Two control voltages are generated, one increasing and another decreasing, in response to the rise of the voltage of a condenser of a common time-constant circuit. Based on the two control voltages, a first and a second currents are formed, from which four different combinations of currents are established. By feeding one of four combinations to each of multiple differential output amplifier circuits each having a feedback resistor, the multiple output amplifier circuits can be individually started up or shut down without generating popcorn noise, using only one single time constant circuit.