電気共振峰を利用したレコードピックアップ A Phonograph Pichup Utilizing Electrical Resonance Peak
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.
日本音響学会誌 34(6), 342-347, 1978