How to convert a NAD 120 V amplifier to 220 V?

In most cases the transformation to the 220V or back to 120V should be done by a qualified tech since rewireing may take place. Normally in the manuals there should be some words about such capabilities so you may want to ask if the manual exists.

I live in England and have to use a transformer for some of my US made stuff. Most US nonexport stuff cannot work on 220V 50 hz, and some will ... my Mark Levinson 360DAC will not. An amplifier should not be a problem. You will have to open up the amp and see if there is a switch. If not, you should call NAD and ask them, as not all US products have dual power transformers in them. You can still use a step down transformer and the amp may work off 50hz.