Big Devil Expression is mainly an echo pedal with a send-return loop. You can connect an external pedal to these jacks to make the new pedal set a colour to the repeated sound. If nothing is connected to the loop it behaves like a regular echo (without a loop). Big Devil Expression is almost identical to Strange Devil Expression, but Big Devil is built in the much bigger 1590DD box which made some difference in layout necessary.
The Repeats knob is connected in such a way that it gives one repeat (exactly one and no “trails”) if it is turned all the way left.
Big Devil Expression has just like Strange Devil Expression a jack for an expression pedal. By connecting an expression pedal to the jack you can change the interval time for the echo with your foot. At the same time a pitch distorting effect is achieved.
A “regular” Strange Devil is shown below that is the same circuit as this one but without the expression function and without a mix control.