What makes a circuit AC or DC?

[ZipSnipe]

I am looking at an audio amp board, and it says it requires AC 12V; 30W 2A power transformer. So I am trying to figure out if I can use DC aka battery power. I know ac is alternating current and dc is direct current. But what makes a circuit determine which to use?

 

[zigzagflux]

Most likely the first thing the source sees is a full wave rectifier, which would also pass through DC if that's what you feed it.
With 12VAC, you hit peaks around 17V, so internally the amp is probably working with something in the range of 16-17V. Therefore, you might want to feed it 17VDC if you want equivalency in sources.
Short answer is 12VAC or 17VDC.

 

[Enzo]

But that short answer doesn't include units that need both positive and negative voltages internally. If you feed DC to such a unit which expects AC, then you wind up with only one of the polarity supplies inside the unit.

The best advice is just use what the unit asks for.

 

[DF96]

But what makes a circuit determine which to use?

Circuit details, such as whether the power input goes straight to the rest of the circuit or first has a rectifier and reservoir capacitor(s).

 

[ZipSnipe]

Thank you gentlemen, I have always been an electronic hobbyist but never really understand how it all exactly works. I mean I know what resistors, diodes and caps do but never really understand the entire picture.

But thanks to the internet and good teachers like you guys I am sooo much more educated