We have just moved from Hungary to the US, and we brought our digital piano with us. We faced the same issue as you do, with converting the voltage from 220V to 110V. I contacted the piano maker, and they recommended this step up-step down converter:
http://www.voltageconverters.com/itemdesc.asp?ic=VC100W. I ordered it (not expensive at all), and it works great! You don't even need the plug adaptor, as you plug your machine into the converter, which then plugs into the wall. I hope this helps.