Abstract:
A transducer for a stringed instrument comprises a first uppermost coil and a second lowermost coil with the axes of the coils coincident. Permanent magnet pole pieces are arranged in the first coil and metallic non-magnetized pole pieces are arranged in the second coil. Oppositely directed U-shaped shields each having a web and outwardly directed opposed flanges are arranged back to back and receive the coils to shield the coils from each other both magnetically and inductively.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation-in-part of U.S. patent application Ser. No. 08/821,084 filed Mar. 20, 1997, now U.S. Pat. No. 5,834,999, which is a continuation-in-part of U.S. patent application Ser. No. 08/616,569 filed Mar. 15, 1996, now U.S. Pat. No. 5,668,520. 
    
    
     BACKGROUND OF THE INVENTION 
     This invention relates to transducers or pickups for stringed musical instruments whose output is intended to be amplified. In particular, the invention provides an improved noise cancellation pickup. 
     The invention will be described by way of example with reference to the musical instrument to which the pickups are fitted as being electric guitars. It should be appreciated that this is by way of example only and that instruments other than guitars may also be fitted with pickups according to the invention. 
     Electric guitars typically have at least four strings which when vibrated produce an output for amplification. The vibration of the strings is converted to electrical signals by pickups. The frequency of the electrical signals produced by the pickups corresponds to the frequency of vibration of the strings. 
     Pickups typically consist of a single bar magnet within a coil or a plurality of permanent magnets within a coil, the strings of the guitar are made of a magnetically permeable material typically a ferromagnetic material and the magnetic lines of flux developed by the permanent magnets are intercepted by the vibrating strings. This causes variations in the field pattern and an induced alternating current is caused to flow in the coils. The frequency of the current corresponds to the frequency of vibration of the strings. 
     The output from the coils is also subjected to noise. Noise is produced by lighting, electric motors and appliances and other sources. This noise, or hum adversely affects the quality of the sound reproduced by the pickups. The fundamental frequency of the electrical supply voltage, typically 50 Hz or 60 Hz, is converted into an audible hum in the amplifying equipment. 
     Many attempts have been made at ways of reducing or eliminating this noise but these attempts have introduced other undesirable effects. 
     Leo Fender in the 1940s was responsible for developing a single coil pickup. His design was particularly noise prone and equated basically to a long antenna for extraneous noise such as 50 Hz or 60 Hz hum and buzz caused by electric motors, lighting and other electrical appliances. 
     U.S. Pat. No. 4,442,749 issued to DiMarzio discloses one such earlier attempt at reducing noise. DiMarzio disclosed an electrical pickup device for stringed instruments. The device had a pair of superimposed coaxial bobbins each axially wound with a coil having its axis perpendicular to the strings of the instrument. An integral shield of magnetic material was present and had a base disposed between the two bobbins perpendicular to the coil axis and two side walls extend upwardly and perpendicularly from the base to at least immediately below the top face of the upper bobbin. A plurality of rod-like permanent magnets extended through the upper and lower coils and through the integral shield. Thus, a plurality of magnets common to both coils were arranged within the coils. 
     The shield extended around three sides of the pickup coil, that is the opposed sides parallel to the coil axis and the underside of the coil. The shield was not particularly effective and allowed the magnetic field in the pickup coil to influence the lower noise reducing coil to affect the inductance of the lower coil and the electrical signals induced into that coil. The tonal structure of the pickup as a whole was adversely affected when the inductance was reduced below an acceptable level and one way to remedy this was to overwind the coils. 
     DiMarzio in a first device employed magnetic pole pieces common to both coils and this prohibits attaining a suitable overall inductance value because of inductance cancellation between the two coils. 
     DiMarzio in a second embodiment discloses a pickup having an upper coil with a plurality of magnetic pole pieces arranged within it. A lower noise canceling coil is also shown. However it does not have a core within the lower coil. Although a channel-shaped member extends around the upper coil noise cancellation is achieved at the expense of tone quality. 
     An attempt at noise cancellation in pickup design was also made by Seymour Duncan. His design used full length Alnico V magnets which extended vertically through two coils. Like the DiMarzio design, the Duncan design also caused inductance and signal cancellation. Duncan did not employ any kind of magnetic barrier to separate the upper and lower coils. 
     A company known as EMG produced a pickup design known as Strat Vintage of SV. EMG employed full length magnets which extended through both an upper and a lower coil. Each coil was separately buffered into a two input differential operational amplifier but the inductance was less than 2.5H since the inductance of the top half coil was 0.8H. The lower coil was of similar inductance. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide an improved transducer or pickup for stringed musical instruments which provides for effective noise or hum cancellation while not sacrificing tone quality. 
     According to one aspect of the invention, there is provided a transducer having a first coil, a second coil arranged with its axis coincident with the axis of the first coil and in use spaced below the first coil, a metallic shield made of magnetically permeable material arranged between the coils, the shield having an outwardly directed wall extending over a side of one of the first coil or the second coil, at least one permanent magnet pole piece associated with the first coil and at least one metallic magnetically permeable pole piece associated with the second coil, whereby the coils are inductively and magnetically decoupled from one another by the shield. 
     There may be a unitary metallic magnetically permeable pole piece associated within the lower coil. In an alternative construction, a plurality of metallic magnetically permeable pole pieces are present. 
     There may be a unitary permanent magnetic pole piece associated with the upper coil. Preferably, a plurality of permanent magnet pole pieces are associated with the upper coil. The or each magnetic pole piece may be in contact with the associated shield. 
     In a further aspect this invention resides broadly in an electric guitar pickup having: 
     a lower coil assembly and an upper coil assembly supported coaxially above the lower coil assembly; 
     a continuous metallic shield made of magnetically permeable material supported between the upper and lower coil assemblies; 
     a plurality of permanent magnet pole pieces extending through the core of the upper coil assembly, and 
     a plurality of magnetically permeable pole pieces extending through the core of the lower coil assembly. 
     Preferably the continuous metallic shield includes at least one outwardly directed wall extending over a side of one of the upper coil assembly or said lower coil assembly. The upper and lower coil assemblies may each be supported by respective dielectric supports which locate the pole pieces within the respective cores of the coil assemblies. In one embodiment the dielectric supports includes an apertured dielectric wall adjacent each end of each coil assembly, the apertures therein being aligned with the respective pole pieces which extend therethrough for location and support. 
     In yet a further aspect this invention resides in a transducer comprising: 
     a first coil having a first axis; 
     a second coil having a second axis coincident with said first axis and spaced below the first coil; 
     a magnetic shield made of magnetically permeable material arranged between the coils, said shield having at least one outwardly directed wall extending over a side of one of said first coil or said second coil; 
     at least one first permanent magnet pole piece associated with said first coil, and 
     at least one magnetically permeable pole piece, separate from said first permanent magnet pole piece, associated with said second coil. Suitably said at least one first permanent magnet pole piece and said at least one magnetically permeable pole piece has a base contacting said magnetic shield. 
     With respect to any one of the above defined inventions the upper and lower coils may be wound with the same or different gauge of wire. Preferably, each of the coils has between 1000 to 7000 turns. More preferably, each coil has about 5000 turns. The coils need not have the same number of turns. 
     It is preferred that the coils be impedance matched or balanced and tuned so that the inductance of each coil is the same. This may be achieved by adopting a suitable wire gauge and number of turns for the coils and by the desired choice of the metallic pole pieces for the lower coil as discussed below. 
     The or each metallic pole piece for the lower coil are preferably made of mild steel although other metals are not excluded and may be in contact with the associated shield. Typically, the metallic shield made of magnetically permeable material and arranged between the coils, is made from mild steel and may have a thickness of about 0.4 mm. Respective non-metallic plates may be arranged on both sides of the lower coil. The non-metallic plates may have a plurality of apertures for receiving the pole pieces located within the lower coil. The metallic shield may be present as a tray having a base and a continuous upstanding wall. Alternatively, the shield may be U shaped having a base and two opposed upstanding side walls. The shield may be H shaped in transverse cross section and the lower coil may also be received between the downwardly directed side flanges. 
     As with the lower coils, respective non-metallic plates may be arranged on both sides of the upper coil. Of course, if the shield is H shaped in transverse cross section the upper coil is received between the upwardly directed side flanges. 
     The H shaped shield may be made as a unitary component or from several pieces. 
     Permanent magnet pole pieces of a number commensurate with the number of strings of the instrument to which the transducer is fitted are preferably arranged within the upper coil. Preferably, the non-metallic plates associated with the upper coil have apertures for receiving the magnetized pole pieces. Preferably, the pole pieces project through the apertures in the plate nearest to the instrument strings. 
     The magnetic pole pieces may be made from ALNICO II or ALNICO V or any other suitable magnetic material. 
     The two coils, because of the arrangement defined are both magnetically and inductively isolated from one another. The upper coil is subjected to the influence of the movement of the strings and noise while the lower coil is subjected only to noise. Because of the close proximity of the coils to one another, they respond equally to the effects of noise. By connecting the coils together either in parallel or series but out of phase, noise can be effectively canceled from the signal. 
     In yet a further aspect this invention resides broadly in an electric guitar pickup having a first coil, a second coil arranged with its axis coincident with the axis of the first coil and in use spaced below the first coil, a metallic shielding wall or walls made of magnetically permeable material and arranged between the coils for inductively and magnetically decoupling said coils from one another, at least one permanent magnet pole piece associated with the first coil and at least one metallic magnetically permeable pole piece associated with the second coil. 
     Preferably the electric guitar pickup has a plurality of permanent magnet pole pieces arranged within the upper coil and a plurality of metallic magnetically permeable pole pieces arranged within the lower coil and more preferably the first and second coils are wound with a different number of turns. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     A particular preferred embodiment of the invention will now be described by way of example with reference to the drawings in which: 
     FIG. 1 is an exploded perspective view of a transducer according to the invention; 
     FIG. 2 is an assembled perspective view of the transducer of FIG. 1; 
     FIG. 3 is a transverse sectional view of the transducer of FIG. 2; 
     FIG. 4 is a transverse sectional view of part of the transducer of FIG. 3; 
     FIG. 5 is a sectional elevational view of that part of the transducer shown in FIG. 4; 
     FIG. 6 is an exploded perspective view of a transducer according to another embodiment of the invention, and 
     FIG. 7 is a perspective view of an alternative shield for the pick up for the invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1 shows a transducer 10 having a non-metallic nonconductive base plate 11. Plate 11 has a series of holes 12 for receiving mild steel non-magnetized pole pieces 13, 14, 15, 16, 17, 18. Although all of these pole pieces are shown being the same length only the two outermost pieces 13 and 18 are full height. The innermost pole pieces 14, 15, 16 and 17 may be one third of the height shown and retained by top plate 19. Plate 19 is constructed of the same material as plate 11. A lower coil 20 extends around pieces 13 to 18 and is located between plates 11 and 19. Shield 21 has a web 22 and two opposed downwardly directed flanges 23, 24. These flanges extend over sides of the coil 20. Web 22 has rounded ends 25 (only one of which is visible in this view). Flanges 23 and 24 terminate half way across the outermost pole pieces 13 and 18 although they may extend beyond them if desired. 
     An upper coil 30 is arranged between plates 31 and 32. These plates are constructed of the same material as plates 11 and 19. Plates 31 and 32 have holes 33 for receiving magnetic pole pieces 34, 35, 36, 37, 38, 39. A shield 40 having a web 41 and opposed flanges 42, 43 together with shield 21 magnetically separate coil 30 from coil 20. Web 41 overlies and abuts against web 22. Flanges 42, 43 extend upwardly and over sides of the coil 30. Web 41 has rounded ends 44 (only one of which is visible in this view). Flanges 42, 43 terminate midway over the outermost pole pieces 34 and 39. 
     FIG. 2 shows an assembled perspective view of the transducer 10. The orientation assumed by strings 50, 51, 52, 53, 54, 55 relative to transducer 10 is shown. Coil 30 is shown closest to the strings while coil 20 is lowermost with the coils being coaxial with one another. The U shaped shields 21 and 40 effectively ensure that coil 20 is not subjected to the magnetic field of pole pieces 34, 35, 36, 37, 38, 39 and the magnetic field is directed towards the strings of the instrument to which the transducer 10 is fitted. 
     FIG. 3 is a transverse sectional view of the transducer 10 shown in FIG. 2. The shields 21 and 40 are shown surrounding the respective coils on three sides. The flanges 23 and 24 of shield 21 extend downwardly over sides of lower coil 20 while flanges 42 and 43 of shield 40 extend upwardly over the sides of coil 30. 
     Magnetic pole piece 37 is held between plates 31 and 32 as indeed are the other pole pieces not visible in this view. Webs 22 and 41 separate the coils from one another. Base plates 11 and plate 19 receive metallic pole piece 16 between them as indeed are the other pole pieces not visible in this view. Magnetic pole piece 37 extends a short distance beyond plate 31. So do the other magnetic pole pieces. 
     FIG. 4 shows a transverse sectional view through the shields 21 and 40 with only the permanent magnet pole piece 37 and the metallic magnetically permeable pole piece 16 shown. These shields may be made as a unitary H shaped field. 
     FIG. 5 is a front elevational view of that part of the transducer shown in FIG. 4. The shield 40 has a web 41 and upwardly extending flanges 42, 43 which terminate halfway over outermost permanent magnet pole pieces 34, 39. Shield 21 has a web 22 and flanges 23, 24 which extend downwardly over the metallic magnetically permeable pole pieces 14, 15, 16, 17 and halfway over pieces 13 and 18. 
     FIG. 6 shows an exploded perspective view of another transducer according to an embodiment of the invention. The transducer 60 has a base plate 61 constructed of a non-metallic material. Plate 61 has a slot 62 which receives a single mild steel core piece 63. A lower coil 64 locates about piece 63 and a plate 65 is positioned over the coil 64. A shield 66 extends over the coil 64 and has a base 67 with two opposed walls 68, 69. Walls 68, 69 extend over sides of the coil 64. 
     An upper coil 70 is present and rests upon lower plate 71. The coil 70 is received within shield 72. Shield 72 has a base 73 and opposed walls 74, 75 which extend over sides of the coil 70. A plate 76 extends over coil 70 and has a slot 77 for receiving permanent magnet pole piece 78. 
     In this embodiment, coil 70 has a single magnetic pole piece and a single metallic magnetically permeable pole piece is arranged within coil 64. 
     FIG. 7 shows an alternative shield construction. Shield 80 is tray shaped and has a base 81 and a continuous upstanding wall 82. Pole pieces 83, 84, 85, 86, 87, 88 are shown and may either be permanent magnets or may be metallic magnetically permeable depending upon whether shield 80 is used for an upper or lower coil. 
     It is not necessary for the shields in a transducer to be both as shown in FIG. 7 or both of the type shown in FIG. 6. Only one or one of each may be used. Likewise, a plurality of pole pieces may be present within one of the coils and a single pole piece may be present in the other of the coils. 
     It is preferred that the inductance and impedance of the two coils be matched by proper choice of number of turns, wire gauge and size of the pole piece or pieces within the coils with the object of achieving a Q factor equivalent to a vintage pickup such as a an early FENDER STRATOCASTER pickup while at the same time minimizing noise from the output signal thereof.