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250
H. J. HASBROUCK
[J. S. M. P. E.
Some reduction in transfer loss can be effected by departing from standard groove dimensions, for example, by reducing the radius from 0.0023 inch to 0.001 inch. A further reduction could be made by increasing average record speed. In view of the extensive duplication of equipment neither solution appears to be economical.
The MI-4856 reproducer is intended primarily for use on nonabrasive high-fidelity records but may be used on all lateral records having standard groove dimensions, including composition coated disks used for immediate playback. It is equipped with a permanent diamond point, the radius of which corresponds to the 0.0023-inch standard. This radius is held to limits not exceeding plus or minus 0.0001 inch to insure an even distribution of pressure over the bottom of a standard groove.
The frequency response characteristic of the reproducer from a standard test pressing twelve inches in diameter and running at 33.3
-15
50 IOO 500 1000 5000 10,000
FREQUENCY CYCLES PER SECOND FIG. 7. Recording and reproducing amplifier charcteristics.
rpm. is shown in Fig. 6. The internal construction of the pick-up is shown schematically in Fig. 8. The armature is of the clamped reed type. The two upper air-gaps are inactive, being filled by nonmagnetic spacers. Since the armature impedance is too high to be directly coupled to the record a linkage having a 6 to 1 leverage ratio or a 36 to 1 impedance ratio, is employed. The diamond point is secured in the lower end of an extremely light pivot arm spring supported vertically but rigid laterally. Thus, the pivot arm is permitted to rise, as during "pinching," without lifting the entire pickup. In the direction of useful motion being transmitted to the armature, the linkage has a minimum of compliance and the upper cut-off is high or about 9000 cps. This peak is reduced by means of a block of loaded rubber arranged as a selective damper tuned approximately to the peak frequency.
The response of the pick-up working into a resistance load would droop at high frequencies because of the inductance of its winding