US1906615A - Electric pick-up - Google Patents

Description

May 2, 1933. J. M. KENDALL 1,906,615

ELECTRIC?- IPICK-UP' Filed April 21', 1928.

Inventor- I Janwes M. Kc-snclaH. Joy

Hi8 Attorney.

Patented May 2, 1933 UNITED *sr TEs.

PArE-Nr oFtFIcEfl JAMES M. KENDALL, or scnENEq'rnnY, NEW YORK, ASSIGNOR .'].0 GENERAL ELECTRIC COMPANY, 'A 'COBPORATIONOF. NEW vonx ELECTRIC PIcK UP Application filed April 21,

My invention relates to apparatus for producing sound, and has for-its principal object the provision of an improved sound producing apparatus which possesses low impedance, has a comparatively wide range of frequency response, and operates with greater flexibility than the sound producing apparatus heretofore provided. 1

Various types of apparatus for the interconversion of sound and electrical vimpulses have been provided in the past. Many of these apparatus :comprise a mag-, netic armature which ismoved either in accordance with electrical impulses dependent on sound or in accordance with the contour of a sound record. In the'operationofsuch apparatus difliculty is frequently encountered because of the: sound distortion which results from the tendency of the armature to vibrate at a frequency dependent on its free vibration. period. Other difiiculties encountered are undue wear of" the sound record and failure of the apparatus to respond f' properly to lowfrequency impulses. In accordance with my invention, these difficulties are minimized or altogether. avoided by the provision of a sound producing apparatus which includes an armature arranged. to-move with a high degree offlexibility and an operating coil of much lower impedance thanthe operating. coils heretofore utilized in such devices.

My invention will be better understood from the following description when. considered in connection with the. accompanying drawing andits scope will be pointed out in the appended claims.

Referring to the drawing, 'Figs. 1 and 2 illustrate a phonographicreproducer or electrical pick-up devicewherein my invention has been embodied; and Fig. 3 illustrates a system wherein this reproducer may be utilized. I

The sound producing appartus or phonographic reproducer illustrated by the drawing includes a magnet 10 provided with bipartite pole piecesll and 12which are arranged to support and partlyenclose a low impedance coil 13' interposed between the upper extensions Hand 15 and the lower 1928. Serial No. 271,813.

extensions 16 and 17 of the pole piecesg A movable armature I8flattened. at its upper 'end and provided with shaft 19 and with and. 15 of the pole pieces 11 and 12 either in response to movementof a stylus attached to the lower end of the armature-by means of a thumb screw 21 or in response to the electrical impulses supplied to the coil13 through theleads 22 and 23. I

In order to center the upper end of the armature 18' between the upper pole piece extensions 14,.a11d15 and to control the movement of the armature, a resilientblock I T 2 1 of rubber or the like and an adjusting plate 25 are provided. The block 245 may be grooved at its lo'wersurface to receive the upper end of the armature, preferably rests on small ledges 40 at the top oftheupper pole piece extensionsll and l5so asto oppose little frictional force to the movement of the armature, and is clamped to these ex tensions by means of screws 26 and the adjusting plate 25 which may be moved slightly in one directiouor the other for the purpose of centering the armature between the pole pieces. It will be apparent that'the resilient member 24 is subject to substantially no compressive and frictional forces due to thefact that it is held by its edges which are clamped between the ledges 40 and the adjusting plate 25. With this construction, the armature moves through a distance which is small as compared to the dimensions of the rubber block along the direction of travel ofthe armature and moves freely. .As clearly indicated by Fig. 1, the casing ofthe apparatusincludes a member 27 to which theam agnet 10-is attached by means of a screw 28fland a 'clampingplate '29 .and towhich the pole pieces 11' and 12 are attached by means of bolts 30 and 31. This oasingalso includes a cover member 32 which 'isattached to the clamping-plate 29 by means of screws 33. It will be seen that pole pieces 11, 12 areprovided with upper surfaces which are in engagement with the surfaces of the bottom ends of magnet 10. This construction avoids the necessity of bolts between the pole pieces and magnet and permits lateral adjustment of the pole pieces. Assembly of the unit is simplified since means entirely independent of the contacting surfaces of the pole pieces and magnet maintain the magnet and pole pieces in proper relation. The magnetic attraction holds the magnet to the pole pieces, this being supplemented by holding means associated with the casing, such as clamping plate 29.

As will be apparent from Fig. 3, the coil 13 is connected to current supply terminals 34 through a volume control potentiometer 35 and a transformer 36 which may possess a high transformation ratio.

It will be readily understood that, with the exception of the low impedance coil 13, the electrical features of the improved ap paratus are similar in some respects to some types of sound reproducing apparatus heretofore provided. The utilization of this low impedance coil is advantageous in that it makes possible the use of a small coil and a short armature which possesses a comparatively low moment of inertia and has a better response, especially at low frequencies, than the armature heretofore utilized.

Other practical advantages resulting from the use of a low impedance coil will be apparent from the following considerations. With the high impedance sound producing apparatus heretofore utilized, it has been necessary to use a volume control potentiometer which had a resistance of approximately 20,000 ohms and to ground the metal part of the apparatus. In the case of a low impedance coil having about 300 turns and an impedance of approximately 26 ohms at 1000 cycles, a volume control potentiometer of only 60 ohms is satisfactory and. grounding of the metal parts is rendered unnecessary. The comparative cost of these two features for the old and improved apparatus is in the ratio of 12 to 1. Further advantages of the low impedance coil are a reduction of alternating current hum and the simplification of the problem of transmitting energy from the reproducer to the loud speaker.

The flexible armature mounting made possible by the member 24 has the advantages of ease in the adjustment of the apparatus, greater amplitude of the armature motion in the air gap, higher voltage per turn, greater reliability in operation due to the fact that the centering and damping member, because of the extremely small amount of compression, does not take a permanent set and operates with substantially no friction and uniformity in production due to the fact that the comparatively large damping member may be cut with greater accuracy than the comparatively small resilient stops heretofore used, and less wear on the phonograph record both at high and low frequencies.

It has been found as the result of extensive tests that the above advantages are realized if the coil 13 has from 50 to 1200 turns and has an impedance which may vary from 9 to 4640 ohms at 10000 cycles depending on the number of turns. The armature 18 may vary somewhat in length but preferably has a moment of inertia which is less than .15 gm cms. and may readily be made as low as .08 gm cmsF. For convenience of eXpl'ession the terms low impedance and short are used herein to designate a coil and an armature having these characteristics.

The embodiment of the invention illustrated and described herein has been selected for the purpose of clearly setting forth the principles involved. It will be apparent however that the invention is susceptible of being modified to meet the different conditions encountered in its use and I therefore aim to cover by the appended claims all modifications within the true spirit and scope of the invention.

WVhat I claim as new and desire to secure by Letters Patent of the United States is:

1. In a sound producer, the combination of a pair of magnetic pole pieces, a relatively broad resilient member clamped to said pole pieces along limited portions of said member adjacent opposing edges thereof, said member having a portion of sub stantial extent intermediate said clamped portions, said intermediate portion being unconfined over both upper and lower surfaces thereof, whereby said intermediate portion of said member is subjected to substantially no compressive and frictional force, and an armature pivoted between said pole pieces near one of its ends and mechanically coupled to said intermediate portion of said resilient member at the other of its ends.

2. In a sound producer, the combination of a pair of magnetic pole pieces, a resiiient member, an armature pivoted between said pole pieces near one of its ends and mechanically coupled to said resilient member at the other of its ends, and a low impedance coil interposed between said pole pieces and arranged to surround said armature.

8. The combination of a pair of magnetic pole pieces, a resilient member, an armature pivoted between said pole pieces near one of its ends and mechanically coupled to said resilient member at the other of its ends, a

coil surrounding said armature, and a volume control. potentiometer having a resistance of less than one hundred ohms connected to the terminals of said coil.

4. In a sound producer, the combination.

of a pair of magnetic pole pieces, a resilient member, and an armature pivoted between said pole pieces at one of its ends and mechanically coupled to said resilient member at the other of its ends, said armature having a moment of inertia which is less than .15 gm. cms

5. In a sound producer, the combination of a pair of magnetic pole pieces,-a resilient member, an armature pivoted between said pole pieces at one of its ends and mechanically coupled to said resilient member at the other of its ends, and acoil surrounding said armature, said coil having an impedance between 9 and 4640 ohms at 10000 cycles. 1

6. In a sound producer, the combination of a pair of magnetic polepieces, an armature pivoted between said pole pieces near one of its ends, and a relatively wide resilient block clamped to said pole pieces along relatively narrow portions of said member adjacent opposing edges thereof and bridging said pole pieces, said member having a central portion unconfined over both upper and lower surfaces thereof, said central portion of said resilient block engaging the other end of said armature.

7. In a sound producer, the combination of a pair of magnetic pole pieces, a single resilient block clamped along relatively narrow portions thereof adjacent two opposite edges thereof on the tops of said pole pieces, said block having a central portion unconfined over both upper and lower surfaces thereof, and an armature pivoted between said pole pieces near one of its ends and arranged to engage said resilient block at the other of its ends at a point of said central portion of said block intermediate said edges.

8. In a sound producer, the combination of a pair of magnetic pole pieces, an armature pivoted between said pole pieces at one of its ends, a resilient block, and means to clamp said resilient block along relatively narrow portions thereof adjacent two opposite edges thereof, said block having a central portion unconfined over both upper and lower surfaces thereof, the other end of said armature being in contact with said block at a point of said central portion intermediate said edges, the dimension of said central portion of said block between said edges along the direction of travel of said armature being large as compared to the length of said travel.

9. In a sound producer, the combination of a pair of magnetic pole pieces each having aledge formed thereon, a resilient member mounted on said ledges of said pole pieces, and an armature pivoted between said pole pieces near one of its ends and arranged to engage said member at the other of its ends.

10. In a sound producer, the combination of a pair of magnetic pole pieces having extensions forming an air gap, an armaturev adapted to vibrate in said air gap, a ledge formed on each of said extensions and spaced from the end of said extension adj acent said armature, a resilient member mounted on said ledges, said armature being pivoted between said pole pieces near one of its ends and arranged to engage said member at the other of its ends.

11. Ina sound producer, the combination of a pair ofmagnetic pole pieces each having a ledge formed thereon, a resilient member mounted along two opposite edgesthere of on said ledges, means including-an adjusting plate connected to said pole pieces to clamp said resilient member to said ledges, and an armature pivoted between said pole pieces near one of its ends and arranged to engage said resilient member at the other of its ends.

12. In a sound producer, the combination of a casing member, a magnet having a pair of pole pieces forming an air gap therebetween, each of said pole pieces having an upper plane surface adapted to engage with.

said pole pieces being laterally adjustable through a small distance relatively to each other across said plane ends of said magnet to vary the length of said air gap, and means to fasten said pole pieces to said casing member. a V

13. In a sound producer, the combination of a magnet having two plane end surfaces, a pair of pole pieces each having a plane contact surface, each of said contact surfaces being laterally slidable across a different one of said end surfaces to adjust the distance apart of said pole pieces, and means independent of said magnet surface and said pole piece surfaces to maintain each of said a lower plane end surface of said magnet,

pole piece surfaces in adjusted position in contact with a diiferentone of said magnet surfaces.

14. In a sound producer, the combination of easing means, a magnet having a. pair of pole pieces, each of said pole pieces havingan upper plane surface engaging a plane end surface of said magnet, each of said said casing means to assist the magnetic attraction between said magnet and said pole pieces in maintaining said pole piece surfaces in adjusted position in contact withsaid magnet surfaces.

In witness whereof, I have hereunto set my hand this 20th day of April, 1928.

JAMES KENDALL.

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