How to make a proper DC Power + Battery circuit for audio effect pedals?
Is there any reason something like this can't be used:



D1 is there just in case; it provides switching from battery to external power even if the jack doesn't break the contact, or if it momentarily fails to do so.

All switching is on the ground side. The J_PWR jack breaks the ground-side connection of the battery when external power is applied.  The J_PWR jack and battery share a common ground-side network which goes through J_IN. The ground for both the battery and J_PWR is interrupted if nothing is plugged into J_IN.

How J_IN works is that when a mono phone plug (TS) is plugged into a stereo jack (TRS), it bridges the sleeve and ring because the two are fused in a mono plug. 

AUDIO BOX should still have a power switch to prevent draining the battery when something is plugged into J_IN, and nothing into J_PWR.


  Note that this is a simplified design. Transformer-based AC adapters usually put out only a half-wave rectified AC: it is DC, but doesn't have a flat voltage. The device has to provide regulation for itself.  
  
  Thus, between the positive input terminal of J_PWR, and D1, we would typically have a three-terminal voltage regulator, flanked by some capacitors. The battery drives the internal, regulated voltage line.