US2518861A

US2518861A - Phonograph pickup

Info

Publication number: US2518861A
Application number: US783114A
Authority: US
Inventors: Joel A Burtch
Original assignee: Brush Development Co
Current assignee: Brush Development Co
Priority date: 1947-03-21
Filing date: 1947-10-30
Publication date: 1950-08-15
Anticipated expiration: 1967-08-15

Description

Aug. 15, 1950 J. A. BURTCH PHONOGRAPH PICKUP Filed Oct. 30, 1947 2 Sheets-Sheet 1 INVENTOR.

JOEL A. BURTCH,

RNEY

ATT

Aug. 15, 1950 J. A. BURTCH 2,513,361

PHONOGRAPH PICKUP Filed Oct. 30, 1947 2 Sheets-Sheet 2 INVENTOR. JOEL A. BURTCH ATTOR N EY Patented Aug. 15, 1950 UNITED STATES PATENT OFFICE PHONOGRAPH PICKUP Application October 30, 1947, Serial No. 783,114 In Great Britain March 21, 1947 mounting means other than the semi-solid material.

This invention provides mounting means other than the semi-solid material to facilitate accurate and rapid mass production of the transducer device shown and described in the aforesaid Lynch application and to prevent cold-flow of the semisolid material over a period of time from adversely affecting the performance of the transducer device.

In the prior art, transducer elements such as piezoelectric crystal elements for phonograph pickups have been enclosed in semi-solid embedment materials such as petroleum jelly (Vaseline) lanolin and the like in order to moistureproof the crystal and in order to damp resonant vibrations of the transducer system. These crystal elements were contained in a housing and the housing was substantially filled with the semi-solid material. Due to the characteristics of the semi-solid materials which had no stifiness and exerted only a mechanical resistance force against the crystal element throughout its range of vibration, it could not act as a mounting means and it was necessary to hold the crystal element within the housing by the usual mounting pads which are well known in the art.

In accordance with this invention there is provided a phonograph pickup comprising a cupshaped housing having an open end and having a shoulder near its lip which is of smaller diameter than the diameter of the lip of the cup-shaped housing. Planar closure means are provided for the open end of the housing which engage the shoulder, and its plane extends substantially perpendicular to the direction of the axis of the housing. There is a flexing-type piezoelectric crystal element adapted to generate an electric signal upon being flexed and harness means which is connected to the closure means supports the crystal element within the housing. There is a mass of semi-solid material within the housing and in engagement with a substantial area of the crystal element for resiliently resisting the flexing of the element, and driving means are connected to the element and extend through the housing.

An object of the invention is to provide a new and novel mounting for a piezoelectric transducer element.

Still another object of the invention is to provide a mounting for a crystal element embedded in semi-solid material.

A further object of the invention is to provide new and novel electrical connections for a crystal element in a phonograph pickup.

Yet another object of the invention is to provide a phonograph pickup which is easil assembled.

Other objects and a fuller understanding of the invention may be had by referring to the following description, claims and drawings wherein:

Fig. l is an isometric view of an assembled pickup; Fig. 2 is a top view of the pickup; Fig. 3 is a bottom view of the pickup; Fig. 4 is an isometric View, on a larger scale, of a portion of the pickup; Fig. 5 is an isometric view of the crystal assembly used in the pickup; Fig. 6 is a cross-sectional view taken along line 6-6 of Fig. 2; and Fig. '7 is a cross-sectional view taken along line 'l! of Fig. 6.

In Figures 1, 2 and 3 there is shown a phonograph pickup comprising a cup-shaped housing portion 85 having a larger diameter at its lip so that a shoulder 86 is formed. Against this shoulder there is positioned a base 5! formed of several washers connected together by portions of the back end of a stylus guard as. The base 5! substantially closes the end of the cup-shaped housing 85. A harness, shown in detail in Fig. 4, is connected to the base 5| and a piezoelectric crystal element 29 is mounted in the harness for free rotary motion but is restrained from any lateral displacement. A quill 35 is connected to the lower edge of the crystal element and extends to the outside of the housing 85. A long thin vertically flexible stylus arm carrying a stylus tip 9! is connected into the quill 35.

As the stylus tip Si is moved laterally by a phonograph record the crystal element 29 is rotated in its harness.

Within the housing to there is a semi-solid mass of rubber-like material 18 contacting a broad area of the crystal and exerting a stifiness force against the crystal I3 as the crystal is twisted by a phonograph record groove laterally moving the p f t e S ylus i 6. Because the material I8 is compliant and yieldingl tends to hold the crystal in place, a force applied to the chuck end of the crystal is stresses the crystal and causes the crystal to generate an electromotive force representative of the mechanical force applied to the crystal. Contrary to the prior art, no mounting pads or other means for forcibly restraining localized portions of the crystal element is are utilized.

The material !8, when properly chosen in accordance with specifications presented later, acts as a stillness-controlled mounting for the piezoelectric crystal element l3 at frequencies below a certain frequency which varies in accordance with its composition. With the crystal element vibrating at frequencies up to this certain frequency, a force potential independent of frequencyis produced across the length of the crystal element causing the element to generate a voltage which is also independent of frequency. At frequencies above this certain frequency the material is, because of its molecular friction, begins to act as a mechanical resistance to the motion of the crystal element so that the crystal element now has produced across its length a force potential proportional to the frequency of its vibration. Thisgencrates for a given amplitude of stylus displace ment a voltage output rising at a rate of about 6 db./octave starting at the certain frequency. If the mounting material l8 hassomewhat more internal friction the pickup response can be made to start rising at a lower frequency, and if it has somewhat less internal friction the pickup response can be made to start rising at a higher frequency. The point at which the pickup response starts to rise may, by analogy to the terms used in the phonograph art, be called the turn-over point. Below the turn over point the force exerted against the crystal element by the material I3 is predominantly a stillness force althrough there ma also be exerted a mechanical resistance force. Above the turn-over point the force is predominantly a mechanical resistance force although there may also be a stiffness force exerted.

Commercial phonograph records usually cut constant amplitude to about 490 cycles per second and constant velocity above that frequency. If such a record is reproduced by a phonograph pickup having no equalization an output response fiat to about e cycles second (the turn-over point) is obtained, and the response thereafter falls at a rate of about 6 rib/octave. A graph of the output voltage from a phonograph pickup embodying the present invention as the pickup reproduces a commercial phenograph record is hat to about 400 cycles :and falls at a rate of about 6 db./octave between ail-'3 and 700 cycles. At this frequency the becomes flat and stays fiat to well out beyond 7,000 cycles. This provides, without any electri a1 equalization, a base boost which for many applications desirable.

The material It! within the housing is very effectively seals the transducer element is against moisture. It further provides :a very effective shock mount thereby, if it is so d permitting the use of very fragile, plate crystal elements. In addition to providing an excellent mounting and in addition togproviding internal equalization the material it also (lamps resonances which may occur in the crystal or in the crystal system comprising the crystal element l3 and the chuck it.

Satisfactory results from a honogr ph p p of the type shown in Fig. 1 are obtained if the material i8 is of the plasticized polyvinyl resin type. One such material is sold under the trade name Korogel. Rubber-like plastic elastomers of the vinylite type are also satisfactory. One such elastomer is made b copolymerising polyvinyl chloride and. polyvinyl acetate and dispersing the resultant resin in a plasticizer such as dibutyl or dioctal phthalate or a mixture of the two.

Many of the materials which are suitable for mounting piezoelectric crystal elements in accordance with this invention will cold-flow with time thereby slightly dislodging the crystal eleent. In order to reduce the effects of cold--flow and in order to assemble the pick-up with the crystal element and its attached stylus assembly accurately in position a harness, illustrated in Fig. 4, is provided.

The crystal element assembly, indicated generally by the reference character 23, is shown in detail .in Fig. 5. It comprises two plates of piezoelectric

crystalline material

25 and 26 connected together and to the leads 2! and 28 to form a multiplate flexing element identified generally by the reference character 22. The crystal element 29 is provided at its top with a saddle element 3B of inverted channel form which is straddled'across the thickness of the element. At its bottom end the crystal is provided with a cradle element 3!, also of channel form, within which the lower end of the crystal is firmly held. The center portion 32 of the saddle it is raised-slight- 1y above the main portion, thereof and a pivot 33 extends slightly above the raised portion The bottom channel 3! carries a pivot point 35.

From the underneath side of the crystal 3! there extends a downwardly inclined laterally projecting hollow quill 35, the inner end of which is secured to the-cradle 3! by sweating or othersuitable means, and the cradle carries a projection 37 (Fig. 6) which fits into the end of the hollow quill in order to accurately position the quill. The cradle 3! includes an integrally connected locating member 38 which extends downwardly from one of its ends. The member 38 has a hole in it through which the quill 35 extends, and. a drop of solder may be applied to the quill at the location where it extends through the member in order to firmly connect the two together. the quill 35 is firmly connected'to andaccurately aligned with respect to the crystal element The sub-assembly comprising the crystal element 29 with its connected quill 35 is mounted in a frame, or harness, which is shown in Fig. 4. The crystal element within the harness is free to rotate but is restrained against movement both lengthwise and laterally to prevent coldflow of the semi-solid material from dislodging the crystal element and moving the stylus out of its position. The mounting frame comprises two upstanding side portions ill, 6| in the form of flat metal strips having their upper ends 42, 43 inserted through

holes

44, 45 in afiat top plate 46 which is made of insulation material. The ends 42, 43 of the metal strips are turned over to connect the top plate 45 to the side legs .8, 45. The base of the harness comprises a pair of superimposed flat circular disks 58, 5! also made of electrical insulation material. The lower ends 41, 48 of the side legs 40, ll extend downwardly through slots in the upper base disk 5i! and then turn and extend outwardly in opposite directions within

shallow channels

52, 53 provided in one or both of the'opposing faces of the superimposed base disks 50, 5|. The lower extremities of the side legs 40, 4| after passing outwardly from between the base disks 5!], 5! are turned upwardly to lie substantially parallel to the legs M], 4! and these up-turned

extremities

55, 56 constitute conducting tongues through which electrical connection may be made to the leads 2?, 28 of the crystal element 23 when the crystal element is mounted within the harness. The two superimposed base disks 5%, 5! are connected together by means of a portion of a fiat stylus guard member 6! which is formed from a spring material. The end of the stylus guard 55 which lies underneath the base 5| is slotted at 6!, 62 to provide three

tongues

53, 6d, and 55. As may be seen in Fig. 7 the end of each of the

tongues

63, 65 is slotted for a short distance at 65 to form two tabs 51, 61' at the end of each of the tongues '63, 65. longues 6B, 55 extend upwardly through registering slots 58, $8 in the base disks 5%, 5i and the two tab portions Bl, ill of each of the tongues are folded over in opposite directions against the upper face of the disk lit! to clamp the two disk portions 58, El together thereby firmly holding legs 45, l! place. The central tongue portion 84 of the stylus guard {55 is bent upwardly to form an upstanding portion is and is then again bent to form a horizontal portion 3' l. The horizontal portion H has a small detent l2 in it and the top plate it has a small detent is (shown in Fig. 7) located near its center. The crystal assembly shown in Fig. 5 fits into the harness shown in Fig. 4 with the saddle 3% extending substantially perpendicular to the direction of the top plate 46 of the harness. The pivot point 33 fits into the detent l3, and the pivot point as fits into the detent 12. Due to the springiness of the stylus guard til, the tongue portion 54 biases the crystal assembly upwardly against the top plate 46 thereby resiliently holding the crystal sub-assembly in place. It is preferable to resiliently hold the crystal assembly in place by means of a spring urging the crystal assembly upwards against a stiir top member rather than by a spring urging the crystal assembly downwardly against a stiff base as a sudden blow on the stylus arm, such as by dropping the pickup arm onto the stylus will not dislodge the pivot points from the pivots when the former construction is used. The crystal lead Z'l is wound around the metal leg ll and soldered to it, and crystal lead 28 is wound around and soldered to the metal leg A li, as shown in Fig. '2. The quill 35 of the crystal sub-assembly extends in a direction toward the outer end of the stylus guard 58 which is bifurcated to establish two leg portions and 82 with a notch there etween. Each of the legs at its outer end is dimpled to establish on its lower face a

smooth bearing surface

82, 83 and, as is shown best in Fig. 6, each leg is arched upwardly from the main portion of the stylus guard A cannister, or housing, 85 which is deeply cup-shaped is provided. This cannister 35 is filled with semi-solid crystal-supporting material l8 and th crystal sub assernbly comprising the crystal assembly 23 mounted in its harness is positioned in the cannister with the upper disk 5d of the base engaging a narrowed shoulder 86 of the cannister and closing the open end of the cannister.

The cannister 85 is adapted to be connected to a phonograph pickup arm, so in order to assure true alignment of the stylus arm with respect to the pickup arm the stylus arm must be accurately aligned with respect to the cannister. The crystal assembly, including the quill 35, is mounted in the frame and a stylus assembly Si] is connected into the quill 35. The deeply cup-shaped cannister is filled with liquid material which, when processed, becomes semi-solid. While the material is is in a liquid state the crystal assembly, in its frame, is pushed into the filled cannister until the upper disk 58 of the base engages a narrowed shoulder 86 of the cannister. This accurately places the frame with respect to the cannister 85. The stylus assembly is firmly held in the notch S t in the stylus guard thus orienting the crystal 2% with respect to the frame and the stylus with respect to the cannister while the material l8 solidifies. The cannister 85 is notched at three places ill, 88 and 89 around its lip, and the extremity 55 of the metal leg 40 extends outwardly from the cannister 85 through slot 81. When the crystal assembly is in place in the cannister 85 the top surface of the leg extremity 55 engages the can" nister 85, thereby making an electrical contact. This is the ground side of the electrical circuit to the piezoelectric crystal 29 and the cannister 85 thereby becomes a grounded electrical shield. The leg extremity 56 extends outwardly from the cannister 85 through the slot 89, but is spaced from the cannister to prevent electrical contact and the quill 35 extends outwardly from the cannister 85 through the slot 88.

The stylus assembly is easily replaceable. It comprises a long, thin strip of metal 96 into one end of which is connected a jewel stylus tip 9!. The end of the stylus arm 90 opposite the stylus tip 9! is bent slightly as at 92 so that as the stylus arm is pushed into the hollow quill 35 the bend in the arm is flattened to cause it to firmly engage the walls of the quill. This holds the stylus arm into the quill by friction alone, obviating the use of thumb screws and the like and greatly reducing the mass of the stylus assembly. By so mounting the stylus arm in the quill 35 it may easily be replaced. A pull on the stylus arm in a direction substantially parallel to the direction of extension of the quill 35 easily removes the stylus arm from the quill and to replace it with a new one, the new stylus arm is merely pushed back into the quill. The stylus is arched slightly at 83, and when the stylus arm is in place in the quill 35 with the stylus tip 9! lying between the legs at, st of the stylus guard 68, the stylus tip 9! projects just slightly below the

bearings

82, 83. As is shown in Fig. 6 the arched portion of the stylus arm 99 is located substantially at the point where the stylus arm extends between the legs 3B, 8! of the stylus guard Bil. When the pickup is not in engagement with the record the stylus arm 52 rests in the notch 95 between the legs 8! thereby holding the stylus arm and preventing cold-flow of material l8 over a period of time from dislodging the stylus. When the pickup is bein used to transcribe a record, the operator lowers the pickup arm supporting the cannister 35 until the stylus tip 9| engages the record 96 and supports the unbalanced weight of the pickup arm. This unbalanced weight lifts the stylus arm 98 slightly with respect to the stylus guard 5d, thereby releasing the stylus arm till from the notch 94 so that the arm can move laterally a limited distance under the influence of the grooves in the phonograph record 96. When the pickup is lifted from the record the stylus arm 9:) once more is locked by the notch 95. If the stylus arm is carelessly dropped onto the record the stylus arm .90 bends readily until the smooth bearings 32, 3.3 at the extremity of the stylus guard to hit the record .96. The spring guard 68 is much stiiferzthanthe spring'pickup arm and it easily absorbs the force of dropping the pickup arm without damage to'itself, and, due to the large area of the

bearings

82, 83 compared to the small area in the tip of a phonograph stylus, the record is not damaged by the guard. The greater vertical flexibility of the stylus arm allows the stylus ill to retract under the force of the fall thus preventing damage to the record by the stylus tip.

This stylus construction eliminates the usual stylus chuckwith its clamping screws. It eliminates soldering operations during the replacing of the stylus and, most important of all, it greatly reduces the stylus assembly weight and consequently the dynamic inertia forces involved-when a record is reproduced. The stylus arm assembly weighs approximately .0015 ounce.

In order that the crystal element 23 shall move freely under the influence of driving forces from the stylus system it is important that shear forces in the mounting material it be kept to a minimum. These shear forces exist in the semisolid material between the crystal .saddle 3i! and the top cross-bar '46 of the frame and b tween the crystal cradle Si and the top surface of the base disk 59. In order to reduce the shear force at the top of the crystal element to prevent locking of the top .edge of the movable crystal element to the stationary frame, the saddle '36 is provided with a step 32 between the pivot 33 and the top surface of the saddle. This step 32 spaces the main portion of the saddle sufficiently far away from the cross-bar 46 of the frame that an appreciable depth of the semi-solid material will exist between the saddle and the cross-bar. Because of the appreciable depth of the semisolid material the shear forces produced therein by motion of the crystal element with respect to the harness is kept to a minimum value which does not disturb the operation of the device. The lower edge ofthe crystal element 2'9 moves with greater amplitude than the upper edge so it is of even greater importance that high shear forces be prevented in the material l8 which lies between the lower edge of the crystal element 29 and the top face of the base disk Ell. This is achieved by providing registering slots I in the base disks 55, to accommodate the quill 35 which extends downwardly and outwardly from the center portion of the lower edge of the crystal element 29. The semi-solid material !8 covers the lower edge of the crystal element 29, as is shown in Fig. 6, and fills the quill slot Hid thereby damping the vibrations of the quill, and because of the relatively large amount ofsemi-solid material 18 keeping the shear forces low. In the top surface of the base disk '56 immediately below the back corner 29' of the crystal element 25 there is provided a groove ltl which is'filled with semi-solid material IS. The depth of the material is between the cradle 3i and the bottom of the groove It! is sufiicient to prevent high shear forces in the material I8.

From 6 it will be noted that the lower pivot point 3c is not on the vertical center line of the crystal element but is set back a short distance toward the back corner 2?? of the crystal element. If the crystal element were suspended in the semi-solid mounting material l3 without a harness, lowfrequency vibrations would cause it to vibrate about a line running from somewhere in the vicinity of the lower-back corner 29' of 8 thecrystal plate up toward-the central portionof the top edge of the crystal plate. As the crystal element vibrated at'higher frequencies thelower pivot point would move in toward the vertical center line of the crystal element.

While there have been described what areat present considered to he the preferred embodiments of this invention, it will be obvious to those skilled in the art that various changes and modifications may be made therein without departing from the invention, and it is, therefore,zaimed-in the appended claims to cover all such changes and modificationsas fall within the true spirit and scope of the invention.

What is claimed is:

1. A phonograph pickup --comprising, a cupshaped housing having a-nopen end and having a shoulder near the lip thereof which is of smaller diameter than the diameter of the lip of :said cup-shaped housing, planar closure means for the open end of said housing engaging .said shoulder with the plane of said closure meansextending substantially perpendicular to thElfliI'fiC- tion of the axis of said'housing, a flexing type electromechanical transducer element, harness means mounted on said closure means and positioned within said housing and "connected'to .said transducer element for supporting said transducer element within said housing, a mass of semi-solid material having stiffness less thanzthe stiffness of said transducer element'within said housing and in engagement with substahtially'the entire area of said transducer element for resiliently resisting the flexing of said element, and driving means connected to saidelement and eX tending through said housing for flexing said transducer element.

2. A phonograph pickup as set forth inclaim 1, further characterized by said harness means comprising two legs connected to and upstanding from said closure'meana'said legs being electrically conductive and'insulated'from each other by said closure means, and two electricalleads each of which is connected to said transducer element and to one of the said legs ofsaid harness.

3. A phonograph pickup as set forth inelaim 2, further characterized by means connecting one end of each of said two legs to saidclosure means, a cross-bar insuiatedly interconnecting the other ends of said two upstanding legs, and pivot'means carried by said cross-bar and by said closure means for pivotably supporting said transducer element.

A phonograph pickup as set forth in claim 1, wherein said driving means includes a stylus, further characterized by said closure means comprising two plates each having a slot therethrough superposed in face-to-facerelationship with the said slots in registration, and'by guard means one'end of which protects said stylus and the other end of which extends through said registering slots in said two superposed plates and is bent over an angle to secure said two plates together.

5. A phonograph pickup as set forth in-claim 4, further characterized by a first and a second channel extending between said two plates from the edge of said closure means toward the center thereof, the upper of said two plates having first and second slots through it each of which communicates one of said channels at its inner end, said harness means including two spaced-apart legs each of which is connected to said closure means by one end thereof extending down through one of said two slots in said upper plate and out through said channel between said two plates.

6. A phonograph pickup as set forth in claim 5, further characterized by said two harness legs extending from said two channels out beyond the edge of said closure means forming electric terminals for said pickup.

7. A phonograph pickup comprising, a base member, a frame having two upstanding electrically conductive legs connected to and extending away from the base member and a crossbar connected between and insulated from said legs, a flexing type elec romechanical transducer element having two electrical leads, pivot means pivotally supporting said transducer element between said base member and said frame for substantially free pivotal motion, each of said leads being electrically connected to one of said legs, housing means enclosing said frame and said transducer element, two terminals extending through said housing means and electrically connected, respectively, to said two legs, driving means connected to said transducer element and extending to the outside of said housing, and means Within said housing and in engagement with said transducer element for resiliently resisting the flexing of said transducer element.

8. A phonograph pickup comprising, a flexing type transducer element having two leads, a housing including a cup-shaped portion and a 10 closure portion of insulating material secured to the open end thereof; harness means secured to said closure portion and comprising a pair of spaced-apart electrically conductive legs one end of each of which is connected to said closure portion; said legs extending into said cup-shaped portion, and a cross-bar of insulating material secured to the other end of each of said legs; means pivotably securing said transducer element between said closure portion and said cross-bar, each of said transducer element leads being electrically connected to one of said legs, and terminal means connected to each of said legs and ex tending outside of said housing for connection in an electric circuit.

JOEL A. BUR'ICH.

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

UNITED STATES PATENTS Number Name Date Re. 20,680 Sawyer Mar. 29, 1938 1,827,919 Van Wagenen Oct. 20, 1931 2,208,940 Dunning July 23, 1940 2,313,129 Dohan Mar. 4, 1943 2,363,493 Begun Nov. 28, 1944 2,363,497 Begun Nov. 28, 1944