Aren't the microphone input jack both high and low impedance on the Yamaha? Just because the jack is a quarter inch jack doesn't mean that it isn't balanced. This give people the ability to use either a cheap high impedance mic, or a low impedance microphone. I haven't used the Tyros microphone jack, but from what my memory of the PSR 9000, I didn't need a impedance matching transformer to use a low impedance microphone.