US2469933A

US2469933A - Electromagnetic pickup

Info

Publication number: US2469933A
Application number: US623277A
Authority: US
Inventors: Harvey F Schwarz
Original assignee: Individual
Current assignee: Individual
Priority date: 1945-06-18
Filing date: 1945-10-19
Publication date: 1949-05-10
Anticipated expiration: 1966-05-10
Also published as: GB615421A; FR935455A; NL69948C

Description

May 10, 1949. H. F. SCHWARZ ELECTROMAGNETIC PICKUP Filed'Oct. 19, 1945 milk . INVENTOR. wen/5y Fc/WAEZ 14f or/26y Patented May 10, 1949 UNITED STATE$ OFFICE 14 Claims.

the vibrations recorded on the sound track. It i must have extreme rigidity in the axial direction or lengthwise of the sound track to minimise distortion. In pick-ups of the electromagnetic type it is also required that the magnetic flux vary linearly with the from an intermediate or neutral position. Furthermore, to prevent resonance effects and similar forms of distortion, it is required that the natural period of lateral vibration of the stylus be well below the lowest frequency desired to be reproduced and it is also necessary that the pick-up assembly and stylus each have a mini mum weight and inertia to minimise the loading on the walls of the record grooves, particularly at high frequencies or sounds having steep wave fronts, and in the event the record is not quite flat. As a matter of convenience it is also preferable to arrange for the ready replacement of the stylus.

The reproducers of the character referred to known prior to my invention do not meet these desirable conditions. In most constructions the stylus support depends upon the use of trunnions or upon the relative stiffness in two directions of a thin, fiat member to provide rigidity in the longitudinal direction together with lateral flexibility. such a construction provides at best a compromise in which insufiicient longitudinal rigidity is obtained and in which the stylus has greater rigidity in the lateral direction than is desirable. This leads to distortion resulting from the longitudinal displacement of the stylus, and from the resonance of the stylus when reproducing low frequencies. A linear relation between the rate of change of flux and the displacement of the stylus is seldom obtained because of the prior attempts to use highly efficient magnetic circuits. Also in the prior constructions provision was made for replacing the stylus by a thumb screw type of clamp which added greatly to the weight of the stylus assembly and resulted in excessive loading on the sound track walls and rapid wearing of the records.

It is accordingly an object of my invention to provide a reproduceror pick-up of the electrodisplacement of the stylus Great Britain June 18, 1945 magnetic type which overcomes the above noteddisadvantages by reason of a novel magnetic circuit and an improved stylus mounting.

It is another object of my invention to provide a pick-up of the character set forth in the preceding paragraphs in which a material reduction in weight is obtained by the use of a greatly simplified magnetic circuit.

It is also an object of my invention to provide a pick-up of the character referred to in the preceding paragraphs-in which the stylus holder is made extremely light in'weight and constructed to have high intrinsic rigidity.

It is an additional object of my invention to provide a pick-up of the character referred to in the preceding paragraphs in which the stylus assembly is formed with a T-shape, lateral vibration of the stylus being accomplished by pivotal movement of the assembly about the longitudinal axis of the bar of the T, such movement being permitted by trunnion blocks of rubber or similar resilient material adhesively bonded to the extremes of the bar of the T-shape, whereby the relatively great length of the bar of the T effectively prevents movement of the stylus in a longitudinal direction.

It is also an object of my invention to provide a pick-up of the character heretobefore described which includes a pair of simple opposed permanent bar magnets for establishing the required magnetic field.

It is a still further object of my invention to provide a pick-up of the character referred to hereinbefore in which the permanent magnet pole pieces are disposed between the rock bar and the record engaging end of said stylus, and as close to said stylus as possible,

It is an additional object of my invention to provide a pick-up of the character set forth in the preceding paragraphs which is formed as a pair of separable hinged halves normally held in closed position by a detent mechanism and which includes means operative in the closed position to hold the stylus in the proper relation to the other parts of the pick-up through clamping pressure exerted on the rubber trunnion blocks by the two halves of the assembly.

Other objects and advantages of my invention will be apparent from a study of the following specification read in connection with the accompanying drawings, wherein;

Fig. 1 is a perspective drawing illustrating the general form and appearance of a pick-up constructed in accordance with the preferred embodiment of my invention;

Fig. 2 is a longitudinal sectional view through the pick-up shown in Fig. 1 and illustrating the details of construction of a coil support and stylus mounting;

Fig. 3 is an underside view of the device shown in Fig. 2 and further illustrating the relative positions of the parts;

Fig. 4 is a fragmentary cross sectional view taken along the line IVIV of Fig. 2; and

Fig. 5 is a perspective view of the stylus assembly showing the details of construction.

Referring to the drawings, I have illustrated therein the preferred embodiment of my invention as comprising a pick-up assembly formed as two separable halves I and 2 joined to each other by a hinged construction comprising a tongue 3 on the lower member 2 disposed within a longitudinally extending slot 4 formed in the upper member I, a hinge pin 5 being passed longitudinally through the tongue 3 and the bosses 6 and 1 disposed on either side of the tongue 3 and defined by the slot 4. The upper part I of the assembly constitutes a cover for the lower part and serves also as a clamping mechanism for holding the stylus in an operative position as will subsequently appear.

When in use, the hinged halves are moved into juxtaposition as illustrated in Fig. 2 and are held in such position by a detent mechanism which may comprise a projection 8 formed on the upper half I and which enters a similarly contoured depression formed in the lower half 2.

The rear portion 9 of the upper half I is givena cylindrical form and is bored as indicated at II] and I to provide for attachment of a tone arm or swivel mounting providing for the traversal of the pick-up over the surface of the record during reproduction.

The bore I communicates with an interior space I2 permitting the electrical conductors comprising the output circuit of the pick-up to be connected respectively to a pair of connector elements l3 extending in a generally vertical direction through the lower half 2 and extending into a recess I4 formed in the under surface of the lower half. This recess and an upwardly extending bore serve to house a pick-up coil l6 and may be normally closed by a dust cover l1 removably secured .to the underside of the lower half 2 as by means of fiat head countersunk screws IS.

The bore I5 preferably extends somewhat beyond the mid line of the lower half 2 at which point its diameter is reduced as indicated at I9 to provide a shoulder against which the pickup coil l6 may be seated. The pick-up coil I6 may be secured within the recess I 5 as by means of shellac, varnish or othe suitable adhesive.

As appears in Figs. 2 and 4, the reduced diameter bore I9 communicates with a longitudinally extending recess 2| which is preferably given a rectangular cross section and which serves to house a stylus assembly indicated generally by the reference character 22. The stylus assembly is preferably constructed along the lines illustrated in Figs. 2, 4 and 5 and comprises a formed T shaped member consisting of a rock bar portion 23 from the center of which depends a tapered cylindrical tube 24.. The tube 24 is preferably contoured as shown in Fig. 4, to provide a short, straight cylindrical section 25 merging into a conical section 26 which is secured to or formed integrally with the transverse bar portion 23. The depending tubular portion 24 is preferably bored as indicated at 21 to receive a 4 sound track engaging stylus 28' formed preferably of sapphire Or a similar jewel.

The bar portion 23 is by preference given a channel shaped cross section defined by a horizontally extending web 29 from opposite sides of which rise narrow flanges 30 and 3|. The flanges 30 and 3| at the extreme ends of the bar portion 23 are given a somewhat greater width and are turned inwardly toward each other as shown at 32 and 33 to provide a quasicylindrical section .to which trunnion blocks 34 and 35 formed of rubber or other like resilient material are secured, preferably as by means of a vulcanising or adhesive bonding process.

I have found that the complete stylus holder can conveniently be formed by a drawing operation-as for example by a machine similar to an eyelet forming machine.

The stylus assembly is mounted within the lower half 2 in the manner shown in Figs. 2 and 4 with the trunnion blocks 34 and 35 seated within the channel 2|. The upper half I includes downwardly extending

bosses

36 and 31 disposed to engage the upper surfaces of the blocks 34 and 35 respectively when the halves are in their closed positions to thereby exert a clamping pressure on the blocks to hold the stylus assembly firmly within the recess 2 I. With the parts so positioned the tubular portion of the stylus passes through the center of the coil I6 and extends below the lower cover I! a suflicient distance to permit the stylus to engage the record surface and allow: adequate clearance between the cover and the surface of the record.

By properly locating the channel 2| and by carefully controlling the location and alignment of the blocks 34 and 35 as through use of a jig or fixture during bonding or vulcanizing, proper alignment upon assembly is assured.

The ends of the coil I6 are preferably carried through the recess I4 and secured as by soldering to the exposed ends of the connector member I3, thus completing the electrical circuit between the coil and the output conductors.

The magnetic field for the device is furnished by a pair of simple rectangular

permanent bar magnets

38 and 39 secured in any suitable manner within rectangular recesses 40 and 4| formed in the under surface of the lower half 2 and aligned with the coil supporting bore I 5. The depth of th recess I4 is chosen to place the upper surface of the

magnets

38 and 39 substantially in contact with the lower surface of the coil I6. While the

magnets

38 and 39 can be secured within the recesses 40 and 4| by any suitable means, as 'by means of screws or clamp-s, I have found that an adequately secure mounting is provided by making the recess and magnet dimensions conform closely and by using a suitable adhesive for holding the magnets in place.

The magnets are placed with unlike poles on opposite sides of the stylus assembly 22 and are spaced apart a distance substantially equal to the internal diameter of the coil IS, a distance which is somewhat greater than the maximum amplitude of vibration required of the stylus 22. Furthermore the magnets are given a width in the longitudinal direction (at right angles to thedirection of stylus motion) which is large compared to the diameter of the stylus. By this means there is produced across the gap between" the opposed faces of the bar magnets a substantially uniform magnetic field of considerable strength. The external magnetic circuit comprises the leakage path through the air and throughthe-hody oithepick-up fromthe extreme ends of the magnet... Thestuhular: portion; 2! oi. the stylus-assembly constit= tesia shunt: magnetic circuit; joiningrthe midpoin-ts of: the air gap -and external'portions of the magnetic! circuit. There. fore, when the stylus portion: N positioned centrally between adjacent laces oi the magnets 3.8. and; 38 substantiallyno time will flow through the stylus However, a; lateral displacement oi the stylus to bringthe stylus nearer oneofz. the magnets: and: further irom the: other will urn balance the magnetic field. and cause: a to flow in one direction the stylus A displacement the opposite direction will result in: a. flux flow through the-stylus in the reverse direction. Because the magnetic. field extending from pole to: pole at the: magnets. 38; and: 39' uniform and because the extent of the: lateral movement of. the is nonnally smaller than: the between the adjacent magnet faces the: flux changethroughthci stylus: hearse substantially' linear relationship to the magnitude oi. the displacement:

I have found that with 'a. pick-up-constructed as; described, the stylus has: great flexibility in the laterai direction and a period of viluration in that: direction which-is substantially lower than the. lowest: recordnfire ueuey desired to be reproduced. Furthermore, the stylus. pre sents extreme: rigidity inthe longitudinal direc ti-cmand is-not subdected to any material dis.- placement in the iongitudinal direction. during: operation. Faithfiul reproduction may be ohtained up to: iremrenciesas as 12,060 cycles per second.

Attention is. iurthenno-re directed to the ease. with which the stylus may be replaced as may he required by thewear resulting from continued use of the device. Replacing a: stylus is accom.-- plished simplyaxndi-eaisily hyhingedly 'movingithe halves l' and 2 awayfrom each. other whereby the complete stylus assembly 27." may lie-lifted upwardly from the channel 21 and replaced by a new stylus of like construction- Als'c:r,..t;lr1ev total weight of the pick-up is com siderazbly reducedv by the simplified magnet struc:-- ture disclosedherein;

While Ihaveshown and'descrihed the. preferred embodiment of my invention 1. do not desireto: be limited to the details have been: shown and describedherein except as defined in the appended claims.

I claim l. In an electromagnetic phonograph pick-up,

the combinationofi: abody member formed of a light weight non-magnetic material; a pick-up coil mounted in a recess formed. in one surface of said body; an armature of magnetic material extended axially through said coil; and a pair of simple permanent bar magnets disposed onopposite sides of said armature. and extending along a. line normal to the length or said armature, said magnets being secured within an. elongated. channel formed in said one surface of said body memher, said magnets extending over the surface of said coil to hold said coil within said recess.

2. In a stylus assembly for an electromagnetic phonograph pick-up, the combination of: :a, tubular stylus shank formed 'of magnetic material; a pointed record engaging stylus secured in one end of said shank; a rock-bar secured to the other end of said shank at right angles thereto and extending substantially equal distances from said shank; and a pair of trunnion blocks secured to the ends of said rock bar, said trunnion of construction which blocks being. termed of 1 a. resilient materiai. ham a. low: modulus" or rigidity and great elasticity permitting angular movement, or Sliiifdizlflfilfi bar in response to lateraldis Taoism-cut: said stylus.

' 3. In a. stylus; assembly for an. electromagnetic ph-cnograsphpiclc-up, the: cembmation of: arm bular stylus shame termed of magnetic: material;

a. pointed record engaging'stylus secured. in one end oi said shank; a rock-bar secured to other end. of said shank. at right angles thereto and: extending '"substantially equal distancesafrom said shank; and apairi of trunnion blocks -ad hesively' honded' to. the ends oi said rock; bar, said trunniomblocks being formed: of a. resilient? mm terialv having alovw mod; elasticity: permitting angular movement. 01? said rock bar in response to lateral displacement oi said-stylus d; In -a;.-styl:us essemloly ioran electromagnetic phonographzpick-up, the combination of: a tuslisularstylus shank: formed or magnetic material: a pointed record engaging stylussecured: in one end or said shank; a. rock bar secured t c-the other end-ofsaidshank' at right anglesthcreto and extending substantially equal distances: from said shank, said roch bar-having a-channel shaped section disposed withthe plane of the. web thereof at rightangl'es to said shank; aspair o1 trunnion blocks secured to the en de of said rock bar, said trunnion: blocks being formed of" a. resilient-material -having a low-modulus of rigidity and? great elasticitypermitting angular movement ofsaid rock bar in response to laterali dli'splacement of said stylus.

5; In a; stylus assemblyfor anelectromagnetic phonograph pick-up, the combination of: a: rock her of channel shaped cross section; trunnion blocks formed of "a resilientmateriali having physteal properties similar to those of vulcanized rubberadhesively bonded to opposite ends-ci-said' rock bar: and a; tubular stylus shank secured tosaid rock bar intermediate the ends thereof-and at right angles te -the length thereof, saidshanlc having a generally tapered form disposed with thelarger' end at said rock bar, said shank haw"- ing asubstantially constant wall thickness alon its length;

6. In a stylus assembly for an electromagnetic phonograph pick-up, the combination of": a rock bar of channel shaped cross section; trunnion blocks formed or a resilient material having physs lead properties similar to those of vulcanized rubber adhesively bonded to opposite ends of said rockbar: and a tubularstyius shank secured tosaidrcch bar intermediate the ends th-ereoi and at right angles to the length thereof; said shank having a cylindrical section spaced from said-rochbarhy a-frusto-conical section disposed with the largerbase at said rack bar,- said shank having a substantially constant wall thickness along-itslength.

7; In a styius assemblyfor an electromagnetic phonograph pickup, the combination of: atu hu-iar stylus shank: formed of magnetic material: a pointed record engaging stylussecu-red in one end of said shank; a, rock bar secured to the other end of said shank at right angles thereto and extending substantially equal distances from said shank, said rock bar having a channel shaped cross section disposed with the plane of the web thereof at right angles to said shank, the ends of said rock bar having a quasi-tubular form defined by upwardly and inwardly curving leg extensions. of said channel shape; and a pair of trunnion blocks secured to the ends of said rock his of rigidity andgreat bar, said trunnion blocks being formedof a re- 'silient material having a low modulus of rigidity and great elasticity.

8. In an electromagnetic phonograph pick-up, the combination of: a body member formed of a light weight non-magnetic material; a pick-up coil mounted in a pocket extending inwardly from one surface of said body; a stylus assembly comprising a rock bar, a shank portion extended at right angles to said rock bar and disposed intermediate the ends thereof, and elastic trunnion blocks'bonded to the ends of said rock bar; recess means in the other surface of said body for receiving said blocks and holding said stylus assembly in a position extending said shank axially'through said coil: and means clamping said trunnion blocks in said recess.

9. In an electromagnetic phonograph pick-up, the combination of: a body member formed of a light weight non-magnetic material; a pick-up H coil mounted in a pocket extending inwardl from one surface of said body; a stylus assembly comprising a rock bar, a shank portion extended at right angles to said rock bar and disposed midway of the ends thereof, and elastic trunnion blocks bonded to the ends of said rock bar; and means for clamping said blocks against the other surface of said body in positions extending said shank axially through said coil and disposing the length of said rock bar at right angles to the direction of desired movement of said stylus shank, whereby said desired movement is permitted by the angular movement of said rock bar torsionally distorting said elastic blocks.

10. In an electromagnetic phonograph pick-up,

the combination of: a body member for-med of a light weight non-magnetic material; a cover member hingedly attached to said body member for swinging movement between open and closed positions; a pick-up coil mounted in a pocket extending inwardly from One surface of said body; a stylus assembly comprising a rock bar, a shank portion'extended at right angles to said rock bar and disposed midway of the ends thereof, and elastic trunnion blocks bonded to the ends of said rock bar; recess means in the other surface of said body for receiving said blocks and holding said stylus assembly in a position extending said shank axially through said coil; means on said cover member disposed to engage said blocks in said closed position of said cover and clamp said trunnion blocks in said recess; and means for releasably holding said cover in said closed position, whereby moving said cover to said open position permits removal and replacement of said stylus assembly.

11. In an electromagnetic phonograph Dick-up, the combination of: a body member, a stylus assembly comprising a tubular stylus shank formed of magnetic material, a pointed record-engaging stylus secured in one end of said shank, and a rock bar secured to the other end of said shank at right angles thereto and extending substantially equal distances from said shank; and resilient mounting means interposed between each of the ends of said rock bar and said body flexibly se curing said stylus assembly to said body in a position disposing the length of said rock bar at right angles to the direction of desired movement of said stylus to thereby permit angular movement of said rock bar in response to lateral dis placement of said stylus.

12. In an electromagnetic phonograph pickup, the combination of a body member, a stylus assembly comprising a tubular stylus shank formed of magnetic material, a pointed record-'- engag-ing stylus secured in one end of said shank, and a rock bar of channel shaped cross section disposed with the plane of the web thereof at right angles to said shank and extending substan ti-ally equal distances from said shank; and resilient mounting means interposed between each of the ends of said rock bar and said body flex ibly securing said stylus assembly to said body in a position disposing the length of said rock bar at right angles to the direction of desired movement of said stylus to thereby permit angular move-' ment of said rock bar in response to lateral displacement of said stylus.

13. In an electromagnetic phonograph pick-up, the combination of: .a body member; a stylus assembly comprising a rock bar of channel shaped cross section, and a tubular stylus shank secured to said rock bar intermediate ends thereof at right angles to the length of said shank having a generally tapered form disposed with the larger end at said rock bar, said shank having a substantially constant wall thickness along its length; and resilient mounting means interposed between each of the ends of said rock bar and said body for flexibly securing said stylus assembly to said body.

14. In an electromagnetic phonograph pick-up including a body member carrying means defining a pair of spaced magnetic pole facesjthe combination of? a stylus assembly including an armature portion and-a rock bar portion extend ing transversely of said armature portion; resilient blocks of polygonal cross section bonded to the ends of said rock bar portion; and means on said body member for engaging said blocks and normally holding said assembly in a position disposing said armature portion centrally between said pole faces and disposing said rock bar in a position disposing the length thereof at right angles to the direction of desired movement of said armature portion.

HARVEY F. .SCHWARZ.

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

UNITED STATES PATENTS