Patent ID: 7973598

Claim:
A pre-amplifier for controlling a voltage gain of an audio signal, comprising: an operational amplifier having a first input terminal receiving a reference voltage; and an input signal controller connected between an output terminal and a second input terminal of the operational amplifier, wherein the input signal controller receives a voltage signal whose voltage level gradually decreases when an audio power amplifier is powered on, wherein the input signal controller comprises: a first transistor having a first drain/source connected to the output terminal of the operational amplifier, a second drain/source connected to the second input terminal of the operational amplifier and a gate receiving the voltage signal; a resistor connected between the output terminal and the second input terminal of the operational amplifier; a second transistor having a first drain/source connected to the output terminal of the operational amplifier, a second drain/source connected to the second input terminal of the operational amplifier and a gate receiving an inverted signal of the voltage signal; and an inverter connected to the second transistor.