Anything from a audio amplifier (a crap one at that) to a audio filter, to a analog buffer, a comparator, many things are possible. For more information check o

Anything from a audio amplifier (a crap one at that) to a audio filter, to a analog buffer, a comparator, many things are possible. For more information check out http://en.wikipedia.org/wiki/Operational_amplifier http://en.wikipedia.org/wiki/Operational_amplifier