SIMPLE SURROUND USING OP-AMP CIRCUIT SCHEMATIC DIAGRAM
Although there have been similar circuits published over the years, this is a little different in a few areas. I wanted to avoid having any active electronics in the main Left and Right channels, since this eliminates any possibility of sound degradation due to the introduction of the opamps. The input impedance of 50k will not pose a problem for any preamp (including valve types), and the main signal is simply in parallel with the additional circuitry.
No volume control has been included, since you already have one in the preamp. It would just become another component to fiddle with, and since it would be rarely used, would probably become noisy over time.
