PURPOSE: To correct optionally a sound volume characteristic by connecting a series circuit comprising a resistor and a diode in parallel with a resistor smoothing a pulse width modulation (PWM) output signal for sound volume control.

CONSTITUTION: A PWM output signal for sound volume control is pulled down by a resistor, smoothed and fed to a base of a transistor (TR) 5, the collector of said TR is connected to a power supply, the emitter of the TR5 is connected to a sound volume control terminal and the smoothing characteristic is changed by connecting the series circuit comprising the resistor 10 and the diode 9 in parallel with a smoothing resistor 3. Thus, the sound volume curve is made linear by correcting a control voltage to the number of step of the PWM output signal into a characteristic inverse to the characteristic of a sound amplifier circuit and other characteristics are set optionally.