Patent ID: 8331584

Claim:
An impedance converter for a capacitor microphone, the converter comprising: a vacuum tube that receives an output signal from a capacitor microphone unit through a grid and with which the signal is output as an output from a cathode follower; an FET in cascade connection with the vacuum tube and that defines a current flowing in the vacuum tube; a bias circuit that applies a bias voltage to the grid of the vacuum tube, wherein the bias circuit includes a first diode and a second diode that apply the bias voltage to the grid of the vacuum tube, the first diode and the second diode being connected in inverse parallel, and a bias resistor for applying the bias voltage at a constant level to the grid of the vacuum tube via the first diode or the second diode; and a resistor for controlling a plate current of the vacuum tube connected to the cascade connection of the vacuum tube and the FET.