voltage regulator - Regulated AC power supply?

I've noticed mains voltage tends to be 120V +/- 10% for 108-132VAC.

How would you go about building a regulated AC-to-AC power supply which works off of mains and outputs 120VAC with a much lower (less than 1%) variation?

The output must be a sine wave at 120Vrms.

Edit: For those asking, the output would be fairly low, 1.5A - 2A, 2.25A max.

Circuit to be powered: Expected input: 0V-5.5KVrms sine wave at 60Hz

Simulated output: +/- 0-100KV DC.

The objective here would be to connect components in the order:

mains --> power conditioner --> variac --> 1:46 center-tapped transformer insulated for high voltage --> Cockcroft Walton Multiplier outlined above.

To achieve a variable 0-100KV DC (effectively 0-200KV DC with both channels factored in) power supply.

I am not certain the power conditioning stage is the way to go but assumed it would be since it is much easier to stabilize low voltage (120Vrms) than it is to stabilize high voltage (5.52KVrms.) Relatively minor ripple of 5% will grow into +/- 10% by the end of the CW stage at full power, yielding around 20KV off of the expected value - not an insignificant amount.

Answer

The classic solution adopted by the TC community is a Variac, but they can tend to get quite heavy for kW throughput.

If you only want to cope with a small range of input variation, say 10% = +/- 12v, then you might consider a small Variac, followed by a 120v->12v transformer, the secondary of which is put in series with your mains supply.

Let's say you had a 1A variac and a 120VA transformer. The transformer could deliver 10A, so could trim a 1200 watt supply, using 120 watt components. There's no free lunch of course, the increase in power throughput goes together with the decrease in range.

If you wanted to be clever, you could custom wind the 12v transformer to give you (say) 0.5v taps, and electronically switch them with (say) back to back MOSFETs, depends what your level of skill and requirement for automatic operation was. A binary tapped 12v transformer and some manual switches would make a cheap half-way house.