Adding Phase Reversal Switch

[SpaceCommandant]

Hi,
I’m being asked by a friend to build two preamps to be used simultaneously with the outputs from the two pickups in his acoustic guitar. He wants independent EQs for each channel and also requested a phase reversal switch. Supposing I use a single-transistor design like the Echoplex or Tillman, what would be the simplest/most effective way to achieve this?
https://tagboardeffects.blogspot.com/2014/05/a-pair-o-preamps-echoplex-and-tillman.html?m=1

[BetterOffShred]

Parasit studios has a layout for a buffer with phase switch out of a TL072.  It's pretty basic.  

[SpaceCommandant]

Very nice, thanks for that!