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There is provided a preferable impedance converter for particularly to a condenser microphone having higher input impedance without being limited by a grid resistance, and suppressing the impedance converter including an amplifier tube 2 with plate-ground, and a bias circuit 1 for generating a bias voltage applied to a grid of the amplifier tube 2, said bias circuit 1 including a diode 1A for generating a bias voltage, and a diode 1B for applying the bias voltage to the grid and thereby flowing a current toward the grid of the amplifier tube 2 so that a ham noise and a variation of a bias can be suppressed.

An impedance converter for a condenser microphone having an input terminal for inputting a voice signal from outside; an amplifier tube with plate-ground for amplifying said voice signal to output it therefrom to outside, the amplifier tube comprising a cathode, a grid and a plate; a power terminal for supplying power to a cathode of said amplifier tube; a bias circuit for applying a bias voltage to a grid of said amplifier tube; a condenser connected to a plate of said amplifier tube, a load resistance connected in series with said condenser; an output terminal for outputting a voice signal amplified by said amplifier tube.

In an impedance converter used for a condenser microphone, a voice signal input at a high impedance is output from a condenser microphone to an amplifier at a low impedance. Thereby, in a condenser microphone with a small value in a an effective electrostatic capacity, a frequency response at a low frequency band is obtained therefrom. Further, in such a kind of impedance converters, a vacuum tube is mainly used as amplification means for maintaining a suitable voice generated from a condenser microphone.

A conventional impedance converter as described above is shown in a circuit of FIG. 10, which was used for various condenser microphones. The circuit of FIG. 10 includes input terminals 141, 142, a bias circuit 110 provided therein with a gird resistance 111 connected in parallel with each other, a bias resistance 112 and a condenser 113, a vacuum tube for amplification (hereinafter called as an amplifier tube) 120, a load resistance 130, a power terminal 143, an output terminal 144, and a ground terminal 145.

A condenser microphone portion 100 includes a voice signal generating circuit 101 for generating a voice signal, and a condenser 102 which comprises a vibration plate (not shown) in which vibration is caused by sound applied thereto from outside thereof and a fixed electrode (not shown), and connected with input terminals 141 and 142 of an impedance converter. In the voice signal generating circuit 101, a voice voltage is generated, based on an electrostatic capacity corresponding to a change of distance between the vibration plate and the fixed electrode thereby a voice signal is produced. The voice signal produced by the voice signal generating circuit 101 is output to the impedance converter through the input terminal 141 and 142. The condenser 102 comprised of a vibration plate and an electrode has an electrostatic capacity of about 5 to 100 [pF].

The condenser microphone portion 100 is connected with the impedance converter through the input terminals 141 and 142, of which the input terminal 141 is connected with the amplifier tube 120, and of which the input terminal 142 is connected with the ground terminal 145 which is an earth.

In the amplifier tube 120, a vacuum tube for amplification is used for impedance conversion. That is, the amplifier tube 120 comprises a cathode connected with the power terminal 143, a grid connected with the input terminal 141, and a plate connected with the output terminal 144. In such a kind of the amplifier tube 120, a voice signal applied at a high impedance to the grid side is output to the plate side at a low impedance.

A power current Ip flows from the power terminal 143 to the cathode of the amplifier tube 120. When the plate side is an anode, the current flows inward from the grid side. When the plate side is cathode, the current flows from the plate side. Thus, the voice signal input into the impedance converter through the input terminal 141 from a condenser microphone is amplified by the amplifier tube 120 to be output from the plate side. That is, in the amplifier tube 120, when a voice signal is applied, the plate side is at ground so that it is operated as a cathode follower.

The bias circuit 112 is disposed between the grid and the plate in the amplifier tube 120, and comprises a grid resistance 111 and a bias resistance 112 connected in parallel therewith each other. The bias resistance 112 of the bias circuit 110 is connected in series with a load resistance 130, and connected with a grid/ground terminal 145 through the load resistance 130.

The flow of the current Ip to the amplifier tube 120 is lowered in voltage by the bias circuit. In the amplifier tube 120, the plate side is loaded through the bias resistance 112, and the grid side is loaded through the grid resistance 111. An AC component of the power current Ip from the plate side of the amplifier tube 120 is bypassed by the condenser 113.

In the impedance converter with the triode as described above, a voice signal applied to the amplifier tube 120 from the condenser microphone portion 100 is not applied to the grid at the input impedance enough to a bias voltage, since the applied voice signal is affected from the grid resistance 111. As a result, a sufficient signal can not be taken out of the plate side.

In view of the above-described problems, an object of the present invention is to provide an impedance converter for a condenser microphone, in which an input voice signal from a condenser microphone portion through an input terminal can be impressed at a sufficient impedance to a grid of an amplifier tube with a high mutual conductance and a sufficient voice signal can be taken out of a plate without a ham noise affected from a cathode.