You need two main components, the first is a full wave rectifier, which can be bought as a single component, or constructed using four diodes. This will effectively cause your Vac signal to become the absolute value of itself. Next you need a capacitor connected between V+ and GND to smooth out the "humps" of the signal. I can't remember the equation off the top of my head, but in general, you need a fairly large capacitor. It's probably best to feed this into a voltage regulator, regulated at the voltage which your require to power your amp.