Dubbing mixer (2):

With reference to the circuit diagram. Fig. 2 in Part 1 of this article, the microphone signals are applied to the circuit either via BU1 or BU7 and fed to the non-inverting (+) input, pin 3, of IC1A. Capacitor a suppresses noise, while potential divider R2-R3- R4 sets the bias voltage via Rs. Capacitor D acts as a buffer.