set to 48v as in curve 104 while on the other hand if consumption current iptm is smaller than the determination current imic the power feed voltage vout is set to 24v as in the curve 108 fig1 b also shows output variation for power feed voltage vout when the voltage change processing 2 from vin to 48v or 24v shown in fig1 is executed for example if consumption current iptm is larger than the determination current imic 1 the power feed voltage is set to 48v as in curve 104 while on the other hand if consumption current iptm is smaller than the determination current imic 1 the power feed voltage is set to 24v as in curve 106 with the example shown in fig1 b the minimum unit rate is made the same regardless of the consumption current iptm but it can also be made different in this manner with the first and second modified examples the power feed voltage vout fed to the external mic is switched depending on the consumption current of the external mic if described using the functional block diagram shown in fig1 the output current detection section 2 detects output current that is output from the boost section 1 to the power feed section 5 and more specifically the output current detection section 2 detects current that is circulating in the boost section 1 by means of the power feed section 5 balanced transmission line bw and audio input section 6 also the control section 4 controls rate.