Active Mixer stages that use Op-Amps are generally known as virtual earth pre-amps. These are inverting in nature. 180 degrees out of phase. IE: The signal coming out of the mixer is upside down as compared to that which is entering it. You then need to use another inverting pre-amp to recover the phase.
Note here that Mixers are more repetitious than complex. The circuits are relatively simple it's just that there's a lot of them. Especially in large recording consoles.