mono jacks have a tip and sleeve. ie 2 contacts.stereo have tip,ring and sleeve. ie 3 contacts, sleeve is ground. tip and ring are left and right. cant remember which one is left though..lol
you can use a stereo jack for mono by just using the tip and sleeve.
It's possible to wire a stereo jack so that when you plug a cable into it...the circuit powers on...haven't tried it yet but read about doing this Ina book I have....Junkyard Jam Band...excellent book on electronics and stuff
you use the ring and sleeve to complete the negative side of the power supply. when a mono plug is inserted it will contact both ring and sleeve completing the circuit.
wire the battery negative to the ring . have the amp negative go to the sleeve contact and hook the positive direct to the amp.. without a plug in the bat negative is floating and not powering the amp.
thanks for the heads up on the book.. its a free pdf download at IT ebooks...