The circuit act as a two stage amplifier to take a small input signal and amplify it to Modular Synth levels. The capacitor and initial resistor are used to remove any DC offset. The two Op Amps then amplify the circuit. The Op Amps use simple voltage dividers to provide the required negative input. You can replace a voltage divider with a potentiometer to give you control over the gain. Replacing the sine signal input with a noisy transistor (2N3904) will allow you to create a classic Noise Module used on many classic analogue synths.
