Moving Coil-Type Cartridge

This invention relates generally to phonograph cartridges and, more particularly, to improvements in such cartridges of moving coil type.

Generally the cartridges of the moving coil type (which shall be referred to as MC-type cartridges hereinafter) have been excellent in frequency characteristics but defective in that their output has been smaller as compared with cartridges of moving magnet type (MM-type cartridges). In FIG. 1, there is shown a general structure of the MC-type cartridges for reproducing stereo sound, in which 101 is a base member consisting of a synthetic resin mold, 102 is a permanent magnet secured to said base member by means of a bonding agent, 103 is a pair of pole pieces respectively secured to each of different polarity ends of the permanent magnet 102, 104 is a bobbin having coils would therearound and supported within a gap between the respective pole pieces 103 by means of dampers 105, and 106 is a cantilever of a rod shape having at an end a stylus and fixed at the other end to the center of a side wall of the bobbin 104. In the MC-type cartridges of the structure of the kind as illustrated, the fixing position of the cantilever 106 to the bobbin 104 is made to be on the central axis of the bobbin which intersecting at right angles distribution direction of magnetic fluxes between the respective pole pieces 103 so that the center of rotations of the coils on the bobbin 104 responsive to variations of sound record groove in its width on phonograph disk (not shown) will be coincided with actual center of the coils, whereby rotational moving directions of respective coil sections facing each pole piece will be opposite to each other with respect to the flux direction and rotary radiuses of such coil sections will be substantially equal to one another, consequent to which electromotive forces substantially of the same magnitude and mutually of opposite polarities will be produced in the respective said coil sections, and electric currents generated due to such forces in the respective coil sections will be superposed on each other so as to be obtained at output ends of the coils as an output of the cartridge. In practice, however, it is very difficult to have the coil center and rotary center of the bobbin and coils coincided with each other on physical and manufacturing technological viewpoints, for example, due to possible errors in dimensions of the respective dampers for supporting the coil bobbin and, even when the coincidence of the coils' and their rotary centers with one another is achieved, the radius of the coil rotation is thereby limited to be small anyhow, so that the obtained electromotive forces and consequential output of the coil will be inherently limited to be small. It may be possible to render the coil rotation radius to be larger by increasing, for example, the thickness of the bobbin in the direction of fluxes flowing. In this case, however, it becomes necessary to enlarge the gap between the pole pieces and, consequent thereto, it will be unavoidable that the size and weight of the cartridge have to become larger as a whole, and that the magnetic field intensity in the gap is weakened and thus the electromotive forces to be obtained are ill influenced. The present invention has been suggested in view of such defects in conventional MC-type cartridges and thus to provide an improved MC-type cartridge wherein the coil rotation center is positioned outside the coil bobbin so as to enlarge the coil rotation radius while rendering the manufacture thereof to be easy and further the distribution of the magnetic fluxes is caused to vary with reference to the coil roration axis so as to be able to obtain effectively large electromotive forces.

A principal object of the present invention is, therefore, to provide an MC-type cartridge which can be manufactured easy and yet is capable of obtaining higher outputs.

Another object of the present invention is to provide an MC-type cartridge of which reliability is high with less fluctuations in output performances .

Other objects and advantages of the present invention will be made clear upon reading the following explanation of the present invention detailed with reference to certain preferred embodiments thereof as shown in accompanying drawings, in which:

FIG. 1 is a side elevation with a part removed of an exemplary one of conventional MC-type cartridges;

FIG. 2 is a perspective view with a part removed of a preferred embodiment in somewhat magnified scale of an MC-type cartridge specifically for stereo recorded-sound reproducing use according to the present invention;

FIG. 3 is a sectioned view of a main part of the embodiment shown in FIG. 2;

FIG. 4 is a fragmentary magnified section showing a support mechanism for coil bobbin in the embodiment of FIG. 2; and

FIGS. consisting of 5A and 5B, and 6 are explanatory views for showing schematically basic operational principle of the present invention in its different aspects, wherein FIG. 5A is a fragmentary magnified section at the part of the pole pieces and coil bobbin in an aspect, FIG. 5B is a schematic perspective view of a part thereof, and FIG. 6 is a similar section to FIG. 5A showing the other aspect.

Referring now to FIGS. 2 and 3 showing the most preferrable embodiment of the present invention, a stylus 1 is secured at a tip of a cantilever 2 of a nonmagnetic material in the present instance, a coil bobbin 3 substantially in a square box shape of a nonmagnetic and electrically insulative material is fixed at its central axis to the cantilever 2 adjacent the other end of the same, and two pairs of coils 4, 4' and 5, 5' for coordinating with right and left channels in stereo record groove in the present instance are wound respectively adjacent each peripheral edge of the bobbin 3 so that each of the pair of coils 4 and 4' or 5 and 5' will be in parallel with one another. The respective coils 4 and 4' or 5 and 5' of each pair are connected with each other in series, and the respective ends of the coil pairs 4, 4' and 5, 5' are forming output terminals 15, 15' and 16, 16', respectively. Pole pieces 6 and 7 made of a magnetic material are respectively formed substantially in an L-shape and are disposed so as to hold at a side surface adjacent one end of them a bar-shaped permanent magnet 8 and to form a magnetic gap in a magnetic circuit of the pole pieces 6 and 7 and permanent magnet 8 between opposing the other end surfaces of the respective bent parts. These bent parts of the pole pieces 6 and 7 forming the magnetic gap are provided with through holes 9 and 10, respectively, which are extending along central axis of the gap. The cantilever 2 penetrates through the hole 9 so as to extend at its end having the stylus 1 out of the pole piece 6 while positioning the coil bobbin 3 at the center of the magnetic gap. The cantilever 2 is supported in position by means of a support member 11 made of a nonmagnetic material and fixedly fitted in the hole 10 in the other pole piece 7, in such that the other end of the cantilever 2 adjacent which the bobbin 3 is supported is fixed to the center of the member 11.

There is shown in FIG. 4 detailes of an example of the above referred supporting structure of the cantilever 2 and coil bobbin 3, in which the cantilever 2 consists of a hollow pipe in the present instance, a filler member 12 made of a nonmagnetic material and having a central hole is fitted tightly in the hollow end of the cantilever 2 extending out of the bobbin 3, and such a string 13 having a proper resiliency as a piano wire is fixed at an end in the central hole of the filler 12 and at the other end in a central hole of the support member 11 fixed in the pole piece 7. Therefore, the cantilever 2 with the coil bobbin 3 is supported on the aligned central axes of the hole 9 in the pole piece 6 and of the magnetic gap in a manner rotatable about a fixing base point 14 of the string 13 as fulcrum.

That is, in the operation of the embodiment of FIGS. 2 and 3, the coil bobbin 3 is to be rotated about the fulcrum 14 which is positioned at an external point extended out of the central axis of the bobbin 3 in the direction in which magnetic fluxes flow from the pole piece 6 to the opposing pole piece 7 since in the present embodiment the pole piece 6 through which the cantilever 2 penetrates is magnetized in N-pole and the other pole piece 7 on the fulcrum side of the cantilever 2 as shown, in response to variations of the width of sound recorded groove which the stylus 1 traces. Therefore, the rotation radius of the respective coil sections 4a, 4'a and 5a, 5'a (see FIG. 3) of the respective coils on the bobbin 3 which are positioned on the side remote from the fulcrum 14 of the bobbin 3 is caused to be larger than that of another coil sections 4b, 4'b and 5b, 5'b positioned on the side closer to the fulcrum 14, consequent to which the intersecting velocity of the coil sections 4a, 4'a and 5a, 5'a through the fluxes in the gap in each rotation becomes larger than that of the coil sections 4b, 4'b and 5b, 5'b. If the respective rotation radiuses of the respective coil sections are the same with each other and hence the intersecting velocities are also the same, the magnitude of electromotive forces generated in such coil sections will be also the same on the respective sides of the bobbin and there can be obtained none of output at the coil terminals since the directions of electric currents produced are identical on the both sides. According to the present invention, on the other hand, it is possible to obtain an output at the coil terminals due to the difference in the velocities of the respective coil sections on both sides of the bobbin at which the coils intersecting the magnetic fluxes in the gap, even though the directions of the electric currents resulting from the electromotive forces generated in such coil sections are the same with each other, since the electromotive forces generated in the coil sections 4a, 4'a and 5a, 5'a on the side remote from the coil rotation fulcrum 14 are larger than those generated in the coil sections 4b, 4'b and 5b, 5'b on the other side due to the difference in the flux intersecting velocities.

In order to improve such effect of the particular fulcrum positioning as described above, in the embodiment of FIGS. 2 and 3, the diameter d.sub.1 of the hole 9 in the pole piece 6 is made smaller than the diameter d.sub.2 of the hole 10 in the opposing pole piece 7, while outer diameters of the both pole pieces are substantially the same. With this arrangement, the magnetic fluxes flowing from the pole piece 6 magnetized to be N-pole to the pole piece 7 magnetized to be S-pole are caused to be distributed in such that, as shown in FIG. 3, the fluxes in the outer peripheral part in the gap will be parallel to the central axis of the magnetic gap but the fluxes in the inner part will be gradually converged outward at the side of the pole piece 7 as they approach the central axis so as to pass more diagonally through the bobbin 3. Consequently, the coil sections 4a, 4'a and 5a, 5'a which facing the pole piece 6, i.e. N-pole, are almost all positioned inside the magnetic fluxes whereas the other coil sections 4b, 4'b and 5b, 5'b facing the pole piece 7, i.e. S-pole, are only partly positioned inside the fluxes with certain parts of them positioned outside the fluxes. That is, the respective coil sections facing the N-pole are disposed in the fluxes so that almost all of them will be effective to generate the electromotive force but those facing the S-pole are disposed so that they will be only partly effective to generate such force.

FIGS. 5A and 5B are to show a basic aspect of such flux distribution as described above. It should be appreciated here that, while in the embodiment of FIGS. 2 and 3 there is shown a case where the pole pieces having the same outer diameters but different inner diameters are employed, both the outer and inner diameters may be made larger on the S-pole side of the pole piece 7 as shown in FIG. 5A under the basic principle of the present invention, while the wall thickness of the both pole pieces is kept substantially equal. In this case, the magnetic fluxes flowing from the N-pole to the S-pole are not converged at the S-pole side but, as shown by respective two parallel arrows, they are directed outward at the side of the fulcrum 14 with respect to the central axis of the magnetic gap which is here represented by a chain line 2 denoting axial line of the cantilever 2, so that the fluxes will pass all diagonally through the coil bobbin 3, whereby there are caused certain parts of the coil sections 4b, 4'b and 5b, 5'b on the S-pole side to be positioned outside the fluxes consequentially, in the same manner as in the case of FIGS. 2 and 3 and thus the same effect can be achieved.

FIG. 5B is a perspective view of an upper half part of FIG. 5A with the respective pole pieces shown schematically in flat plate shape for simpler explanation. In the drawing, the diagonal magnetic fluxes as defined by four arrows pass diagonally through each coil 4 or 5 defined by chain lines so as to enclose in the fluxes substantially all of the coil section on the N-pole side represented by a hatched area a but to allow only such a part of the other section on the S-pole side as represented by a hatched area b to be enclosed in the fluxes. It will be here understood that, when the coil is moved within the fluxes in vertical direction in the drawing, the electromotive forces of the same direction denoted by arrows e and e' are generated in the respective hatched parts a and b effective to generate such electromotive forces of the coil sections facing the N and S poles, respectively, and that the effective part a is much larger in the area and hence in the number of coil turns than the other effective part b. For this reason, the generated electromotive forces e and e' of the same direction will be caused to have a relation e > e' and, consequently, an output corresponding to their difference e - e' is obtained.

It is thus possible to generate effective electromotive forces similarly to the case where the rotation fulcrum of the coil bobbin is positioned outside the bobbin, also by constructing the magnetic gap in such manner that, as has been described above, the magnetic fluxes in the gap between the pole pieces will flow in the directions in which they will be away from the coil rotation fulcrum and thus will be diagonal with respect to the coils at least in the range closer to the central axis of the coil bobbin positioned at the center of the gap. The effect obtained by such magnetic gap construction enables it possible to obtain a larger cartridge output in conjunction with the effective electromotive forces obtained as a result of the difference in the rotation radiuses of the respective coil sections on both sides of the bobbin.

FIG. 6 shows in the similar sectional view to FIG. 5A an aspect of the present invention wherein substantially the same magnetic gap construction as in the embodiment of FIGS. 2 and 3 is employed. In this aspect of FIG. 6, the difference in the inner hole diameters of the respective pole pieces 6 and 7, that is, the difference of the diameter of the hole 9 in the pole piece 6 from that of the hole 10 in the other pole piece 7, is made as large as possible, whereas the outer diameters are made identical with each other. This will result in that the magnetic fluxes from the N-pole 6 to be remarkably converged toward the narrow end surface of the S-pole 7 so that the fluxes in the range closer to the central axis of the bobbin will pass through the coils diagonally at larger angles with respect to the axis. The coil sections on the N-pole side of the bobbin are thereby caused to be exposed to the fluxes having much wider distribution range and, on the other hand, the coil sections on the S-pole side are to be exposed only to the fluxes of much narrower distribution range. Therefore, it becomes possible to wind the coils on the bobbin with much more turn number than in the case of FIGS. 2 and 3 or of FIG. 5 so that the difference between the respective electromotive forces e and e' generated on the N-pole and S-pole sides will be remarkably larger, whereby it is made possible to obtain a larger output.

Further, in the case where the supporting fulcrum 14 for the cantilever 2 and thus the rotation center of the coil is provided at a more remote position as shown in FIG. 6 than in the cases of FIGS. 2 and 3 and FIG. 5, it is possible to have the rotation radius of the coils enlarged. In practice, however, the maximum rotation radius specifically at the position of the stylus on the tip of the cantilever is determined by the width of the sound recorded groove and, therefore, as regards the effect of the fulcrum positioning at more remote point from the bobbin, it should be noted that rotational directions of the whole coils can be made more closer to directions intersecting at right angles the central axial line common to the cantilever and coils in their stationary position than in the case in which the fulcrum is closer to the coil bobbin. Consequent to this fact, the respective coil sections facing the N and S poles can be moved to intersect the magnetic fluxes at an angle closer to the right angle with respect to the flux flowing directions and, further, the coil sections specifically on the N-pole side which having larger rotation radius are provided with optimum conditions effective to produce larger electromotive forces therein as compared with those sections on the S-pole side since converging degree of the fluxes to which the N-pole side coil sections are exposed is smaller and thus more fluxes are substantially parallel to the magnetic gap axis than on the S-pole side, so that the desired effect of the invention becomes more remarkable.

While the explanation has been made with reference to the particular arrangement of the foregoing embodiments wherein the fulcrum 14 for the coil bobbin 3 is disposed outside the bobbin on the S-pole side and the magnetic fluxes flowing through the magnetic gap from N-pole of the pole piece 6 to S-pole of the other pole piece 7 are distributed so as to be away from the fulcrum 14 in said position as the fluxes approach the fulcrum, this is for the reason that the provision of the fulcrum 14 at the above described position, in other words, on the side opposit to the tip on which the stylus 1 is provided of the cantilever 2 with respect to the coil bobbin 3 is more advantageous in the practical manufacturing of the cartridge. Theoretically, it is possible to provide the fulcrum or supporting point 14 of the cantilever 2 on the same side with the stylus 1 with respect to the coil bobbin 3. In this case, it is necessary to render the inner diameter of the N-pole side pole piece 6 to be larger and also the inner diameter of the S-pole side pole piece 7 to be smaller in contrast to the foregoing embodiments so that the magnetic fluxes will be distributed so as to be away from the fulcrum 14 in the region adjacent the fulcrum and will approach the central axial line while flowing through the gap and the cantilever as well as the bobbin, as the fluxes advance away from the fulcrum. With this arrangement, the effect that the electromotive forces generated in the coil sections on a side of the bobbin 3 which is, in this case, the side facing the S-pole may be made larger than those generated on the other side can be achieved.

While in the foregoing the present invention has been explained mainly with reference to the illustrated embodiments, the intention is not to limit the invention only to those aspects shown but is rather to cover all modifications, alternatives and equivalent arrangements to be included in the scope of appended claims.

For example, the embodiment shown in FIG. 2 represents an aspect which is considered most advantageous in the viewpoint of manufacturing and utility in practising the present invention, in which the pole pieces 6 and 7 are shown as having substantially an L-shape. However, it will be readily appreciated that the magnetic circuit may be properly constituted with elements including the pole pieces and the permanent magnet as well of other shapes and in any other arrangement than those shown, as long as the basic principle of the present invention explained with reference to FIGS. 5 or 6 is performed.

Further, the MC-type cartridge according to the present invention is not necessarily restricted to the one for stereo recording or reproducing. Rather, the present invention is applicable also to the cartridge for monoral recording and reproducing when the coil structure and arrangement are properly modified.

In the respective embodiments described, further, the cantilever has been referred to as being in the form of straight rod or pipe and passed through a hole in one of the pole pieces respectively having the through hole. However, it is not necessary to form and dispose the cantilever and pole pieces always as disclosed but is possible to provide such hole only in one of the pole pieces on the side of the supporring fulcrum for the coil bobbin and cantilever as long as the stylus on the cantilever can be positioned out of the magnetic gap in any proper manner.