I don't know if the SD1 is like Yamaha PSRs but I think the intention of Yamaha was that you should use a step up transformer with a dynamic mic. The input impedance is high and there isn't enough gain in the amplifier for a low impedance mic so you'll get a lot of hiss if the gain control is turned up. So it might be worth getting a cheap transformer and trying that before you spring for a mic preamp.