PURPOSE: To obtain an input signal which has desired frequency characteristics by providng a low-pass filter, a differentiator, and an adder in regard to a sound quality adjusting circuit for a speaker.

CONSTITUTION: Resistances R1, R2, and R3, capacitors C1 and C2, and an operational amplifier op1 constitute an LPF1, and resistances R4, R5, and R6, a capacitor C3, and an operational amplifier op2 constitute a differentiator 2. The adder 1 consists of a resistance R7 and an operational amplifier op3. In this case, respective constants are so set that A=1/a2-1 and 2π-f0'R4C3A=1/aQ0-1/Q0', thereby obtaining desired frequency characteristics. When a speaker in used is changed, the gain of the LPF1 and diffeentiator 2 is only varied. In this case, A=R3/R1, fo'= low resonance frequency of the desired frequency characteristics, and a =f0'/f0.