Predicting voltage reduction of transformer under high load?

[hanit]

Hi,
I'm new to the forum and this is my first post.
I am an audio enthusiast (pro studio and consumer hifi) and recently I've been looking to expand my knowledge to be able to do cooler, better things.
Right now I'm trying to power a subwoofer amp in my home but it's meant to be put in a car and run off a car battery. In other words I need to build a power supply that transforms AC wall outlet to 12v dc AND is capable of very high current. I'm talking up to 100 amps

The amp needs a 12v dc power supply which I'm building using a microwave oven transformer, a bridge rectifier capable of 100amps, and giant smoothing caps. I'll be winding my own secondary to get the voltage I need.

My problem is that I have no idea what the output voltage of the transformer should be. I'm now finding out that at high loads the voltage output of a transformer decreases. Amps typically accept voltage of 10-16v dc, 12 being optimal. My amp will be drawing a RMS 700 watts with an unknown peak value if that matters. This means the psu needs to be able to dish out 60 continuous amps, but I'd like it to be able to do 100amps if possible so I can maybe do more with it down the road.

How can I predict how much the decrease in voltage will be? I feel like a few physics equations should do.the trick. My gut tells me that I should build the transformer top output 16 volts so that once the load hits, the voltage drops to my happy #12, but I'd much rather have a more concrete starting point because I honestly don't know what will happen to my amp if the voltage isn't accepted. Hoping it would just not power up, or simply turn off if it's already on.
Any help is appreciated.

Thanks in advance guys!
Hanit