Audio Research Reference 3 voltage conversion

Anyone who knows how to convert an Audio Research Ref3 preamp from 120 volt to 220-240 volt? I know that some products needs to change the transformer and other products just needs to switch a couple of wires.

Is it possible to convert an Reference3 to 220-240 volt? I don't want to use an outboard voltage converter.

Looking forward to your responds. Thanks.