JP4399371B2 - Microphone device - Google Patents

Description

  The present invention relates to a microphone device that uses an output transformer as a load when a plurality of electret condenser microphones are connected in parallel to a load (daisy chain connection).

  Among electret condenser microphones, drain output (2-wire type) electret condenser microphones that obtain output from the drain side of an FET (field effect transistor) are offered to the market under the name of “plug-in power system” (for example, Patent Document 1). An example is shown in FIG.

  According to this, the electret condenser microphone 10 includes a microphone capsule 11 having an electret material on one of the diaphragm and the fixed pole, and an FET (N-channel junction field effect transistor in this example) 12 as an impedance converter. In the plug-in power system, the load resistor RL and the DC power source Vcc of the microphone are connected in series with the drain D of the FET 12 so that the voltage change at both ends of the load resistor RL is obtained from the output terminals T1 and T2. I have to.

  Since the FET 12 used here is a built-in bias type in which a diode and a high resistance are connected between the gate G and the source S, the drain current is Idss (when the voltage between the gate G and the source S is 0). Of the drain current).

  Although only one electret condenser microphone 10 is shown in FIG. 3, by connecting a plurality of electret condenser microphones 10 in parallel to the load resistance RL, it is possible to synthesize audio signals from the respective microphones. it can.

  However, when a plurality of electret condenser microphones 10 are connected in parallel to the load resistance RL, the DC both-ends voltage of the load resistance RL changes depending on the number of connected. That is, when the number of connected terminals increases, the voltage across the DC of the load resistor RL approaches the power supply voltage of the DC power supply Vcc, thereby reducing the voltage between the drain D and the source S of the FET 12. There is a problem of not working.

  This problem can be solved by appropriately switching the resistance value of the load resistance RL according to the number of microphones to be connected. However, it is quite difficult for an installer or user to perform the switching operation. .

  Another method is to use an output transformer for the load. According to this, the voltage between the drain D and the source S of the FET 12 does not decrease depending on the number of microphones to be connected (current consumption). However, when a direct current is passed through the output transformer, another problem arises in that direct current magnetization occurs and the transformer performance deteriorates.

JP-A-8-33090

  Accordingly, an object of the present invention is to prevent DC magnetization of a transformer when an output transformer is used as a load in a microphone device in which a plurality of electret condenser microphones are connected in parallel to a load.

  In order to solve the above problems, the present invention includes a plurality of electret condenser microphones each including a microphone capsule having an electret material on either one of a diaphragm and a fixed pole, and an FET as an impedance converter. A microphone device in which a microphone is connected to a load in parallel includes an output transformer used as the load and a current mirror circuit, and a center tap is provided in a primary winding of the output transformer, and the primary side The drain of the FET included in each electret condenser microphone is connected in parallel to one winding portion of the winding, the source of the FET is connected in parallel to the current input side of the current mirror circuit, and the primary The other winding part of the side winding is connected to the current mirror circuit. While connected to the output side, it is characterized by being connected to the center tap of the DC power source of the microphone through the load resistor.

  In the present invention, the current ratio of the current mirror circuit is preferably 1: 1. Further, an AC blocking inductor can be used instead of the load resistance.

  According to the current mirror circuit, a current substantially equal to the current flowing on the current input side is generated on the current output side. According to the present invention, due to the action of such a current mirror circuit, the primary winding of the microphone output transformer has a current in the opposite direction in one winding portion and the other winding portion as viewed from the center tap. Therefore, direct current magnetization is not generated.

  Moreover, even if the number of microphones connected in parallel is increased or decreased, the current flowing through one winding section and the other winding section is balanced by the current mirror circuit, so no adjustment is required and the number of microphones is increased or decreased. However, a microphone device capable of daisy chain connection that operates normally can be realized.

  Next, an embodiment of the present invention will be described with reference to FIGS. 1 and 2, but the present invention is not limited to this. FIG. 1 is a circuit diagram schematically showing the configuration of a microphone device according to the present invention, and FIG. 2 is a schematic diagram showing a connection example of a daisy chain.

  As shown in FIG. 1, this microphone device includes a device main body 20 as a drive unit and an output unit of a microphone, and a plurality of, in this example, three electret condenser microphones connected in parallel to the device main body 20. 10a to 10c.

  The electret condenser microphones 10a to 10c may have the same configuration as the electret condenser microphone 10 described with reference to FIG. 3, and include a microphone capsule 11 having an electret material on either the diaphragm or the fixed pole, and an impedance converter. FET (field effect transistor) 12. The fixed pole is connected to the gate G of the FET 12, and the diaphragm is connected to the ground together with the source S of the FET 12.

  The apparatus main body 20 includes a microphone DC power supply Vcc, an output transformer 21 for outputting an audio signal as a load, and a current mirror circuit 24. The primary winding 22 of the output transformer 21 is divided into one winding portion 22a and the other winding portion 22b by a center tap 22c, and the electret condenser microphone 10a with respect to the one winding portion 22a. -10c are connected in parallel to the drain D of each FET12. The secondary winding 23 of the output transformer 21 is connected to a sound output circuit (not shown).

  The current mirror circuit 24 includes, as a basic configuration, a current input side transistor Q1 and a current output side transistor Q2, and the current input side transistor Q1 has a diode connection in which a base and a collector are coupled, and a collector voltage thereof is set. By applying the current to the base of the transistor Q2 on the current output side, a current having the same direction as the current input to the current input side is output (generated) between the collector and emitter of the transistor Q2 on the current output side.

  In this type of current mirror circuit, the current ratio can be arbitrarily set by selecting the number of transistors on the current input side and the current output side. A simplified current mirror circuit in which the transistor Q1 on the current input side is replaced with a diode may also be applied to the present invention. In this example, electrolytic capacitors C are connected in parallel on the current input side and the current output side, respectively, but this is a capacitor for AC grounding.

  The source S of each FET 12 included in the electret condenser microphones 10 a to 10 c is connected in parallel to the current input side of the current mirror circuit 24, and the primary side winding of the output transformer 21 is connected to the current output side of the current mirror circuit 24. The other winding part 22b of the wires 22 is connected.

  The DC power supply Vcc of the microphone is connected between the ground of the apparatus body 20 and the center tap 22c of the primary winding 22 of the output transformer 21. In this case, the DC power supply Vcc and the center tap 22c A load resistor RL is connected between them. An AC blocking inductor can be used instead of the load resistor RL.

  In the configuration described above, when the current I1 is supplied from the one winding portion 22a of the primary winding 22 of the output transformer 21 to the drain D of each FET 12 included in the electret condenser microphones 10a to 10c, the current I1 is The current flows between the drain D and the source S of the FET 12 and is input to the current input side including the transistor Q1 of the current mirror circuit 24.

  Then, a current I2 having the same direction as the current I1 is generated on the current output side including the transistor Q2 of the current mirror circuit 24. This means that a current I2 in the opposite direction to the one winding portion 22a is generated in the other winding portion 22b of the primary side winding 22 of the output transformer 21. DC magnetization is prevented.

  In order to prevent the DC magnetization of the output transformer 21, the current I1 and the current I2 are required to be substantially equal, so that the current ratio between the current input side and the current output side of the current mirror circuit 24 is 1: 1. It is preferable.

  As shown in the schematic diagram of FIG. 2, this microphone device is provided with a plug 31 and a jack 32 in each electret condenser microphone 10a to 10c to form a unit, thereby daisy chaining the necessary number of units to the device body 20. It can be connected (a daisy chain connection).

  In this case, even if the number of connected units is increased or decreased, the current mirror circuit 24 always balances the current I1 flowing through the one winding portion 22a and the current I2 flowing through the other winding portion 22b. A microphone device which is absolutely necessary and operates normally can be realized.

  A microphone device in which a plurality of electret condenser microphones according to the present invention are connected in parallel is used, for example, in a security system used in combination with a microphone device in a conference hall or a security camera.

1 is a circuit diagram schematically showing a configuration of a microphone device according to the present invention. The schematic diagram which shows the example of a connection of a daisy chain. The circuit diagram which shows the electret condenser microphone of the conventional drain output (2-wire type).

Explanation of symbols

10 (10a to 10c) Electret condenser microphone 11 Microphone capsule 12 FET
20 Device body 21 Output transformer 22 Primary winding 22a, 22b Winding portion 22c Center tap 24 Current mirror circuit 31 Plug 32 Jack

Claims (3)

A plurality of electret condenser microphones including a microphone capsule having an electret material on either one of a diaphragm and a fixed pole and an FET as an impedance converter, and each of the electret condenser microphones connected in parallel to a load In the microphone device,
An output transformer used as the load and a current mirror circuit are provided, a center tap is provided in the primary winding of the output transformer, and each electret condenser microphone is provided in one winding portion of the primary winding. The drain of the FET provided is connected in parallel, the source of the FET is connected in parallel to the current input side of the current mirror circuit, and the other winding portion of the primary winding is connected to the current mirror circuit. A microphone device, wherein the microphone device is connected to a current output side, and a DC power source of the microphone is connected to the center tap via a load resistor.   The microphone device according to claim 1, wherein a current ratio of the current mirror circuit is 1: 1.   The microphone device according to claim 1, wherein an AC blocking inductor is used instead of the load resistance.

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