control voltage mixer
http://www.musicfromouterspace.com/analogsynth/dcmixer.html
DC voltage virtual earth circuits
http://sound.westhost.com/project43.htm
This is simple way to power some 5v logic from a 240vac source. If a 120vac power adapter is used, the circuit will also work for 120vac power lines.
http://www.discovercircuits.com/DJ-Circuits/240VAC5VDC.htm
This is a classic example of a regulated DC power supply that produces both a positive 15v and a negative 15v from a 20vac wall adapter.
http://www.discovercircuits.com/DJ-Circuits/SPLITSUPPLY1.htm
If you need about 12,000 volts DC for an ion generator this circuit might be the ticket.
http://www.discovercircuits.com/DJ-Circuits/12v.htm
Taking only a cursory look at the input-voltage ratings of your power-supply IC can limit the usable input-voltage range.
http://www.edn.com/article/CA282635.html