This is a classic signal mixer that is used for audio purpose. VR1 & VR2 are inputs audio volume controlers and VR3 is used to control the balance between Input1 and Input2. connect a dynamic microphone to Input1 and a music without vocal to Input2, then adjust the VR1, VR2 and VR3 for obtaining a good mixed audio signal from the circuit Output. VR4 is an optional volume control for output audio signal.