How can I fix a turntable to work from a 12V DC plug?

My dad has just bought my brother a Pioneer PL-Z82 turntable for his christmas. It is meant to work by connecting to a system and drawing power from there. My brother's system won't support this so we are trying to connect it to a 12V DC supply. When we do this it works.....but constantly runs at one speed. I am assuming we are giving it too much power, am I right?

Can anyone explain the best way to convert this turntable?