1. Why order one from the UK when THR10 are available in the US? Are the C models getting hard to find?
2. No voltage convertor necessary. You just need that right power cord.

They use an external 15VDC/3A power supply, it looks similar to a laptop PSU.

Input voltage on the original Yamaha PSU should be universal 100-240VAC, but double check the label on the PSU. All you should need is the right North American power cable. I'm pretty sure that it uses the same cable as a Playstation.

If it's NOT a universal 100-240 power supply, then just order the one I linked on eBay (but you'll still need the power cable). There are generic copies available for less on Amazon, but some are unreliable.