Thanks for jumping in and answering guys!
This remains unanswered so I'll give it a shot.
how does the DC power from a battery get switched to AC to power the preamp?
First, I think we need to straighten some things up.
The amplifier/preamp circuit operates on DC voltages. This voltage/power is supplied by a battery or a DC power supply unit.
The miniscule AC signal you have at the input, think of it as a "control" signal. That's all it is.
To really understand what is going on, you have to go back to the basics... in this case, all about Transistors.
You may be thinking... But we're not using transistors. We're using IC chips... opamps to be exact. Well, Op amps are just a bunch of transistors arranged in a specific way.
So going back to basics, a transistor device allows you to control the large flow of current at it's collector by feeding it a small input current at it's base terminal.
The relationship between how much Collector current you get, by how much base current you feed to it is called the Current gain Hfe of the transistor. For some transistors, this could vary by 10x to 100x or even more.
Still with me? Let's simplify things. Let's look at a single transistor.
The Base is connected to your mic signal. The Collector is where you get your output. The Collector gets it's power source from a battery.
So now, the varying, miniscule input voltage (coming from the mic) signal is fed to the base of your transistor which in effect causes a varying but larger voltage change at the collector of your transistor (which is where you get the output).
So what's really happening here is this... your preamp's output voltage comes from the DC power supply. (Not from the inputted ac signal.)
But now you're probably more confused... you're probably thinking if the output voltage is supplied by the DC power supply, how did it become an AC signal then?
Ahhh. because this varying DC voltage induced at the collector goes to another output capacitor, or output transformer. <--- I bet you didn't see that coming.
What happens next is the DC component at the output is blocked, and only the varying voltage level is passed through by the output transformer or output capacitor. Except this time, we're talking about voltage levels that are much higher! But nevertheless, almost an exact copy of the input signal! (Remember the Hfe, Current gain capability of the transistor we discussed above?)
The above is just a simplification of things, but that's the gist of what happens.
So by knowing the above things, you can see how
a) Any hum on the DC supply voltage will show as hum on your output signal.
b) The maximum output voltage capability of your preamp is dependent on your DC supply voltage.
c) If you feed a large input signal to your amplifying device, and multiply this by the gain of the preamp, but there is not enough DC supply voltage to properly represent that input signal at the output, then you get a flat top waveform, i.e. maximum reached (see note B above). .... we call this "clipping"
OF course, there are some tricks to get around some of these problems.
*You can get rid of the output capacitor by using DC Servos. (which is what I'm using on the TS-2)
*You can further increase the output signal for "free" (i.e. not dependent on the DC supply voltage) by using an output transformer with a 1:2 ratio or higher.
*You can use DC-DC converters so that you can convert a small DC supply voltage (say 9V or 12V) to a higher DC supply voltage for your preamp, thus giving you more headroom and more maximum output voltage capability. (which is again, also what I'm doing with the TS-2 preamp)
Hope this explains a little some of the things happening, of how/why it's happening.