An impedance adjusting device includes a power output port, a ground terminal, a capacitor assembly including at least one capacitor, and a power output port. The power input port is electronically connected to a switching power supply, and the capacitor is electronically connected between the power input port and the ground terminal. The capacitor is connected in parallel to an output capacitor of the switching power supply to regulate a flat impedance of the switching power supply when an audio signal is played at full 20 Hz to 20 KHz frequencies at the same output audio voltage level.