Back in project IC power amplifier we built a differential amplifier. We're again going to build a differential amplifier, but this time the amplifier uses three op amplifiers to achieve the same.
The schematic shows you that Q1 is an oscillator that is already familiar to us. It gives out a highpitched '"beep."
U1:A and U1:B generate in-phase signal when S1 is off, opposite phase signals when S1 is on. U1:C works as a differential amplifier.
When you finish wiring up the circuit, switch power ON, and try to listen to the sound with S1 released. Do you hear a faint beep and a noise?
That's because in-phase input signals offset each other. Now press S1 and see what happens. You can hear a louder beep. That's because the opposite phase signals are applied to the differential amplifier, activating the speaker.