A summing amplifier, or an adder, uses an operational amplifier to combine multiple input signals into a single output. Conversely, a difference amplifier, or a subtractor, amplifies the difference between two input signals. Consider an audio mixer circuit with an operational amplifier. When audio signals are fed into the input channels, the input resistors generate currents that flow through feedback resistors, producing an output voltage. Using Kirchhoff's current law at the op amp's input node in conjunction with Ohm's Law, the resultant output voltage is proportional to the weighted sum of the input signals. A basic differential op amp circuit can partially reduce audio track noise in this mix. Applying KCL at the inverting and non-inverting nodes yields two equations. With equal voltages across both nodes, an output voltage expression is obtained. The difference amplifier can nullify signals common to both inputs only when the resistance ratios meet certain conditions. As a result, a differential output is obtained. When the resistance R2 equals R1, and R3 equals R4, the difference amplifier functions as a subtractor.