Abstract:
A ribbon microphone includes two magnets spaced in parallel and generating a magnetic field therebetween, two ribbon diaphragms arranged in parallel at a predetermined distance in the magnetic field, and a step-up transformer raising the voltages of electric signals generated in response to vibrations of the ribbon diaphragms in the magnetic field and outputs the raised electric signals. The step-up transformer includes two primary windings and two secondary windings corresponding to the two ribbon diaphragms, one of the two ribbon diaphragms and one of the two primary windings of the step-up transformer are connected in parallel whereas the others are connected in parallel, and the two secondary windings of the step-up transformer are connected in series so as to have opposite polarities. The ribbon microphone exhibits enhanced shielding effect without shielding a step-up transformer and does not generate noise caused by electromagnetic induction.

Description:
TECHNICAL FIELD 
     The present invention relates to a ribbon microphone and, in particular, to a technique for preventing noise caused by an external induction magnetic field in a ribbon microphone including two ribbon diaphragms (hereinafter simply referred to as “ribbons”) and a step-up transformer. 
     BACKGROUND ART 
     A ribbon microphone includes a microphone case accommodating a ribbon microphone unit, a step-up transformer, a circuit board, a connector, and any other component. The ribbon microphone unit includes, as its main components, two magnets generating a magnetic field and a conductive ribbon. These magnets are arranged on the two sides of the ribbon, and a magnetic field is generated between these magnets. The ribbon is disposed in the magnetic field while two ends in its longitudinal direction are held under proper tension. The ribbon vibrates in the magnetic field in response to sound waves, and a current corresponding to the vibration flows through the ribbon. In this manner, the sound waves are converted into electric signals. Each magnet has a rod shape which has a rectangular cross-section. The two magnets are arranged in parallel with each other while one surface in the width direction of one of the magnets faces that of the other magnet across the ribbon. An aluminum foil has been widely used as the material for the ribbon. Aluminum has higher conductivity and a lower specific gravity than any other metallic material and is thus suitable for a ribbon of a ribbon microphone. 
     A typical conventional ribbon microphone unit is configured such that one ribbon is arranged in one magnetic field generated by magnets. Another commercially available ribbon microphone has two ribbons that are arranged at a predetermined space in parallel with each other in one magnetic field and that are connected in series. With this configuration, the ribbon microphone can produce an output of double magnitude. Such a double-ribbon microphone unit is disclosed in Japanese Patent Laid-Open No. 2009-118118 issued to the assignee of this application. 
     In a ribbon microphone unit including two ribbons as disclosed in Japanese Patent Laid-Open No. 2009-118118, ribbons are arranged at two ends in the anteroposterior direction of magnetic poles, i.e., at positions corresponding to two ends in the thickness direction of magnets. The two ribbons are electrically series-connected as described above. Since aural signals outputted by the two ribbons are weak, the signals are outputted as a microphone output after the voltage of the signals is raised with a step-up transformer. A ribbon microphone is bidirectional, and the front and rear ribbons are set equally in acoustic terms such that aural signals produced by the front and rear ribbons are bidirectional. 
       FIG. 3  shows a conventional ribbon microphone unit provided with two ribbons arranged in one magnetic field. The ribbon microphone unit  10  (hereinafter simply referred to as “unit  10 ”) includes a yoke  12 , two magnets  15 , and two ribbons  16  and  17 . With reference to  FIG. 2 , which illustrates an embodiment of the present invention, the yoke  12  is a vertically long rectangular frame. The yoke  12  has the rod-shaped magnets  15  having a rectangular cross-section and fixed to opposed vertical inner walls, respectively, of the yoke  12  in parallel at a distance. These magnets  15  are magnetized in a direction orthogonal to the opposed surfaces of the magnets  15 , i.e., a direction orthogonal to the sheet surface in  FIG. 3 , and the magnetic poles of the magnets  15  are oriented in the same direction. As a result, a parallel magnetic field with a magnetic flux oriented in one direction is generated between the magnets  15 . 
     In the magnetic field, the two ribbons  16  and  17  are arranged. The two ends in the longitudinal direction of each ribbon  16  or  17  are fixed under proper tension to respective terminal portions provided at the two ends in the longitudinal direction of the yoke  12 . The ends of the ribbon  16  are electrically continuous with terminals  21  and  22  whereas the ends of the ribbon  17  are electrically continuous with terminals  23  and  24 . One end in the longitudinal direction of each of the ribbons  16  and  17 , i.e., the upper end in  FIG. 3  is connected by a wire to the corresponding terminal portion of the yoke  12  via the terminal  21  or  23 . The other end of the ribbon  16  is connected to one end of a primary winding  301  of a step-up transformer  30  via the terminal  22  whereas the other end of the ribbon  17  is connected to the other end of the primary winding  301  via the terminal  24 . Accordingly, the ribbons  16  and  17  are connected in series such that output signals from the ribbons  16  and  17  are inputted to the primary winding  301  of the step-up transformer  30 . The magnetic field extends over substantially the same range as the thickness of the magnets  15  (the lateral direction in  FIG. 3  (the anteroposterior direction)), and the ribbons  16  and  17  are arranged near the two ends, respectively, in the anteroposterior direction of the magnetic field. This is because the unit  10  is not bidirectional unless the front and rear ribbons  16  and  17  are set equally in acoustic terms. 
     As shown in  FIG. 3 , sound waves v 1  entering the ribbon microphone unit  10  from the front face of the ribbon  16  act on the ribbon  17 . For convenience, sound waves acting on the ribbon  17  will be denoted by reference characters v 2  hereinafter. The two ribbons  16  and  17  vibrate in response to the sound waves v 1  and v 2 . Electromagnetic conversion causes currents i 1  and i 2  corresponding to the sound waves v 1  and v 2  to flow through the ribbons  16  and  17 , respectively. Since the upper ends of the two ribbons  16  and  17  are connected in series via the terminals  21  and  23  in  FIG. 3 , the currents i 1  and i 2  flowing through the ribbons  16  and  17  are opposite in direction and are equal in magnitude. The current i 1  (=i 2 ) flows into the primary winding  301  of the step-up transformer  30 . 
     The step-up transformer  30  is an output transformer of the ribbon microphone unit  10 , has a turns ratio of as high as, for example, 1:70, and raises an output voltage of the unit  10  about 70 times and output the raised voltage. Not only a microphone unit including two ribbons as shown in  FIG. 3  but also a microphone unit including one ribbon outputs an extremely low voltage. Accordingly, the step-up transformer has a turns ratio of as high as 1:70. 
     SUMMARY OF INVENTION 
     As described above, the ribbon microphone including the step-up transformer  30  with a high rate of rise of voltage readily generates noise in aural signals by, for example, penetration of an induction magnetic field H from a commercial AC power supply into the step-up transformer  30 . For this reason, penetration of an induction magnetic field is conventionally prevented by covering the entire step-up transformer  30  with a shielding member, a shielding case, or any other shielding means. However, shielding of the entire step-up transformer  30  requires a bulky-shielding member. More secure shielding of the entire step-up transformer  30  requires a higher thickness of the shielding member. This results in a further increase in the size of the step-up transformer  30 . 
     An object of the present invention is to provide a ribbon microphone capable of solving problems with a conventional ribbon microphone, i.e., having enhanced shielding effect and not generating noise caused by electromagnetic induction without shielding a step-up transformer that is an output transformer, utilizing the structural feature of a ribbon microphone including two ribbons. 
     The ribbon microphone of the present invention includes: a pair of magnets spaced in parallel with each other, the pair of magnets generating a magnetic field therebetween; two ribbon diaphragms arranged in parallel with each other at a predetermined distance in the magnetic field between the pair of magnets; and a step-up transformer which raises the voltages of electric signals generated in response to vibrations of the two ribbon diaphragms in the magnetic field and outputs the electric signals, in which the step-up transformer includes two primary windings and two secondary windings corresponding to the two ribbon diaphragms, one of the two ribbon diaphragms and one of the two primary windings of the step-up transformer are connected in parallel with each other whereas the other of the two ribbon diaphragms and the other of the two primary windings of the step-up transformer are connected in parallel with each other, and the two secondary windings of the step-up transformer are connected in series so as to have opposite polarities. 
     The two ribbon diaphragms (hereinafter simply referred to as “ribbons”) vibrate in response to sound waves. Electromagnetic conversion generates electric signals corresponding to the sound waves in the ribbons. The electric signals generated in the ribbons are inputted to the respective primary windings of the step-up transformer, and the voltages of the electric signals are raised by the step-up transformer. Since the two secondary windings of the step-up transformer are connected in series so as to have opposite polarities, even if an external magnetic field penetrates into the step-up transformer, noises generated in the two secondary windings by electromagnetic induction are in opposite phase to each other and cancel each other out. The ribbon microphone thus can exhibit sufficient shielding effect without covering the entire step-up transformer with a magnetic shielding case made of an expensive material such as permalloy. Accordingly, a ribbon microphone including inexpensive compact shielding means can be provided. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a longitudinal sectional view provided with a circuit diagram illustrating a ribbon microphone according to an embodiment of the present invention; 
         FIG. 2  is a front view of the ribbon microphone according to the embodiment; and 
         FIG. 3  is a longitudinal sectional view provided with a circuit diagram showing a conventional ribbon microphone. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     A ribbon microphone according to an embodiment of the present invention will be described below with reference to  FIGS. 1 and 2 . The ribbon microphone unit has the same physical configuration as that of the conventional example shown in  FIG. 3 , and the same components are denoted by the same reference numerals. 
     Referring to  FIGS. 1 and 2 , a ribbon microphone unit (hereinafter simply referred to as “unit”)  10  includes a yoke  12 , two magnets  14  and  15 , and two ribbons  16  and  17 . The anteroposterior direction of the unit  10  corresponds to the lateral direction in  FIG. 1 .  FIG. 2  shows the unit  10  as seen from the front. As shown in  FIG. 2 , the yoke  12  has a shape of a vertically long rectangular frame. The yoke  12  has rod-shaped magnets  14  and  15  that have a rectangular cross-section and are fixed to opposed left and right vertical inner walls, respectively, of the yoke  12  with a predetermined space therebetween in parallel with each other. These magnets  14  and  15  are magnetized in a direction orthogonal to opposed surfaces of the magnets  14  and  15 , i.e., a direction orthogonal to the sheet surface in  FIG. 1  and the lateral direction in  FIG. 2 , and the magnetic poles of the magnets  14  and  15  are oriented in the same direction. A magnetic field with a parallel magnetic flux oriented in one direction is thus generated between the magnets  14  and  15 . 
     The two ribbons  16  and  17  are arranged in the magnetic field. The ribbons  16  and  17  in the illustrated embodiment each have a corrugated cross-section at a large portion extending in its longitudinal direction. The first corrugated portions of the ribbons  16  and  17  each have ridges parallel to the longitudinal direction. The ribbons  16  and  17  with the first corrugated portions have a certain degree of resiliency. Two ends in the longitudinal direction of each ribbon  16  or  17  are fixed under proper tension to terminal portions provided at two ends in the longitudinal direction of the yoke  12 . Each ribbon  16  or  17  has second corrugated portions, each being provided between the corrugated cross-sectional portion and the end fixed to the corresponding terminal portion, the second corrugated portion being oriented perpendicular to the first corrugated portion. The second corrugated portions of the ribbon  16  or  17  each have ridges parallel to the width direction. The second corrugated portions are referred to as resiliently deformable portions  161 ,  162 ,  171 , and  172 , respectively. The ribbon  16  has the resiliently deformable portions  161  and  162  whereas the ribbon  17  has the resiliently deformable portions  171  and  172 . With this configuration, the ribbons  16  and  17  can vibrate in reaction to sound waves. 
     As shown in  FIG. 1 , two step-up transformers  31  and  32  are provided to correspond to the two ribbons  16  and  17 . The step-up transformers  31  and  32 , respectively, raise the voltages of electric signals generated in the ribbons  16  and  17  in response to vibrations of the ribbons  16  and  17  in the magnetic field and output the electric signals. The step-up transformer  31  includes a primary winding  311  and a secondary winding  312  while the step-up transformer  32  includes a primary winding  321  and a secondary winding  322 . The step-up transformers  31  and  32  may be separately provided corresponding to the two ribbons  16  and  17  or may have a common core on which the two primary windings  311  and  321  of the step-up transformers are wound independently of each other and the two secondary windings  312  and  322  are wound independently of each other. The phrase “wound independently of each other” refers to “not wound so as to form a tapped continuous winding.” If the two step-up transformers  31  and  32  are separately provided, these step-up transformers  31  and  32  are arranged in the same orientation and in the same posture so as to be equally affected by an external magnetic field. 
     The electrical connections among the two ribbons  16  and  17  and the primary windings  311  and  321  and the secondary windings  312  and  322  of the step-up transformers will be described. As shown in  FIG. 1 , the two ends of the ribbon  16  are electrically continuous with terminals  21  and  22  whereas the two ends of the ribbon  17  are electrically continuous with terminals  23  and  24 . One end in the longitudinal direction of the ribbon  16 , i.e., the upper end in  FIGS. 1 and 2  is connected by a wire to a negative end of the primary winding  311  of the step-up transformer  31  via the terminal  21  whereas the lower end of the ribbon  16  is connected by a wire to a positive end of the primary winding  311  via the terminal  22 . One end in the longitudinal direction of the ribbon  17 , that is, the upper end in  FIGS. 1 and 2  is connected by a wire to the positive end of the primary winding  321  of the step-up transformer  32  via the terminal  23  whereas the lower end of the ribbon  17  is connected by a wire to the negative end of the primary winding  321  via the terminal  24 . Accordingly, the two ribbons  16  and  17  are connected in parallel with the primary windings  311  and  321  of the two step-up transformers  31  and  32 , respectively. More specifically, one of the two ribbon diaphragms and one of the two primary windings of the step-up transformers are connected in parallel, and the other of the two ribbon diaphragms and the other of the two primary windings of the step-up transformers are connected in parallel. Note that the ribbons  16  and  17  are connected to the respective primary windings at the ends opposite in polarity to each other. The secondary windings  312  and  322  of the two step-up transformers  31  and  32  are connected in series so as to have opposite polarities. In the unit shown in  FIG. 1 , a negative end of the secondary winding  312  and a negative end of the secondary winding  322  are connected, and positive ends of the secondary windings  312  and  322  output signals. 
     The operation of the ribbon microphone according to the embodiment and, more particularly, the operation of the step-up transformers  31  and  32  will be described. Assume that, as shown in  FIG. 1 , sound waves v 1  enter the ribbon microphone unit  10  from the front of the ribbon  16  and sound waves v 2  exits the ribbon microphone unit  10  from the back of the ribbon  17 . The sound waves v 1  and v 2  are substantially the same sound waves and are in phase with each other. The two ribbons  16  and  17  vibrate in response to the sound waves v 1  and v 2 , respectively. The ribbons  16  and  17 , which cross the magnetic flux between the magnets  14  and  15 , output signals corresponding to the sound waves v 1  and v 2 . Currents i 1  and i 2  shown in  FIG. 1  are electric currents which are generated by electromagnetic conversion and flow through the ribbons  16  and  17 , respectively. Since the two ribbons  16  and  17 , respectively, are connected in parallel with the primary windings  311  and  321  of the two step-up transformers  31  and  32  at the ends opposite in polarity to each other, the currents flowing through the primary windings  311  and  321  are in opposite phase each other. 
     At the secondary windings  312  and  322  of the two step-up transformers  31  and  32 , secondary currents are induced by the currents i 1  and i 2  flowing through the respective primary windings  311  and  321 . The currents flowing through the primary windings  311  and  321  are in opposite phase each other. Since the secondary windings  312  and  322  are connected in series so as to have opposite polarities, a current i 0  of one phase, which is the sum of the currents induced at the secondary windings  312  and  322 , flows through the secondary windings  312  and  322 . With the electrical connections among the two ribbons  16  and  17  and the two step-up transformers  31  and  32  shown in  FIG. 1 , output signals can be obtained in the above-described manner. 
     As described above with reference to the conventional ribbon microphone unit, the step-up transformers  31  and  32  are output transformers of the ribbon microphone unit  10 , have turns ratios of as high as, for example, 1:70, and raise output voltages of the unit  10  about 70 times and output the raised voltages. As described above, a ribbon microphone including a step-up transformer having such a high turns ratio (a high rate of rise of voltage) readily generates noise in aural signals by, for example, penetration of an induction magnetic field H from a commercial AC power supply into a step-up transformer. However, according to the illustrated embodiment of the present invention, the secondary windings  312  and  322  of the two step-up transformers  31  and  32  are connected in series with each other so as to have opposite polarities. With this configuration, noises caused by penetration of an induction magnetic field H into the step-up transformers  31  and  32  are in opposite phase each other and cancel each other out. Accordingly, the step-up transformers  31  and  32  can cancel noises caused by an induction magnetic field even if the entire step-up transformers  31  and  32  are not covered with a shielding case or any other shielding means, unlike conventional ribbon microphone units. The step-up transformers  31  and  32  can have a very simple shielding means. 
     Industrial Applicability 
     A ribbon microphone outputs a weak signal in spite of its large physical size and readily generates noise caused by an induction magnetic field. Such a problem prevents the spread of ribbon microphones. Application of the technical idea of the present invention can contribute to the spread of ribbon microphones.