While wall warts aren't too good, Yamaha arrangers are fairly forgiving. On both of mine the audio output chip is good for 25V or more - it's the regulators for the lower voltages that might get a bit hot if the input voltage is a bit high. I get around this by making my own stabilised power supplies which not only solve the problem of over voltage but also ensure good solid bass and no ducking of the audio on loud notes.