most of the op amp talk is over my head
but I do know that both the AD-20 and PS 2 can runn off of 7v - 24v DC
though if my memory serves me
the AD-20 is really only 7v to 18v
or may be its the other way around
I used to power my AD-2- and PS-2 off of one single 12v SLA
in order to get the phantom running for longer that a 9v cell
good luck this sounds like a cool project