Originally Posted by thoglette
...choke input filters...
Choke input filters, lovely for 200 mA at 450 volts, if you don't mind the cost, weight, and bulk. But largely useless when you want 2 amps at 6.3 volts instead...that would require such a low DC resistance that the choke would have to be wound with impractically thick wire on an impractically large set of stampings.
That's the thing, until switchers came along, getting several amps of clean DC at low voltage was ridiculously difficult. Heck, as Max999's struggles show, even getting a clean 600 mA at low voltage was difficult!
One of the local junior colleges has some decades-old 16V lab power supplies rated for up to 5 amps - they weigh at least 35 lbs (~15 kilos) each and are quite enormous, maybe 1ft x 8" x 10" or so (~ 30 cm x 20 cm x 25 cm). A tiny modern laptop SMPS the size of a bar of bath-soap puts out more watts, and far cleaner DC, with maybe one-fiftieth of the volume and weight.
It's absolutely amazing, when you see both power supplies side by side. It's a technological leap about as big as going from an early James Watt steam engine to a modern Formula One race engine.