na u00d7 v a u00d7 v in v max in 2 r 0 b s nb u00d7 v b u00d7 v in v max in 2 r s n 1 u2212 compensation factor u00d7 v a compensation factor u00d7 v b v b equation 5 in equation 5 above s n denotes a noise compensation scale the compensation factor denotes u201c weighted value u00d7 time u201d the maximum value of the compensation factor is 1 and the weighted value denotes a value smaller than 1 in equation 5 above when the compensation factor is added and the supply voltage is instantaneously varied the noise compensation unit 330 operates similarly to the first embodiment however as the time goes by the compensation factor obtained by multiplying the weighted value and the time becomes increased and the noise compensation scale gradually approaches 1 consequently the audio output b is varied when the power is varied and supply voltage variable noise is also removed in equation 5 above the noise compensation unit 330 varies a preset compensation weighted value thereby varying a ratio at which the audio output signal is increased or decreased gradationally according to the cumulative variable time fig4 is a detailed configuration diagram of the noise compensation unit of fig3 in accordance with the embodiment of the present invention as illustrated in fig4 the noise compensation unit 330 includes a noise compensation scaler 331 and a multiplier 332 hereinafter the elements of the noise compensation unit 330 will be described in detail the noise compensation scaler 331 determines the noise compensation scale according to the power variation control information generated by the power variation control.