US2482467A - Phonograph pickup - Google Patents

Description

Sept. 20, 949, R CORBETT 2,482,467

PHONOGRAPH PICKUP Filed May 10, 1946 g ES 1 INVENTOR Pa A Crfie/f 72W; 4. 40m B? ATTORN Y Patented Sept. 20, 1949 PHONOGRAPH PICKUP Ray F. Corbett, Lewisburg, Pa., assignor to Westinghouse Electric Corporation, East Pittsburgh, Pa., a corporation of Pennsylvania Application May 10, 1946, Serial No. 668,659

' My invention relates to sound reproducing devices and, in particular, relates to arrangements for producing an output current varying in amplitude and frequency in correspondence with a groove cut on a phonographic record. One of the principal features of my invention is the em-' ployment of the variations in resistance of metallic wires which are impressed with longitudinal strains by a needle following the groove in the phonograph record.

' One object of my invention is to provide a novel type of sound reproducing head for phonographs.

Another object of my invention is to provide a phonograph sound reproducing head in which the mechanical movements of the needle or stylus following the groove in the record are caused to vary the resistance of a metallic conductor in such a way as to produce an output current in an electric circuit which corresponds in frequency and amplitude with the periodic fluctuations of the sound groove. i

Still another object of my invention is to provide a phonograph sound head in which movements of theneedle or stylus in one direction cause variations in the output current of an electrical network embodying resistors strained by said movements, but in which movements of the stylus or needle in directions at right angles to the first-mentioned direction produce substantially no fluctuations in the output current.

Still another object of my invention is to produce a phonograph pick-up in which the output current is unaffected by movements of the stylus except in the direction of one of three rectangular coordinated axes in space.

Other objects of my invention will become apparent upon reading the following description, taken in connection with the drawing in which:

Figure 1 is a schematic view, partly in perspective, illustrating a preferred embodiment of my invention; and

Fig. 2 is an end view of the arrangement shown in Fig. 1.

Referring in detail to the drawing, my phonograph pick-up comprises a horizontal support tone arm I adapted to be supported on the frame of a phonograph having a turntable rotatin about a vertical axis on which disk-type sound records may be carried in accordance with a practice now conventional in the phonograph art. The arm I is adapted to be swung on the means supporting it adjacent to the phonograph frame so that the stylus or needle 2 rests in the sound groove of a rotating phonograph disk. As the disk rotates, the needle 2 is moved by the curva- 3 Claims. (Gl.179100.41)

ture of grooves in the disk in the direction indicated by the arrows 3 in Fig. 2.

The needle 2 is held in a chuck 4 which is mounted on a small plate 5 of fiber or other suitable material. The plate 5 is supported from the arm I by a pad 6 of rubber or other suitable elastic material which is sufficiently flexible so that the needle 2 can move in the directions of the arrows 3 far enough to follow the fluctuations of the groove in the phonograph disk. The pad 6 is seated between the arms I and 8 of the support member I and is held in position thereon by a top plate 9 of fiber or other suitable insulating material. Supported on the upper surface of the topplate 9 is a wire II of some suitable material and having the configuration shown in heavy outline in Fig; 1. The wire II may conveniently be made up in the form shown from two subdivisions I2 and I3, each in the form of a hair pin. Tw-o arms of the hair pins are connected to each other and to a center lead I4 consisting of any suitable conducting wire, which may be cemented or otherwise fastened for a portion of its length to the support arm I. The hair pins I2 and I3 then have their closed ends bent around the support arms I and 8, as indicated in Fig. 1, and are so proportioned that their closed ends project into contact with the lower surface of the plate 5. These ends are then attached, for example, by a suitable adhesive to the plate 5. The free ends of the hair pins I2 and I3 are connected to suitable lead wires I4 and I5, 2. portion of which may be cemented to the arm I, and which leads are connected to the outer terminals of a transformer winding I 6, preferably through a flexible cord. The lead I4 is connected to one terminal of a battery or other source of voltage H, the other terminal thereof bein connected "to the mid point of the winding IS. A secondary winding I 8 cooperates with the winding I6 to produce output currents which may be fed through a suitable amplifier to a loud speaker or other sound reproducing device.

' When the stylus 2 follows the groove in the rotating phonograph disk, it is alternately moved from side to side in the direction of the arrows 3,

-i. e. along a radius of the record, with a frequency corresponding to the sound pressure modulation of the record groove of the phonograph disk. The hair pins I2'and I3 are mounted on the plates 8 and 9 under tension so that they respectively exert a force tending to move the plate 5 to the left and to the right in Fig. 2. The restposition of the needle 2 is naturally that in which these two forces are equal and opposite to each other. When new the stylus 2 is moved either to the left or to the right in the direction of the arrows in Fig. 3, it will increase the longitudinal stress in one of the hair pins I2, I3 and decrease such stress in the other hair pin. The resistance of the hair pins I2 and I3 and the number of turns in the respective halves of the winding I5 are so proportioned initially that in the undisplaced position of the stylus 2, the currents sent by the voltage source I! through the winding I6 are equal and opposite to each other in magnetic effect on the core associated with the winding I5. As a result, the flux set up in said core is substantially zero.

When now the stylus 2 increases the stress in, let us say the hair pin l2, and decreases the stress in the hair pin I3, the resistance of the hair pin I2 increases and the resistance of the hair pin I3 decreases and, consequently, the current flowing through one-half of the windin I6 increases while that flowing through the other half decreases, with the result that a magnetic field is set up in the core associated with the winding 16.

At a time corresponding to one-half period of the sound pressure wave recorded by the groove on the phonograph record, the stylus 2 will be moved back through its undisplaced position and receive a displacement in the opposite direction, thereby increase the longitudinal stress in the hair pin I3 and decreasing such stress in the hair pin !2. As a result, the current through the respective halves of the winding 16 will have been altered so that these windings produce a magnetic field in the core associated with the winding IE which is in the opposite direction to that described in the presaid paragraph. The dimensions and elastic characteristics of the pad 6 are so proportioned that, for the maximum lateral excursions of the stylus 2 under the impress of the phonograph record groove, the Variations resistance of the hair pins I2 and I3 due to changes in the mechanical stress imposed thereon by plate 5 are substantially proportional from instant to instant to the displacement of the stylus 2 from its zero or undisplaced position. The resistance of the hair pins I2 and I3 is likewise made large enough so that it controls the reactance variation of the winding It at all the frequencies of sound which the phonograph record carries. As a result of this arrangement, the strength of the magnetic field in the core associated with Winding IE will, at any instant, be proportional to the displacement of the stylus 2 from its zero position.

In short, a magnetic flux is produced in the core associated with winding I6 which varies in amplitude and frequency in conformance with the fluctuations of the groove in the phonograph disk. This magnetic flux will set up in the winding I 8 a voltage which likewise varies at the frequency of the lateral displacements of the stylus 2 by the phonograph groove. Where the lateral fluctuations of the phonograph groove are perfectly proportional at all sound frequencies to the instantaneous amplitude of the sound, the voltage produced by the winding I1 will be proportional both to the product of the amplitude of the sound record by the frequency of its pitch; and in such instances it may be desirable to provide a suitable compensating network of a type well known to those skilled in the art in the output circuit of the Winding I8 to render the cur rent therein proportional strictly to the instantaneous amplitude of the fluctuation of the 4 groove on the phonograph record. However, in actual practice, the amplitude of fluctuation of the groove on the phonograph record is not strictly proportional to the loudness of the sound being recorded, but is less at higher sound frequencies than at lower sound frequencies. This fact produces what may be called an automatic compensation for the increase in the output voltage of the winding I8 at high pitches so that for many purposes the above-mentioned compensating network may be greatly simplified or even eliminated.

Fluctuations in the resistances of the hair pins I2 and I3 which result in production of a magnetic flux in the core associated with winding I6 are those which represent differences in the resistances of hair pins I2 and I3. In other words, anything which produces a simultaneous increase in resistance of the hair pins I2 and I3 decreases the current through the two halves of the winding l6 equally and leaves its magnetic effect on the core associated with unaltered. Thus it is that while lateral movements in the direction of thearrow 3 of the stylus 2 increase the resistance of hair pin I2 while they decrease the resistance of hair pin I3 and thereby result in the production of magnetic fields in the core associated with winding I6, movements of the stylus 2 in any direction normal to the arrows 3 produce equal increases or decreases in the resistance of the hair pins I2 and I3 and hence produce nochange in the magnetic field in the core associated with winding I6. In other words, vertical movements normal to the plane of the sound record produce no output currents in the winding I'I and, similarly, any fluctuating drag of the stylus 2 along the phonograph disk produces only equal changes in the resistance of the hair pins I2 and I3 and hence produces no output current from the winding I'I. Thus it is that with my arrangement it is only movements of the stylus 2 along one of three rectangular axes in space that produce output currents in the winding I1; and movements along the other two of the three coordinated axes produce ,no output currents from the winding I1.

While I have shown the resistors I2 and I3 as made up in the form of hair pins, itwill be evident to those skilled in the art that these wires may be given any other configuration so long as lateral movements of the stylus 2 act only to increase the longitudinal stress in one wire and decrease the longitudinal stress in the other wire. Many other configurations meeting these conditions will be apparent to those skilled in the art.

I claim as my invention:

1. ,A sound pick-up device for phonographs comprising a tone arm, a stylus elastically sup,- ported relative to said tone arm, two hair-pin shaped resistorshaving their closed ends mechanically connected to said stylus and having their open ends mechanically connected to said tone arm, means for causing electric currents to flow in multiple through said conductors, 'mea-ns for causing the electrical effect of said currents to neutralize each other in an output circut when said stylus is in a predetermined position relative to said tone arm, and to produce a net effect :in an electrical output circuit in other-positions of said stylus relative to :said tone arm, and means for causing said conductors to exert :oppositelydirected mechanical forces on said stylus.

'2. In a phonograph pick-up a tone arm having a longitudinal axis, a stylus supported substantially perpendicular to said axis and provided with means for permitting its movement ina direction perpendicular to said axis and to the first-mentioned direction, a pair of hair-pin shaped conductors having their closed ends supported on said stylus and having their open ends supported on said tone arm, said hair-pin shaped conductors being positioned so that longitudinal stresses therein exert opposing forces on said stylus in directions parallel to said movement, means for producing current flow through said conductors in multiple with each other, and means for opposing the electrical effects of said currents on an output circuit.

3. In a phonograph pick-up a tone arm having a longitudinal axis, a stylus supported substantially perpendicular to said axis and provided with means for permitting its movment in a direction perpendicular to said axis and to the first-mentioned direction, a pair of hair-pin shaped conductors having their closed ends supported on said stylus and having their open ends supported on said tone arm, said hair-pin shaped conductors being positioned so that longitudinal stresses therein exert opposing forces onsaid stylus in directions parallel to said movement, means for producing current flow through said conductors in multiple with each other, means for opposing the electrical effects of said currents on an output circuit, and means for causing said hair-pin shaped conductors to exert substantial stresses in opposite directions on said stylus in the undisplaced position thereof.

RAY F. CORBETT.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS OTHER REFERENCES Electronic Industries, vol. IV, No. 8, Aug. 1945, page 89, Strain-Gage Phono Pickup, by W. S. Bachman.

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