JP5103130B2 - Power supply for condenser microphone - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To automatically control the drain voltage of an FET to a proper value, when supplying the drive voltage to a condenser microphone, having the drain of the FET as the signal output. <P>SOLUTION: A power unit is provided which has an electret condenser microphone unit 11 and the FET 12 as an impedance converter, and is connected to the condenser microphone 10, including the drain of the FET 12 as the signal output through a single-core shield coated microphone cable 20, wherein a visible light conductive element 35 is used as a load resistance to apply a power supply voltage to a signal output line 31, connected to the side of the drain D of the FET 12 via the core 21 of the microphone cable 20 from a DC power source 33 through the load resistance, a light-emitting diode 36 optically connected to the visible light conductive element 35 is driven by a light-emitting diode driving element 37, according to the drain voltage of the FET 12, and the resistance value of the visible light conductive element 35 is varied, according to whether the drain voltage of the FET 12 is high or low, thereby controlling the drain voltage of the FET 12 to a substantially constant value. <P>COPYRIGHT: (C)2009,JPO&amp;INPIT

Description

  The present invention relates to a power supply device applied to an electret condenser microphone that uses a drain of an FET (field effect transistor) used as an impedance converter as a signal output. The present invention relates to a technique for suppressing distortion.

  Some condenser microphones contain only an electret condenser microphone unit and an FET as an impedance converter in a unit case, and drive power is supplied from an external power supply device.

  This type of condenser microphone normally outputs an audio signal from the drain of the FET. A single-core shielded wire is used for the microphone cable, and the single-headed two-pole plug of the microphone cable is connected to the power supply device side. Since power is supplied by being inserted into a jack, it is called a plug-in power system (see, for example, Patent Document 1).

  A conventional example of a plug-in power type condenser microphone and its power supply device will be described with reference to FIG. The condenser microphone 10 includes an electret condenser microphone unit 11 and an FET 12 as its impedance converter, and these are housed in a unit case (not shown). The unit case is made of metal such as aluminum and serves as a shield case.

  The electret condenser microphone unit 11 is an acoustoelectric converter in which a diaphragm 11a that vibrates with an incoming sound wave and a fixed pole 11b are arranged to face each other via a spacer ring (not shown). Is provided with an electret dielectric film on the diaphragm 10a side, and in the case of the back electret type, an electret dielectric film is provided on the fixed pole 11b side. In either case, the diaphragm 10a side is connected to the ground terminal T2, and the fixed pole 11b side is connected to the gate G of the FET 12.

  The FET 12 normally has two diodes D1 and D2 connected in parallel between the gate G and the source S with the forward direction of current flow reversed, and a high diode connected in parallel to the diode group. A bias built-in type FET including the resistance element R1 is used, and the drain current of this type of FET 12 is fixed at Idss (the drain current value that flows when the voltage between the gate G and the source S is approximately 0 V). The drain D of the FET 12 is connected to the signal system terminal T1, and the source S is connected to the ground system terminal T2.

  The condenser microphone 10 is connected to the power supply device 30 via the microphone cable 20. In this case, the microphone cable 20 has a single-core shielded wire formed by coating a single-core wire 21 with a shield-coated wire 22. Is used.

  The core wire 21 connects the signal system terminal T 1 of the condenser microphone 10 to the signal system terminal T 3 of the power supply device 30. The shielded wire 22 connects the grounding terminal T2 of the condenser microphone 10 to the grounding terminal T4 of the power supply device 30.

  Actually, the microphone cable 20 is provided with a single-headed two-pole plug (not shown), and by inserting the single-headed two-pole plug into a jack (not shown) on the power supply device 30 side, the signal system terminals T1 and T3 are connected to each other. Connected to each other, the ground terminals T2 and T4 are connected to each other.

  In the power supply device 30, the signal output line 31 having the output terminal OUT1 is drawn from the signal system terminal T3, and the ground line 32 having the output terminal OUT2 is drawn from the ground system terminal T4, and is output from the output terminals OUT1 and OUT2. An audio signal is output. The signal output line 31 is connected to a DC cut capacitor C1.

  A DC power supply 33 and a load resistor 34 are connected in series between the ground line 32 and the signal output line 31, and a DC voltage is applied from the DC power supply 33 to the signal output line 31 via the load resistor 34. Accordingly, driving power is supplied from the power supply device 30 to the condenser microphone 10 as the single-headed two-pole plug of the microphone cable 20 is inserted into the jack on the power supply device 30 side.

  As described above, the plug-in power type condenser microphone 10 uses a single-core shielded wire for the microphone cable 20 and can output an audio signal with a single-headed two-pole plug. Widely used as a microphone for sound collection.

  By the way, when the drain D of the FET 12 is used as a signal output, if the voltage (potential) of the drain D is maintained near the center of the power supply voltage of the DC power supply 33, the signal output is distorted even if a large signal is applied. Hateful.

  The power supply device applied to the plug-in power system has no particular standard for the power supply voltage of the DC power supply or the resistance value of the load resistance, and the value differs depending on the manufacturer or model. In addition, since FETs are also manufactured by many manufacturers and sold by Idss rank, an appropriate capacitor microphone is selected from them.

  Therefore, depending on the combination of the power supply voltage of the DC power supply, the resistance value of the load resistance, and the Idss of the FET, the drain voltage may not be set to an appropriate value (near the center of the power supply voltage). The microphone may not work.

  In order to solve this point, the present applicant uses a transformer as a load of a power supply device applied to the plug-in power method in Patent Document 2, and cancels a direct current flowing in the transformer by a current mirror circuit. It has been proposed to keep the drain potential near the center of the power supply voltage while preventing the DC magnetization of the transformer.

  According to this, it is possible to operate a plurality of condenser microphones in parallel and daisy chain connection, but since a transformer is required, it is expensive and the apparatus itself must be large.

JP-A-8-33090 JP 2006-197284 A

  Accordingly, an object of the present invention is to provide a small and inexpensive power supply apparatus that can automatically control the drain voltage of an FET to an appropriate value when supplying a drive voltage to a capacitor microphone that uses the drain of the FET as a signal output. Is to provide.

In order to solve the above-mentioned problem, the invention according to claim 1 is a single core in a condenser microphone having an electret condenser microphone unit and an FET (field effect transistor) as an impedance converter, and using the drain of the FET as a signal output. A power supply device connected via a shielded microphone cable, a signal output line connected to the drain side of the FET via the core wire of the microphone cable, and a source side of the FET via the shield cover of the microphone cable A capacitor that supplies a DC voltage from the DC power source to the drain side of the FET via the load resistor and the signal output line. It can be used as the load resistance in a microphone power supply. A resistance value of the visible light conductive element by driving the light emitting diode according to a drain voltage of the FET appearing on the signal output line, and a light emitting diode optically coupled to the visible light conductive element A light emitting diode driving element for controlling the voltage, a drain voltage of the FET is input to a negative input terminal of the light emitting diode driving element, and a reference voltage obtained by dividing the voltage of the DC power source by a predetermined value is input to a positive input terminal. An input inverting amplifier is used .

According to a second aspect of the present invention, in the first aspect , the reference voltage input to the + input terminal is set to approximately ½ of the voltage of the DC power supply.

According to the first aspect of the present invention, a visible light conductive element is used as a load resistance, and a light emitting diode optically coupled with the load resistance is driven by the light emitting diode driving element in accordance with the drain voltage of the FET. By changing the resistance value of the visible light conductive element according to the voltage level, for example, even if a capacitor microphone is replaced and an FET with a different Idss is used, the drain voltage of the FET is set to a substantially constant value. Can be controlled. Further, by using an inverting amplifier for the light emitting diode driving element, the power supply device can be made inexpensive and downsized.

According to the second aspect of the present invention, by setting the reference voltage input to the inverting amplifier to approximately ½ of the voltage of the DC power supply, even if a large signal is applied, the signal output is hardly distorted. it can.

  Next, an embodiment of the present invention will be described with reference to FIG. 1, but the present invention is not limited to this. FIG. 1 is a circuit diagram showing a power supply device for a condenser microphone according to the present invention in a state where a condenser microphone is connected. In the description of this embodiment, the condenser microphone power supply device may be simply referred to as “power supply device”.

  In FIG. 1, the condenser microphone 10 and the microphone cable 20 may have the same configuration as the conventional example described in FIG.

  In the power supply device 30A of the present invention, a DC power supply 33 is connected between the signal output line 31 and the ground line 32 as in the conventional example described with reference to FIG. 2, but in the present invention, visible light is used as a load resistance. A conductive element 35 is used, and a DC voltage is applied to the signal output line 31 from the DC power source 33 through the visible light conductive element 35.

  The visible light conductive element 35 is a semiconductor sensor using a photoconductive effect in which the resistance value decreases as the amount of received light increases and the resistance value increases as the amount of received light decreases. For example, P1114-01 manufactured by Hamamatsu Photonics Co., Ltd. ) Etc. can be used.

  The power supply device 30 </ b> A includes a light emitting diode 36 that is optically coupled to the visible light conductive element 35 and a light emitting diode that drives the light emitting diode 36 according to the voltage (potential) of the drain D of the FET 12 that appears on the signal output line 31. And a drive element 37.

  The light emitting diode driving element 37 increases the resistance value of the visible light conductive element 35 by decreasing the current flowing through the light emitting diode 36 when the voltage of the drain D increases, and conversely, when the voltage of the drain D decreases, It operates so as to decrease the resistance value of the visible light conductive element 35 by increasing the flowing current.

  In this embodiment, an inverting amplifier 37 a is used for the light emitting diode driving element 37. The negative input terminal of the inverting amplifier 37a is connected to the signal output line 31 via the input resistor R3. In this embodiment, the resistor R2 and the electrolytic capacitor C2 are connected between the input resistor R3 and the signal output line 31. A filter circuit is interposed.

  The reference voltage is input to the + input terminal of the inverting amplifier 37a. This reference voltage is a driving voltage of the condenser microphone 10 applied to the drain D of the FET 12. In this embodiment, the reference voltage is obtained from a voltage dividing circuit having resistors Ra and Rb connected in parallel with the DC power supply 33. By setting Ra = Rb, a reference voltage in which the power supply voltage of the DC power supply 33 is approximately ½ can be obtained. The light emitting diode 36 is connected between the output of the inverting amplifier 37 a and the ground line 32.

  When the voltage of the drain D becomes higher than the reference voltage, the current supplied from the inverting amplifier 37a to the light emitting diode 36 decreases. Thereby, since the light irradiation amount of the light emitting diode 36 is reduced, the resistance value of the visible light conductive element 35 is increased, and the voltage of the drain D is lowered.

  On the other hand, when the voltage of the drain D becomes lower than the reference voltage, the current supplied from the inverting amplifier 37a to the light emitting diode 36 increases. Thereby, since the light irradiation amount of the light emitting diode 36 increases, the resistance value of the visible light conductive element 35 decreases, and the voltage of the drain D increases.

  In this way, the voltage at the drain D is controlled to be substantially constant with the reference voltage as the control target voltage. Therefore, for example, even if the condenser microphone 10 is replaced and the FET 12 having a different Idss is used, the drain voltage of the FET 12 can be controlled to a substantially constant value.

  Further, by setting the resistance values of the voltage dividing resistors Ra and Rb to be equal (Ra = Rb), the voltage of the drain D is maintained near the center of the power supply voltage of the DC power supply 33, so that even if a large signal is applied. The signal output is less distorted.

The typical circuit diagram which shows the power supply device for capacitor | condenser microphones of this invention in the state to which the capacitor | condenser microphone was connected. FIG. 2 is a schematic circuit diagram showing a conventional example of a plug-in power type condenser microphone and its power supply device.

Explanation of symbols

10 Condenser microphone 11 Electret condenser microphone unit 12 FET
20 Microphone cable 21 Core wire 22 Shield coated wire 30A Power supply device 31 Signal output line 32 Ground wire 33 DC power supply 35 Visible photoconductive element 36 Light emitting diode 37a Inverting amplifier Ra, Rb Voltage dividing resistor

Claims (2)

A power supply device having an electret condenser microphone unit and an FET (field effect transistor) as an impedance converter, and connected to a condenser microphone having the drain of the FET as a signal output via a single-core shielded microphone cable, The DC power supply has a load resistance between the signal output line connected to the drain side of the FET via the core wire of the microphone cable and the ground line connected to the source side of the FET via the shield coating of the microphone cable. In a power supply device for a condenser microphone that supplies a DC voltage from the DC power source to the drain side of the FET via the load resistor and the signal output line,
A visible light conductive element used as the load resistor, a light emitting diode optically coupled to the visible light conductive element, and driving the light emitting diode according to a drain voltage of the FET appearing on the signal output line. A light emitting diode driving element for controlling the resistance value of the visible light conductive element,
An inverting amplifier in which the drain voltage of the FET is input to a negative input terminal and a reference voltage obtained by dividing the voltage of the DC power source to a predetermined voltage is input to a positive input terminal. A condenser microphone power supply. The power supply device for a condenser microphone according to claim 1 , wherein the reference voltage input to the + input terminal is set to approximately ½ of the voltage of the DC power supply.

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