Musicians who are used to playing the piano and singing will really enjoy the external mic input featured on a wide variety of keyboards!
This video demonstrates the quality of the Roland FP90 as the singer accompanies herself. The FP90 has a designated gain knob as well as a reverb (echo) option that can be controlled by the artist as they sing through the amp and speakers on board.
Another Roland keyboard that has a mic input is the Juno which requires an external amp/speaker to produce sound. If a mic is attached, the voice comes through the same amp as the keyboard, so it can act as a small portable PA system.
The Yamaha CVP Clavinova not only takes a mic and utilizes the onboard speakers to project the voices but includes a wide variety of vocal effects. The vocal harmonizer adds a great deal of richness for performances or simply in home practice. Vocal harmony modes include chordal, vocoder and vocoder-mono and there are over 40 additional types of harmony structures to build just the right sound!
The popular and portable Yamaha PSR keyboards also have a mic input and models offering over 40 different harmony options as well!
For a piano player/singer these keyboards with mics are just fantastic!