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DESCRIPTION JP2009105523

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DESCRIPTION JP2009105523
An object of the present invention is to automatically control a 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. The power supply apparatus includes an electret condenser
microphone unit 11 and an FET 12 as an impedance converter, and is connected to a condenser
microphone 10 having a drain D of the FET 12 as a signal output via a microphone cable 20 with
a single core shield coating. When applying a power supply voltage from the DC power supply
33 via the load resistance to the signal output line 31 connected to the drain D side of the FET
12 via the core wire 21 of the microphone cable 20, the visible light conductive element 35 is
used as the load resistance. The light emitting diode 36 optically coupled thereto is driven by the
light emitting diode drive element 37 according to the drain voltage of the FET 12, and the
resistance value of the visible light conductive element 35 is changed according to the high or
low of the drain voltage of the FET 12. , The drain voltage of FET 12 almost constant To control
the value. [Selected figure] Figure 1
Power supply for condenser microphone
[0001]
The present invention relates to a power supply device applied to an electret condenser
microphone that uses as the signal output the drain of an FET (field effect transistor) used as an
impedance converter, and more specifically, to the signal output due to fluctuations in drain
voltage (potential). The present invention relates to a technology for suppressing distortion.
[0002]
Among the condenser microphones, there are those in which only the electret condenser
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microphone unit and the FET as its impedance converter are accommodated in a unit case, and
drive power is supplied from an external power supply device.
[0003]
This type of condenser microphone normally outputs an audio signal from the drain of the FET,
and a single-core shielded cable is used for the microphone cable, and the single-headed two-pole
plug of the microphone cable is used on the power supply side. Since power is supplied by
plugging into a jack, it is called a plug-in power system (see, for example, Patent Document 1).
[0004]
A plug-in power type condenser microphone and a conventional example of the 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, which are housed in a unit case (not shown).
The unit case is made of metal such as aluminum and doubles as a shield case.
[0005]
The electret condenser microphone unit 11 is an acousto-electric converter in which a
diaphragm 11a vibrating with an incoming sound wave and a fixed pole 11b are disposed
opposite to each other through a spacer ring (not shown), and in the case of a film electret type.
An electret dielectric film is provided on the diaphragm 10a side, and in the case of the back
electret type, the electret dielectric film is provided on the fixed electrode 11b side.
In either case, the diaphragm 10 a side is connected to the ground system terminal T 2, and the
fixed pole 11 b side is connected to the gate G of the FET 12.
[0006]
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In the FET 12, normally, two diodes D1 and D2 connected in parallel in the reverse direction of
the current flow between the gate G and the source S, and a high connected in parallel to this
diode group A bias built-in 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 flowing 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.
[0007]
The condenser microphone 10 is connected to the power supply device 30 via the microphone
cable 20. In this case, the microphone cable 20 is a single-core shield-covered wire formed by
covering the single core wire 21 with the shield-covered wire 22. Is used.
[0008]
The core 21 connects the terminal T 1 of the signal system of the condenser microphone 10 to
the terminal T 3 of the signal system of the power supply device 30.
The shield covering wire 22 connects the terminal T 2 of the grounding system of the condenser
microphone 10 to the terminal T 4 of the grounding system of the power supply device 30.
[0009]
In practice, a single-headed two-pole plug (not shown) is provided on the microphone cable 20,
and the single-headed two-pole plug is inserted into a not-shown jack on the power supply device
30 side to connect the signal terminals T1 and T3 to each other. The terminals T2 and T4 of the
grounding system are connected to each other.
[0010]
In the power supply device 30, the signal output line 31 having the output terminal OUT1 is
drawn out from the terminal T3 of the signal system, and the ground line 32 having the output
terminal OUT2 is drawn out from the terminal T4 in the grounding system. An audio signal is
output.
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The signal output line 31 is connected to a capacitor C1 for direct current cut.
[0011]
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. Thus, as the single-headed two-pole plug of the
microphone cable 20 is inserted into the jack on the power supply device 30 side, driving power
is supplied from the power supply device 30 to the condenser microphone 10.
[0012]
As described above, this plug-in power type condenser microphone 10 uses a single-core
shielded cable for the microphone cable 20 and can output an audio signal with a single-head
two-pole plug, so that it is inexpensive. It is widely adopted as a sound collection microphone.
[0013]
When the drain D of the FET 12 is used as a signal output, if the voltage (potential) of the drain D
is kept 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.
[0014]
The power supply apparatus applied to the plug-in power system has no specific standard on 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.
Also, since FETs are also manufactured by many manufacturers and sold under Idss rank
classification, among them, an appropriate one is selected as a condenser microphone.
[0015]
Therefore, depending on the combination of DC power supply voltage, load resistance, and FET
Idss, the drain voltage may not be set to an appropriate value (near the center of the power
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supply voltage). The microphone may not work.
[0016]
In order to solve this point, in the patent document 2, the present applicant uses a transformer as
the load of the power supply device applied to the plug-in power system, and cancels the direct
current flowing in the transformer by the current mirror circuit. It is proposed to keep the
potential of the drain near the center of the power supply voltage while preventing DC
magnetization of the transformer.
[0017]
According to this, although a plurality of condenser microphones can be operated in parallel and
daisy chain connection can be made, since a transformer is required, it is expensive and the
apparatus itself has to be large.
[0018]
JP-A-8-33090 JP-A-2006-197284
[0019]
Therefore, an object of the present invention is to provide a compact and inexpensive power
supply device capable of automatically controlling the drain voltage of the 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. To provide.
[0020]
In order to solve the above problems, the invention according to claim 1 has an electret
condenser microphone unit and an FET (field effect transistor) as an impedance converter,
wherein a single core of the condenser microphone uses the drain of the FET as a signal output.
A power supply device connected via a shield-coated microphone cable, the signal output line
connected to the drain side of the FET via the core of the microphone cable, and the source side
of the FET via the shield coating of the microphone cable A DC power supply is connected via a
load resistor to a ground line connected to the capacitor, and a capacitor for supplying a DC
voltage from the DC power supply to the drain side of the FET via the load resistance and the
signal output line In the power supply for microphones, it may be used as the load resistance The
resistance value of the visible light conductive element is driven by driving the light emitting
diode according to the photoconductive element, the light emitting diode optically coupled to the
visible light conductive element, and the drain voltage of the FET appearing on the signal output
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line. And a light emitting diode driving element for controlling the light emitting diode.
[0021]
According to a second aspect of the present invention, in the first aspect, the drain voltage of the
FET is input to the-input terminal of the light emitting diode drive element, and the voltage of the
DC power supply is divided to a predetermined level to the + input terminal. It is characterized in
that an inverting amplifier to which a reference voltage is input is used.
[0022]
The invention according to claim 3 is characterized in that, in claim 2, the reference voltage input
to the positive input terminal is set to approximately one half of the voltage of the DC power
supply.
[0023]
According to the first aspect of the present invention, a visible light conducting element is used
as the load resistance, and the light emitting diode optically coupled thereto is driven by the light
emitting diode driving element according to the drain voltage of the FET. By changing the
resistance value of the visible light conductive element according to the level of the voltage, for
example, even when the condenser microphone is replaced and different FETs having different
Idss are used, the drain voltage of the FET is made substantially constant. Can be controlled.
[0024]
According to the second aspect of the present invention, by using the inverting amplifier as the
light emitting diode drive element, the power supply apparatus can be made inexpensive and
miniaturized.
[0025]
According to the third aspect of the invention, by setting the reference voltage input to the
inverting amplifier to approximately one half of the voltage of the DC power supply, the signal
output is less likely to be distorted even if a large signal is applied. it can.
[0026]
Next, an embodiment of the present invention will be described with reference to FIG. 1, but the
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present invention is not limited to this.
FIG. 1 is a circuit diagram showing the power supply device for a condenser microphone
according to the present invention in a state where the condenser microphone is connected.
In the description of this embodiment, the power supply device for condenser microphones may
be simply referred to as "power supply device".
[0027]
In FIG. 1, since the condenser microphone 10 and the microphone cable 20 may have the same
configuration as the conventional example described in FIG. 2, the description thereof will be
omitted here.
[0028]
In the power supply device 30A according to the present invention, the DC power supply 33 is
connected between the signal output line 31 and the ground line 32 as in the prior art described
with reference to FIG. The conductive element 35 is used, and a DC voltage is applied to the
signal output line 31 from the DC power supply 33 via the visible light conductive element 35.
[0029]
The visible light conductive element 35 is a semiconductor sensor using a photoconductive effect
in which the resistance decreases as the light receiving amount increases, and the resistance
increases as the light receiving amount decreases. For example, P1114-01 (product number
manufactured by Hamamatsu Photonics Co., Ltd. Etc) can be used.
[0030]
Further, the power supply device 30A drives the light emitting diode 36 according to the voltage
(potential) of the light emitting diode 36 optically coupled to the visible light conducting element
35 and the drain D of the FET 12 appearing on the signal output line 31. And a drive element 37.
[0031]
The light emitting diode drive element 37 decreases the current flowing to the light emitting
diode 36 when the voltage of the drain D rises, and increases the resistance value of the visible
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light conductive element 35. Conversely, when the voltage of the drain D decreases, It operates to
increase the current flow to lower the resistance of the visible light conducting element 35.
[0032]
In this embodiment, an inverting amplifier 37 a is used as the light emitting diode drive 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. Filter circuit is
interposed.
[0033]
Further, a reference voltage is input to the positive input terminal of the inverting amplifier 37a.
This reference voltage is the drive 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 divider circuit having
resistors Ra and Rb connected in parallel with the DC power supply 33.
By setting Ra = Rb, it is possible to obtain a reference voltage in which the power supply voltage
of the DC power supply 33 is approximately 1⁄2.
The light emitting diode 36 is connected between the output of the inverting amplifier 37 a and
the ground line 32.
[0034]
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When the voltage at the drain D becomes higher than the reference voltage, the current supplied
from the inverting amplifier 37a to the light emitting diode 36 decreases.
As a result, the light irradiation amount of the light emitting diode 36 decreases, so that the
resistance value of the visible light conductive element 35 becomes high, and the voltage of the
drain D is lowered.
[0035]
On the other hand, when the voltage at the drain D becomes lower than the reference voltage,
the current supplied from the inverting amplifier 37a to the light emitting diode 36 increases.
As a result, the light irradiation amount of the light emitting diode 36 is increased, so that the
resistance value of the visible light conductive element 35 is lowered, and the voltage of the
drain D is increased.
[0036]
Thus, the voltage of 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 FETs 12 having
different Idss are used, the drain voltage of the FET 12 can be controlled to a substantially
constant value.
[0037]
Also, by setting the resistance values of the voltage dividing resistors Ra and Rb 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 a large signal is applied. Signal output is less likely to distort.
[0038]
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The typical circuit diagram which shows the power supply device for condenser microphones of
this invention in the state to which the condenser microphone was connected.
The typical circuit diagram which shows the prior art example of the capacitor microphone of a
plug-in power system, and its power supply device.
Explanation of sign
[0039]
DESCRIPTION OF REFERENCE NUMERALS 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 wire 32 ground wire 33 DC power supply 35 visible light
conductive element 36 light emitting diode 37a inverting amplifier Ra, Rb voltage dividing
resistor
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