The invention relates to a conditioning circuit that measures operating points of a power supply to deduce therefrom the current-voltage characteristic thereof and to determine directly the voltage corresponding to its maximum power point, without using any kind of tracking algorithm that causes the operating point of the power unit to oscillate about the maximum power point. Energy in the direction of suppressing reverse voltages applied to a plurality of rectifier diodes is injected into a rectifier circuit. The sound absorbing unit is provided with an entrance which is always opened without any closing member. A voltage corresponding to a difference between a threshold voltage of the PMOS transistor Q1 and a threshold voltage of the PMOS transistor Q2 is generated at a mutually connected section of the both MOS transistors as a reference voltage.