PURPOSE: To stabilize the characteristic by MFB by detecting the reproducing sound of a speaker including a bass-reflex port by a microphone and feeding back the detected sound to an input of a power supply to detect the sound pressure of the bass-reflex port.

CONSTITUTION: A speaker box 5 is provided with a speaker unit 9 and a bass- reflex port 6, a microphone 8 for detection is fixed by a stand 7, to detect the sound pressure of the speaker unit 9 and the bass-reflex port 6. In feeding back the detected signal of the microphone 8 to an inverting input of the power amplifier 1 via a feedback circuit 4, the accelerating feedback is applied, the phase is changed rapidly near a resonance frequency fp of the bass-reflex port 6 in the acceleration sound pressure frequency characteristic of the speaker unit where the reproducing limit frequency of the speaker is lowered, the characteristic after feedback is unstable, but the bass-reflex port radiating sound is fed back together, then the phase is not changed rapidly, and no instability takes place even after the feedback.