US1986583A - Phonograph reproducer - Google Patents

Description

Jan. 1, 1935. J. H. KING 1,986,583

PHONOGRAPH REPRODUCER Filed Jan. 12, 1933 FIG.

/N VENTOR J. H. KING Br ATTORNEY Patented Jan. I, 1935 UNITED STATES PATENT. OFFICE Telephone Laboratories,

Incorporated, New

7 York, N. Y., a corporation of New York Application January 12, 1933, Serial No. 651,255

2 Claim.

This invention relates to phonograph reproducers and the object of the invention is to reduce the effective mechanical impedance of a reproducer so that it is capable of properly track- 5 ing a record containing a wide range of frequencies and amplitudes with very low stylus pressure.

It is well understood in the art that reproducers of high mechanical impedance at the needle point, that is, those having moving systems with relatively high values of one or more of the mass, stifiness and resistance components of the impedance, require larger vibratory forces for their operation and hence produce greater wear on the I?) record than those of lowimpedance. For certain uses such as playing back from original .soft waxes, a reproducer of very low impedance must be used or the record will be entirely destroyed by a single playing and even this reproduction will be of poor quality due to the inability of the groove to withstand the forces required to displace the vibratable element of the reproducer. Even with reasonably hard records such as those made from cellulose acetate, the pressures involved when using a high impedance reproducer may be great enough to momentarily distort the groove.

In accordance with the general features of this invention the effective mechanical needle-point impedance of known types of reproducers is very materially reduced by driving their moving systems by means of an intermediate lever arm and stylus. In the preferred embodiment of the invention the arm is suitably pivoted and is held in contact with the vibratable element of the reproducer and carries a stylus for tracking the record, the relation of the stylus position and of the point of contact with the vibratable element relative to the pivot being such as to reduce the 40 mechanical impedance as measured at the tracking stylus to the required amount. Thus, with a suitably designed arm the impedance of any reproducer may be reduced sufliciently to permit the satisfactory reproduction from soft waxes and other records. In the drawing:

Fig. 1 shows one type of reproducer equipped with the impedance reducing linkages of this invention; and

Fig. 2 is an enlarged detail view of the moving system.

Referring to Fig. 1 the reproducer is of the type shown in British Patent 363,449 of December 24, 1931 and comprises essentially a magnetic circuit and a dynamic generating element. The

magnetic circuit includes a magnet 11, a central pole piece 12 and a pole piece 13 disposed with respect to the pole piece 12 to form a gap 14 in which is vibrated the coil 15. The coil is mounted on a lightweight cup member 16 which is connected to the stylus 1'? by a light stifl? tube 18. a This moving system is supported by a flat mounting spring 19 and, when used as an ordinary reproducer, by a similar spring 26 (shown dotted in Fig. 2) secured to the bottom of the housing piece 20. A rubber washer 21 is preferably fitted around the tube 18 to exert a slight damping action on the moving system and to exclude dust from the gap 14.

As explained in detail in the British patent referred to above such a reproducer when driven by records of the hill and dale type is capable of responding to a very wide range of frequencies and amplitudesand theoretically, at least, it may be designed in accordance with that invention to have practically any desired needle-point me- 20 chanical impedance. From a manufacturing standpoint, however, when very low impedance is required the moving parts must be so small and light in weight that the reproducer is of fragile construction and very difllcult to build. When built for commercial use on ordinary records the impedance of these reproducers is ordinarily of such value that a needle pressure of the order of 25 grams is required for good tracking.

The reducing linkage shown to illustrate the invention comprises a T-bar 22 of duralumin or other suitable strong, lightweight material mounted in rubber bearings 31 in the block 23. For maximum stiffness and minimum mass the tongue portion of the bar is preferably of I-beam cross-section. The upper surface of the tongue has a small depression 24 into which fits the stylus 17 to form a pivot contact between the reducing linkage and the reproducer. The screws 25 in the block 23 are tightened with the stylus engaging the tongue and this point of contact is maintained during the operation of the device by the same forces that keep the stylus 1'7 on the record groove when the reproducer is used in the ordinary way. Since there is no positive connection between the bar and the moving system of the reproducer the stylus 17 is free to move slightly along the axis of the bar so that the coil. is driven only in a vertical direction and no unnecessary stresses are set up in the moving system.

When attaching this assembly to the reproducer the lower mounting spring 26 is preferably removed as shown in Fig. 1 and the block 23 is secured to the housing 20 by the screws 27 which are ordinarily used for the lower mounting 55 spring. The mechanical impedance of the complete system measured at the point of the stylus 28 will depend not only on the impedance of the standard reproducer used, but also on the location of the depression 24 with respect to the pivot axis of the bar 22.

The effective mass measured at the outer end of a pivoted T-bar of the type shown is approximately one-third of its actual mass and the effective impedance of the moving system of the standard reproducer measured at the end of the T-bar varies inversely as the square of the ratio of where a and b are the lever arms as indicated in Fig. 2. The effective mass of the T-bar and stylus measured at the stylus point may be readily made less than 10 mgms. and the efiective mass of the moving system of the standard reproducer may be of the order of 40 mgms. (referred to its own stylus point), but when this latter mass is referred to the tracking stylus, it becomes The stiffness and resistance components of the impedance are also reduced by the same ratio. Hence, by the addition of the small eifective mass of the T-bar a very great reduction in total effective impedance, with a corresponding reduction in the stylus pressure required, may be obtained. For example, with z"? the stylus 28 will track the record 30 satisfactorily with a pressure of 1.5 to 2 grams, whereas the standard reproducer without the T-bar requires about grams needle pressure. With this very low needle pressure original wax may be reproduced not only without the record groove, but also with greatly improved response at high frequencies since no appreciable temporary deformation takes place.

While the invention has been described for purposes of illustration with reference to its application to a particular reproducer, it should be understood that it is equally applicable to any other mechanically driven vibratory device. The impedance reduction ratio used will, of course, depend on the requirements of theparticular case and may be greater or less than in the structure described. If desired, the bar 22 may be made up in the form of a lever of the first class with the lever arm b extending beyond the pivot axis of the bar in which case the block 23 is preferably attached to the housing piece 29 with the lever arm b extending forwardly so that the stylus 28 is clearly visible from the front of the reproducer as in the construction shown.-

What is claimed is:

1. In combination, a stylus operated phonograph reproducer for use with commercial records and an impedance reducing attachment therefor comprising a lever arm, a stylus mounted on one end of the arm for engaging original recordings, and a pivot bearing member for the arm adapted to be attached to the reproducer and to hold the arm in sliding contact with the reproducer stylus.

2. In combination, a stylus operated phonograph reproducer having a moving system with upper and lower spring supports, a lever and a stylus thereon eng ging the grooves of an original recording, and a pivot bearing member for the lever adapted to be secured to the reproducer in place of the lower spring with the lever engaging the stylus of the reproducer.

JOHN H. ENG.

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