Magnetic pickup



Dec. 30, 1930. o. M. DUNNING 1,786,935 MAGNETIC PICK-UP Filed July 14, 1923 I/III/I/IAIIl luv/11111111111 1 1 W z 6 i ll ii 2 12 J 3 l, v I I 1N2? OR I W VATI'ORNEYS Patented Dec. 30, 1930 UNITED STATES PATENT OFFICE ORVILLE M. DUNNIN G, 01? STAMFORD, CONNECTICUT, ASSIGNOR T0 ACOUSTIC PRODUCTS 7 COMPANY, A CORPORATION OF DELAWARE i mnenn'rrc MPIGKUP Application filed July 14, The invention relates to electric reproduction from phonograph records. One object of the invention is to provide an improved phonographic pickup of the electromagnetic type. In electromagnetic pickup structures, wave motion becomes inevitable. It has been found that the armature and needle have a natural resonance and that at the frequency where this resonance occurs it will be seen that the reproduction will suffer due to the resonance and vibration of the armature and stylus adding to the vibration in response to the sound groove. It has also beenfound that at all frequencies above the resonant vibration frequency of the assembly that the stylus tracks poorly, resulting in lowered output voltage. It has been one object of the present invention to so design the various parts that the resonant characteristic of the stylus and armature occurs at such a high frequency as to not effect the voltage wave generation in response to sound grooves of normal musical frequencies. In apparatus of this type, it has been previously possible to design a pickup in which the armature and stylus resonant frequency peak occurs at approximately 2500 cycles. With my improved structure however, it is possible to provide a device in which the armature stylus resonance occurs at a considerably increased fre quency, thereby increasing the range of dis tortionless voltage wave generation in response to the sound groove of a record. Another object of my invention is to provide a device in which the power generated in response to a given movement of a stylus driven armature is considerably increased, and the amplitude distortion of the voltage wave generated materially decreased. This is accomplished by the provision of an armature moving between four poles constituting a magnetic field. In the present device, it has been found possible to pivot'the armature midway between two pairs of the poles of the field, whereas it has been previously found necessary to pivot the armature between the two lower poles of the field. In the design of devices of this type in the 1928. Serial No. 292,766. past it has been found impossible to accompllsh this for several reasons. In the first place it is very desirable to design the armature in such a way as to use the standard length needles now in use. Since these are usually about five-eighths of an inch in length, this requirement'means that the needle holding screw must be within one-half mch of the surface of the record. Now, in order that the large mass of the needle holdmg screw may have no effect on the resonance characteristics of the armature system, it is absolutely necessary that the screw be located in the center of rotation of the armature and as close to its center of gravity as posslble. In addition to this requirement it is also necessary that the stylus extend at least one-quarter inch below the bottom surface of the, pickup in order that the pole pieces and their support maynot strike the surface of the record. It is further necessary, if the resonance of the armature system is to occur at a high frequency, to make the armature one-h'alf inch or less in length. These requirements make it necessary that if the armature is to be pivoted midway between the two pairs of poles, then the pole pieces, one-half the coil, and one-half the armature and needle holding screw bearings must be included within a space of one-quarter inch or less, which has heretofore been impossible. In my design these requirements are fulfilled in the following manner: The thickness of the pole piece and its support are reduced to a minimum possible to carry the necessary flux; the coil used consists ofavery smallnumber of turns instead of the usual several thousand turns, and no coil form is used,' the coil being wound directly on the steel of the armature without insulation further than that on the wire. In the present invention, there is provided a moving armature, which is centrally pivoted between two pairs of magnet poles and about which there is directly wound a coil of few turns. The mechanical structure of the armature and its mounting and stylus retaining screw, as previously referred to and as will hereinafter be described in detail, is such that resonance of the armature with the needle Occurs at a very high frequency. It will be seen that a few turns in the armature coil results in low impedance. It is common knowledge that if the inductance is decreased and the resistance also decreased in such asystem, the power available remains constant, while the voltage is decreased in the ratio of turns. However, since the impedance of-thecircuit is lowered,-it is possible to use a higher step up input transformer ratio so that the resulting voltage at the .secondary of the input transformer is independent of the turns or voltage ofthe generator. lVith the low impedanceof the present 'windings the generated voltage decreases. This, however, coupled with the low impedance gives a resultant of power available equal to the high impedance coils of many turns as reviousl used. The resent low im edance P circuit may, therefore, be coupled with a transformer system of high ratio whereby the resultant voltage of the pickup and trans former system remains high. With these and other objects in view, to be more fully set forth hereinafter, the invention consists in the novel construction, combination of elements and arrangements of parts which will be exemplified in the congstruction herein described in the following specification and illustrated in the accompanying drawings. The disclosure made the basis of exemplifying the present inventive concept suggests :a practical embodiment thereof, but the in- 7 present invention. vention is not to be restricted to the exact detalls of this disclosure, and the latter, therefore, is to be understood from an illustrative, rather than a restrictive standpoint. The invention will first be described in connection with the accompanying drawings, wherein similar reference characters are used to designate corresponding parts throughout the several views, and more specifically defined and indicated in the appended claims. Attention is directed to the drawings in which Fig. 1 is a front plan view with the cover removed of one form of my present invention; Fig. 2' is an enlarged sectional view taken on lines 2-2 of Fig. 1; Fig. 3 is an enlarged sectional view taken on lines 3-3 of Fig. 1; Fig. 1 is an enlarged detail view of an armature and windings utilized in my invention. The drawings, forming part of the present application, embody only one form of the It is to be understood that the novel features of the present device a may be readily incorporated in various strucup including a case 1, to which may be secured any suitable type of cover. It is not For providing a pemanently energized mag- 'netlc circuit between the pole p1eces,-there is provided a permanent horseshoe magnet 6 positioned on the upper surface and at the extremities of the pole pieces 2 and 3. The pole. pieces 2 and 3 are permanent y secured to the base 1 by non-magnetic screws 7, together with'a non-magnetic member 8 secured to the back 1 provides a magnetically insulated casing for the horseshoe magnet and its pole pieces. 7 Between the pole pieces 2 and 3 an armature .9 is provided, which is formed as illustrated in Fig. 4, including an upper armature flux conveying surface 10 and a lower flux conveying surface 11. The armature between these flux conveying surfaces 10 and 11 is tubular. The internal cross-section of the tubular portion is preferably provided with straight sides, as illustrated, and is thus adapted to receive and securely hold a con-, in a brass bearing member 15 secured to the pole pieces by the screws 6. The forwardly extending bearing pinion 12 is tubular and internally threaded to provide for the reception of a stylus engaging knurled screw 16. Thus, it maybe seen that the weight and natural resonance of the stylus securing screw by virtue of its mounting at the pivotal point .of the armature will not affect the movement of the armature or the armature and stylus resonant frequency. Between the flux conveying surfaces and thepivotal points of the armature, there is provided a small number of turns of fine wire. By means of the present construction, the turns of wire may be directly wound upon the armature itself, thus avoiding the necessity of heavy coil forms, which would materially alter and lower the resonance frequency of the armature and stylus assembly. The present construction allows for the coils to be closely wound directly upon the armature, thus avoiding flux leakage and resulting in'a high voltage output. It willalso be seen that the provision of only a few turns of the armature coil results in low imlot pedance coils. In the form of the invention illustrated, there is provided approximately ninety turns of wire directly upon the armature resulting in impedance of the order of a few ohms at 1,000 cycles. This may be favorably compared with the extremely high impedance ranging in order of several thou sand ohms found in a single winding type of electromagnetic pickup. It may be readily seen that the low impedance in the present structure makes possible the use of a several hundred to 1 step up transformer ratio as compared with the 3 to 1 transformer ratio required in a single winding type instrument. In devices of the present character, the flexibility of the needle and armature assembly materially decreases the resonant frequency of the assembly, causing a peak to occur in the frequency characteristic of the pickup. By the present structure, the armature is maintained extremely short and the direct magnetic flux is not carried by the armature as a result of the position of opposite poles on the two sides as indicated in Fig. 1, whereby the armature carries only the residual or alternating flux, hence the armature may be extremely light. This saving of weight in the armature does not, however, result in a decrease of magnetic eflect due to the motion of the armature. On the contrary, the present structure increases the effect of movement of the fiux conveying sur faces of the armature in response to the sound groove by providing two variable reluctance circuits for the magnetic flux. The small size and weight of the armature, together with its reenforcing windings and the centrally positioned receiver for the stylus securing screw, results in a reasonably flat voltage output-frequency characteristic up to a high frequency. The flexibility and mass of this assembly controls the frequency of the resonant peak of the assembly. This may be seen when it is remembered that by provision of a perfectly rigid armature and needle assembly that the resonance period would occur at infinitely high frequency and that the resonance period becomes of lower frequency as the flexibility and mass of the assembly increase. Thus, the invention provides for a high resonance frequency armature and needle assembly, and, at the same time, provides decreased amplitude distortion and increased efficiency by increasing the variable reluctance of a plurality of magnetic circuits. By the opposite relation of the poles, as indicated in the drawings, it will be seen that the voltage waves generated are in phase and thus aid to produce an extremely efiicient electric magnetic pickup device. In accordance with the provisions of the patent statute, I have described my invention, but I desire it understood that it is not confined to the particular form shown and described, the same being merely illustrated, and that the invention can be carried out in other ways without departing from the spirit of my invention, and therefore, I claim broadly the right to employ all equivalent instrumentalities coming within the scope of the appended claims, and by means of which objects of my invention are attained and new results accomplished as it is obvious that the particular embodiments herein shown and described are only some of many that can be employed to attain these objects and accomplish these results. Having now described my invention, what I claim and desire to secure by Letters Patent, is: 1. An electromagnetic pickup including a four pole magnet, an armature having a pair of reluctance varying members disposed in reluctance varying position between the poles and a low impedance circuit comprising a few turns of wire directly wound upon the armature intermediate the reluctance varying members thereof. 2. An electromagnetic pickup including a four pole magnet, an armature having a pair of reluctance varying members disposed in reluctance varying position between the poles and a low impedance circuit comprising a few turns of wire directly wound upon the armature intermediate the reluctance varying members thereof, and means to pivot the armature intermediate the pole pieces. 3. An electromagnetic pickup including a four pole magnet, an armature having a pair of reluctance varying members disposed in reluctance varying position between the poles and a low impedance circuit comprising a few turns of wire directly wound upon the armature intermediate the reluctance varying members thereof, and means to pivot the armature intermediate the pole pieces, said circuit including turns of wire on either side of the pivoting means. 4. An electromagnetic pickup including a four pole magnet, an armature having a pair of reluctance varying members disposed in reluctance varying position between the poles and a low impedance circuit comprising a few turns of wire directly wound upon the armature intermediate the reluctance varying members thereof, and means to



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