What is an Audio Mixer?
An audio mixer is a device used in some audio systems to combine signals from two or more signal sources, either before or after preamplification.
An example of mixing occurs in a karaoke system where musical backup from a CD player is mixed with a vocalist's performance at the microphone.
A mixer circuit is based on an op amp adder. Below is the op amp circuit:
Being that mixers combine one or more signals, it makes sense that mixers are based off of op amp adders, which add or combine multiple signals.
If resistors R1 to R4 are variable, the signals can be mixed in any proportion, meaning at any volume levels. And overall volume can be set by a variable resistor in series with Rf.