Have a nice day evos!
Dear,
I have a set of equipment including: PS Audio Gain Cell Dac and Pre M700 and all use 100 volts for the Japanese market.
I did some research and learned that it is possible to convert the voltage from 100v to 220v by changing a jumper.
I live in a country where the voltage is 220v and I don’t want to use a transformer.
Somebody please help me to do this or guide me with guaranteed information so that I can take help from electronics engineers. Many thanks 
I had a similar situation with SGCD and S300 which was dealt with by PS Audio before I left the US. You should try contacting PSA directly for instructions in your situation
Not easily done. See here for a good explanation:
Suggest getting a quote from your local distributor for the modifications. It may be cheaper and faster turnaround, if they are willing to touch it.
There’s also the matter of line frequency. Are they 50Hz or 60Hz and is any change there needed?
As I know,
The voltage used in Japan is 100 V. There are two types of frequencies used: 50 Hz in eastern Japan and 60 Hz in western Japan (including Nagoya, Kyoto and Osaka ). So the devices here accept both types of Hz (I think).
Operating voltage on the electrical system where I live: 220 V; Grid operating frequency: 50 hz.
Unfortunately where I live there aren’t any PS Audio representatives, agents or distributors (Vietnam)