What Determines The Amount Of "Current" In An Amp?

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Thanks Viridian.

In the very big amps of old, they boasted large power supplies. Is it necessary for Class D amps to have a large power supply to deliver continuous high current into low ohms?
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The majority of Class D amps use a Switched Mode Power Supply (SMPS) design. SMPS are also sometimes used with amps in other classes of operation like Class A/B. SMPS designs do not require large transformers as conventional linear PS designs do, since they run much more efficiently. Class D output stages (independent of whether they use an SMPS or linear PS) are also very efficient, so they do not require large heat sinks that other designs do. Class A biased output stages are the most inefficient and therefore require an extremely large amount of heat sink area.

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Yes Bill, but can Class D amps deliver high sustained current into low ohms?
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Ralph, are you saying that the amp is delivering 20 volts to the speakers in the above example? 100=20*(20/4).