and it is known that the transient response characteristic of the new type pickup is better than that of traditional pickups of MM type. To obtain a high Q pickup body, ferrite magnetic poles are used for the body. The frequency characteristics of the body are measured and shown in Fig. 10. When the load impedance of the body is a general value, that is, 100kΩ//100pF, the Q comes to 3. 3. Fig. 11 shows a frequency characteristic of the vibrating system which is largely controlled so that the damping factor becomes 0. 5. An experimental pickup (MM type) was composed with the ferrite body and the largely controlled vibrating system, and the frequency characteristic was measured with a test record and shown in Fig. 12. Although the frequency characteristic above 50 kHz was not measured, it is manifest that a wide frequency range pickup can be produced, which possesses the response far beyond the mechanical resonance frequency.

著者

川西 哲夫

出版者

一般社団法人日本音響学会

雑誌

日本音響学会誌 (ISSN:03694232)

巻号頁・発行日

vol.30, no.11, pp.601-607, 1974-11-01

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In the theoretical analysis of phonograph pickup, there are some researches based on the conception of one degree of freedom system, but it seems that any studies, in which the oscillating system is considered as of infinite degree of freedom have not yet been performed. In this paper the osciiating system of pickup is regarded as a simplified system shown in Fig. 1, and a theoretical analysis of an actual pickup (moving magnet type) as shown in Fig. 2, is carried out using eq. (1), thst is, the equation of motion in a bar with infinite degree of freedom. Under boundary conditions given by eq. (2) and eq (3), eq(5) is a solution of displacement for the pickup cantilever, which is needed to obtain some results, about frequency responses for example. The frequency response and the mechanical impedance characteristic are respectively calculated from eq. (8) and eq. (11), and several constants needed in the calculation are determined as shown in Table 1. Fig. 4 shows calculated freqency responses of an actual pickup, and the dotted line in the figure represents a response in case of very amall viscous damping. Fig. 5 and Fig. 6 show respectively phase angles and mechanical impedance characteristics. From these calculated results it is known that a pickup has generally three resonant points in the normal frequency range. Fig. 7 shows a measured frequency response of the actual pickup, and a true response of the vibrating system which is compensated by an electromagnetic frequency response of the pick up body (Fig. 10) is compared with the response previously calculated shown in Fig. 11. Fig. 8 shows a comparison between measured and calculated mechanical impedance characteristics. From these investigations it is made clear that the theoretical results approximately coincide with the experimental results, but it is known that this analysis does not always give satisfactory results in spite of considering the system as of infinite degree of freedom. Finaly a few shapes of vibration of the pickup cantilever are calculated, and some aspects of the bending cantilever are manifested to a certain extent. In future a study on the difference between theory and experiment will have to be accomplished.