Half of the schematic for a Simple eurorack mixer. You can Use a TL074 or TL072. Depends on how many Op amps you decide for your mixer. 

The First op amp(U1.1) Inverts and Amplifies the mixed voltages with a Gain of 1. 

Then that is fed into the second op amp(U.1.2) Inverts and Amplifies the mixed signal phase back to the Original Phase with a Gain of +1.