| 摘要 |
PROBLEM TO BE SOLVED: To make the efficiency of the amplifier circuit high when an output level of the amplifier circuit is small. SOLUTION: The amplifier circuit is provided with a boosting circuit 2 and a voltage step-down circuit 3. A microcomputer M monitors a divided voltage from a variable resistor BL. When the microcomputer detects a setting position of the variable resistor to be high, the microcomputer turns on a 3rd switching element SW 12 to activate the boosting circuit 2 to apply a boosted voltage (18 V) to an amplifier section 4 as a power supply voltage. When the microcomputer detects a setting position of the variable resistor to be small, the microcomputer activates the voltage step-down circuit 3 to supply a step-down voltage (8 V) to the amplifier circuit section 4 as the power supply voltage.
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