Replaceable stylus-lever generator module for an electromagnetic cartridge
United States Patent 1 1 1111 3,881,073  
 Grado Apr. 29, 1975 [541 REPLACEABLE s s. vE 3.683,128 8/1972 Grado 179/10041 1 GENERATOR MODULE FOR AN 1233323 1311335 3&#39;51 119/100&#39;41 K 1 er ELECTROMAGNETIC CARTRIDGE 3,700,829 10/1972 Anneberg et al. 179/10041 K [76] Inventor: Joseph F. Grado, 4614 Seventh Ave., Brooklyn, N.Y. 11220 [22] Filed: Nov. 12, 1973 [21] Appl. No.: 414,950  
 [52] U.S. Cl... l79/l00.4l K; 179/100.41 Z; 274/37 [51] Int. Cl H04r 11/12 [58] Field of Search 179/100.41 J, 100.41 K,  
 l79/100.4l M, 100.41 Z; 274/37 [56] References Cited UNITED STATES PATENTS 3,546,399 l2/l970 Stanton et a] l79/l00.4l K  
 Primary Examiner-Stanley M. Urynowicz, Jr. Assistant ExaminerStewart Levy Attorney, Agent, or FirmAmster &amp; Rothstein [57] ABSTRACT An electromagnetic cartridge for the reproduction of quadraphonic, stereophonic and/or monaural sound recordings has a balanced magnetic and electrical structure including a replaceable stylus-lever generator assembly which has front and rear pole piece extensions cooperatively positioned with the generator.  
 7 Claims, 7 Drawing Figures REPLACEABLE STYLUS-LEVER GENERATOR MODULE FOR AN ELECTROMAGNETIC CARTRIDGE The present invention relates generally to electromagnetic cartridges for the reproduction of monaural, stereophonic and/or quadraphonic sound recordings and in particular to an improved replaceable module for such an electromagnetic cartridge.  
  In a typical cartridge, a housing supports an appropriate magnetic structure including first and second pairs of coils, with each coil pair being connected in series. A stylus-lever generator assembly is also mounted in the cartridge housing and includes a cantileversupported stylus-lever having the requisite complients such that the stylus thereof may track the usual 45 45 groove of a phonograph record disc. Operatively connected to the stylus-lever is a generator which is disposed in the cartridge within the magnetic gaps defined by a permanent magnet and pole pieces of the magnetic structure. Motion of the stylus in the record groove produces related motion in the generator which causes signals to be induced in the coil pairs. These signals are then processed to provide a reproduction of the signals formed in the record groove.  
  There are numerous design problems presented in the commercialization of pickups or cartridges of this type and in continually improving their performance to assure higher fidelity in record reproduction. One of these problems is to increase the voltage sensitivity to obtain maximum efficiency from the cartridge, by providing a minimum operating gap between the generator and the magnetic structure. This is particularly important in the new 4-channel sound reproduction systems since these systems require low inductance and electrical resistance which reduces the voltage sensitivity of the electromagnetic circuit.  
  The problems are increased still further when a module having the stylus-lever and generator is designed to be removably mounted in the electrical and magnetic structure of the cartridge, since the module is inserted into an existing magnetic gap of the cartridge and precision and control of the tolerance between the magnetic circuit of the cartridge and the generator in the module are difficult to achieve resulting in variations in stylus-lever compliance, variations in voltage output in the channels of the reproduction system and voltage variations between the channels. In addition, the air gap between the generator and the magnetic structure tends to be relatively large making electromagnetic cartridges with replaceable modules relatively less sensitive than cartridges without replaceable modules.  
  In accordance with the invention, a replaceable stylus-lever generator assembly for an electromagnetic cartridge has a generator mounted in gaps between both poles of the magnetic structure which, when assembled within the electromagnetic cartridge forms part of the balanced magnetic structure of the cartridge. The module structure contains pole extensions which are terminations of opposite polarities of the magnetic structure. These include first and second pairs of magnetic members mounted in opposing relationship on a carrier and cooperatively positioned to define the operating gaps therebetween. A stylus-lever operatively mounted on the carrier extends generally parallel to the length dimension of the operating gaps. The generator is positioned in cooperative relation with the magnetic members and the operating gaps to define first and second working gaps; the generator moving relative to the gaps in response to movement of the stylus-lever.  
  When the module is operatively positioned in the cartridge, the magnetic members from front and rear pole piece extensions which are part of the balanced magnetic structure. Since both polarities of the magnetic structure are arranged as part of the replaceable module, high precision in the relationship between the magnetic structure and generator can be assured and greatly reduced gap width can be provided to increase the sensitivity to the signals from the record grooves and reduce sensitivity to stray fields. Since the module is factory assembled, the tolerances can be accurately controlled and variations between modules minimized.  
  Still other objects, features and advantages of the present invention will be understood by reference to the following detailed description of a presently preferred but nonetheless illustrative embodiment of the invention, when taken in conjunction with the accompanying drawings.  
 BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a longitudinal sectional view of a typical elecromagnetic cartridge embodying features of the present invention, shown with the balanced magnetic structure partially removed from the cartridge housing, the dotted lines showing a removed position for the stylusv lever generator assembly.  
  FIG. 2 is a side elevation view of the electromagnetic cartridge of FIG. 1.  
  FIG. 3 is a sectional view on an enlarged scale, taken substantially at 45 to the horizontal and along the lines 3-3 of FIG. 2 and looking in the direction of the arrows.  
  FIG. 4 is a sectional view taken along the lines 4-4 of FIG. 3 looking in the direction of the arrows.  
  FIG. 5 is an enlarged view of a portion of FIG. 3 showing the relationshsip between the generator and front and rear pole piece extensions.  
  FIG. 6 is a partial sectional view taken along the lines 66 of FIG. 3 and looking in the direction of the arrows with the stylus-lever generator assembly removed for simplicity and greater clarity and showing an alternate embodiment of the front pole piece extensions of the invention.  
  FIG. 7 is a sectional view taken substantially along the lines 66 of FIG. 6 and looking in the direction of the arrows.  
 DESCRIPTION OF THE PREFERRED EMBODIMENT Referring now specifically to the drawings, and in particular to FIG. 1, there is shown an improved electro-magnetic cartridge or pickup for monaural, stereophonic and/or quadraphonic reproduction of sound recordings, generally designated by reference numeral 10, which is mounted within an appropriate housing 11, which is in turn removably mounted on the tone arm of a turntable or record player. Housing H includes top wall 12, front wall 13, and spaced side walls 14, 15, which define a downwardly opening cartridgereceiving chamber C into which the cartridge 10 is inserted.  
 I Cartridge 10 includes, a balanced magnetic structure M including four pole pieces, l6, l8 and 20, 22 whose I right angles to the first plane. A first pair of coil l7, 19  
 is mounted on the first pair of pole pieces 16, 18 and connected in voltage adding and hum canceling relation to each other; and a similar pair of coils 21, 23 are mounted on a second pair of pole pieces 20, 22 with the coils likewise being connected in voltage adding and hum canceling relation to each other. Typically, each of the coils includes a coil bobbin 22 fabricated of a non-ferrous material and a coil proper, generally designated by reference numeral 24, wound such that the opposing pairs of coils l7, l9 and 21, 23 are of substantially identical configuration to avoid introducing any electrical imbalance in the coil arrangements and symmetry.  
  The balanced magnetic structure also includes a rear magnetic plate 26, a front magnetic plate 30 disposed in space parallel relation to the rear plate 26 and a permanent magnet 32 interconnected therebetween. The pole pieces are supported on the rear magnetic plate 26 by non-ferrous sleeves 28 which provide compensating gaps through which the rear pole pieces are magnetically coupled to the rear magnetic plate. As explained in greater detail in U.S. Pat. No. 3,694,586 of Sept. 26, 1972, the compensating gaps are adjustable to prevent stray fields from generating hum producing voltages in the cartridge. Cylindrical ferrous ring 29 surrounding the coils provides further hum cancellation.  
  Disposed in the magnetic path afforded by the magnetic structure M is a replaceable module, generally designated by the reference numeral 40, which is removably mounted in a downwardly opening socket or cavity provided intermediate the rear face of the front magnetic plate 30 and the adjacent front faces of the rear pole pieces in common plane P,(see FIGS. 1 and 3). Module 40 includes a carrier 42 fabricated of nonferrous material formed with a central cavity 44 having a shoulder or seat 46 opening toward the front magnetic plate 30 of the magnetic structure M adapted to receive front pole piece extension assembly 70. The front pole piece assembly is adapted to be magnetically coupled and in contact with the rear face of the front magnetic member 30 and is formed of magnetic material with an annular front portion having an inwardly extending shoulder 71 and a rearwardly directed mating ridge 72 adapted to press into seat 46 to provide support for assembly 70. Further support is provided by surface 74 which presses into the outer surfaces of the seat. Arranged symmetrically about the front pole piece extension assembly and extending rearwardly are diametrically opposed front pole piece extensions 16b, 18b and b, 22b (not shown). The radially outer surface of each pole piece extension is tapered radially inward from the front towards the rear of the pole piece extension, terminating in rear faces which lie in a substantially common plane which defines one side of first and second pairs of operating gaps, G,, G (see FIG. 3) and G G (not shown).  
  Cooperatively positioned on carrier 42 in opposing relation to said front pole piece extensions are rear pole piece extensions 16a, 18a and 20a, 22a. The rear pole piece extensions are adapted to be respectively magnetically coupled and in contact with the rear pole pieces and are formed of a magnetic material having generally cylindrical rear portions which terminate in rear faces having a diameter substantially equal to the diameter of the front faces of the rear pole pieces, and tapered forward portions having radially outer surfaces tapered radially inwardly from the rear towards the front of the pole piece extensions terminating in front faces lying in a substantially common plane defining the other side of the first and second pairs of operating gaps, G G and G G By providing both front and rear pole pieces extensions the overall length of the operating gap is reduced thus producing increased flux density in the gap. Further, the tapered front and rear pole piece extensions confine the magnetic flux to a relatively small area and minimize fringing.  
  Mounted on the non-ferrous carrier 42 is the styluslever generator assembly generally designated by reference numeral 50 which includes stylus-lever 54 extending generally parallel to the length dimension of the op erating gaps and having the usual stylus 52 mounted approximate its forward end and terminating at its rearward end in flange 56. A suspension system for the stylus-lever generator assembly includes reduced diameter shaft 58 extending axially from the rearward end of the stylus-lever and connected to flange 60 which, in turn, is anchored to base support 62. interposed between the rearward face of flange 56 and the forward face of flange 60 and surrounding shaft 58 is an elastomeric member 64 which is in axial compression to preload the suspension system as described in greater detail in U.S. Pat. No. 3,683,128 of Aug. 8, 1972 in particular the embodiment shown in FIG. 7 and in my copending application entitled Compressive Pivot Suspension System for an Electromagnetic Cartridge Ser. No. 414,983, filed on Nov. 12, 1973. Operatively coupled to flange 56 of stylus-lever 54 is an annular generator 66 which is symmetrically disposed with respect to the operating gaps and defines equal length, front and rear working gaps, Gwf, Gwr between the front and rear generator faces and the respective front and rear pole piece extension faces as illustrated in FIG. 5. The generator may be disposed with respect to the operating and working gaps as shown to provide flux-bridging operation which is described in greater detail in my U.S. Pat. No. 3,683,128, referenced above, or it may be disposed to provide gap operation which is shown and described in greater detail in my U.S. Pat. No. 3,694,586 issued Sept. 26, 1972.  
  The removable mounting of module 40 is accomplished by appropriately sizing carrier 42 and front and rear pole piece extensions in relation to the cavity, rear pole pieces and the front magnetic plate so that the insertions of the module into the cavity results in generally constant pressure between the module and the cartridge thus maintaining the module in its proper position. Additional support for the module can be ob tained by forming carrier 42 with surfaces which will interengage with housing H, as for example guiding extensions 42a, 42b which engage the lower portion of sidewalls l4, 15. When the assembly is in its operating position, the front and rear pole piece extensions are physically and magnetically coupled to the front magnetic plate and the rear pole pieces, respectively, thus completing a magnetic circuit between opposite polarity faces of the front and rear pole pieces extensions via the rear pole pieces which are magnetically connected through the compensating gaps provided by sleeve 28 to rear magnetic plate 26 which, in turn, is magnetically connected to front magnetic plate 30 by permanent magnet 32.  
  Alternatively, the front pole piece extensions can be four individual magnetic pole pieces independently supported on a carrier. This embodiment is shown in FIGS. 6 and 7. Referring to these figures, carrier 42&#39; is formed with an inward annular support 46&#39; and supports in cooperative relation with the rear pole pieces extensions symmetrically spaced independent front pole piece extensions 80,82 and 84,86. Each independent pole piece extension has a generally cylindrical front portion, a rearwardly directed mating or ridge end 72&#39; and a rear portion tapered radially inward toward the rear of the carrier and adapted to be magnetically coupled to and in contact with the rear face of the magnetic plate 30. The rear pole piece extensions terminate in rear faces lying in a substantially common plane which cooperate with the front faces of the rear pole piece extensions to define operating gaps, 6,, G and G 6,. When the module of FIG. 6 is in place in the cartridge, the independent pole pieces extensions are magnetically equivalent to the pole piece extensions of assembly 70 in FIG. 3 and form part of the balanced magnetic circuit as described above.  
  In operation, the stylus tracks the record groove causing the generator to move with respect to the magnetic field in the operating and working gaps thus inducing a voltage in the coil pairs. The presence of a high magnetic flux density in the gaps insures that the induced voltage will be relatively high for relatively small excursions of the generator. In addition, distortion of the flux path is reduced with a consequent reduction in the distortion of the output signal from the cartridge because of the concentration of the magnetic flux into a relatively small cross-sectional area defined by the cooperating faces of the pole piece extensions. This cooperation of the elements of my electromagnetic cartridge provides increased output signals with decreased distortion which is particularly important for electromagnetic cartridges for the recently introduced quadraphonic sound systems. Furthermore, since both polarities of magnetic structure are present in the replaceable module, existing cartridges can now be updated in performance with replaceable modules of new design.  
  While this invention has been described in some detail, this description does not limit the invention for it will be understood by those skilled in the art that variations and modifications may be made without departing from the spirit thereof or the scope of the following claims.  
 What I claim is:  
  l. A replaceable module for an electromagnetic cartridge comprising a carrier, first and second pairs of magnetic members mounted in opposing relationship on said carrier and cooperatively positioned to define first and second operating gaps, a stylus-lever operatively mounted on said carrier and extending generally parallel to the length dimension of said operating gaps, a generator operatively mounted on said stylus-lever and positioned in cooperative relation with said first and second pairs of magnetic members to define first and second working gaps of substantially equal length,  
 sound recordings, includinga replaceable stylusgenerator module comprising a support, a magnetic structure mounted on said support including a first magnetic member having a face defining one side of a module receiving cavity and first and second diametrically opposed pairs of pole pieces having respective front pole faces disposed .in a substantially common plane parallel to the face of said first magnetic member and defining the opposite side of said module receiving cavity, said module being dimensioned to be removably mounted within said module receiving cavity and including a carrier, a first magnetic extension means mounted on said carrier and adapted to be magnetically coupled to said first magnetic member, said first magnetic extension means including first and second pairs of diametrically opposed first pole piece extensions having pole faces disposed in a substantially common plane within the confines of said carrier and defining one side of first and second pairs of operating gaps, a second magnetic extension means including first and second pairs of diametrically opposed second pole piece extensions mounted on said carrier and adapted to be magnetically coupled to said pole pieces having pole faces disposed in a substantially common plane within the confines of said carrier and defining the other side of said first and second pairs of operating gaps and a stylus-lever generator assembly mounted in said module in operative relation to said first and second pairs of operating gaps and operable to track said record groove.  
  4. The cartridge of claim 3 wherein said generator is operatively disposed proximate said operating gaps and has opposing faces which cooperate with said front and rear pole piece extensions to define front and rear working gaps of substantially equal length.  
  5. The cartridge of claim 3 wherein the crosssectional area of the opposing faces of said front and rear pole piece extensions defining said operating gaps is less than the cross-sectional area of the front faces of said rear pole pieces.  
  6. An electromagnetic cartridge for the reproduction of sound recordings comprising a support, a magnetic structure mounted on said support and including a permanent magnet and a front magnetic member coupled to said permanent magnet, said front magnetic member including first and second pairs of rearwardly extending diametrically opposed front pole piece extensions having pole piece faces disposed in a substantially common first plane defining one side of first and second operating-gaps, first and second pairs of diametrically opposed rear pole pieces mounted on said support in cooperative relationship with said front pole piece extensions, said rear pole pieces having front faces disposed in a substantially common second plane parallel to said first plane and defining the other side of said first and second pairs of operating gaps, a stylus-lever generator assembly operatively mounted on said support and in operative relation to said first and second operating s and operable in response to tracking of said re- 1 groove.  
  An electromagnetic cartridge comprising a supa magnetic structure comprising a permanent ;net mounted on said support, a front magnetic nber having a rear face coupled to said permanent gnet, first and second pairs of diametrically opposed s of pole pieces mounted on said support and havfront pole faces disposed in a substantially common le substantially parallel to a plane defined by the face of the front magnetic member, said first pair ole pieces having their longitudinal center lines coiar at substantially 45 to the horizontal and said )nd pair of pole pieces having their longitudinal ter lines co-planar and at substantially right angles aid first pair, a module adapted to be removably se- :d between said planes defined by the faces of said pole pieces and the rear face of said front magnetic nber including a carrier, first and second pairs of netrically opposed rear pole piece extensions mounted on said carrier and adapted to be respectively magnetically coupled and in contact with said rear pole pieces. first and second pairs of diametrically opposed front pole piece extensions mounted on said carrier and adapted to be magnetically coupled and in contact with the rear face of said front magnetic member, said rear pole piece extensions having front faces disposed in a substantially common plane defining one side of first and second pairs of operating gaps, said front pole piece extensions having rear faces disposed in a substantially common plane defining the other side of said first and second pairs of operating gaps, a stylus-lever movably mounted on said carrier and extending generally parallel to the length dimension of said operating gaps and a generator operatively connected to said stylus-lever proximate said first and second operating gaps for movement in response to movement of said stylus-lever.