I never cease to be amazed at our ability to take a simple question and turn it into a war, and I've been reading this post with some amusement and now frustration so I feel obliged to throw my understanding of the situation into the frame.

My thoughts: Its a mono channel, unbalanced input, use a mono jackplug.

Heres why:

Mono/Stereo: I feel the answer to the question lies in the fact that the manual totally fails to suggest that the microphone input is anything other than a mono signal. Also harmonisers like mono input rather than stereo. Therefore it is mono, because Yamaha would not miss the opportunity to plug such a useful facility as a stereo input. If this was the case, you could use the 2k as a Submixer for a slave module and then not have to use a mixer at all!

The input to the PSR2000 must be unbalanced; again the lack of any mention to the contrary in the manual is adequate proof to me for the same reason, it's too useful a capability not to be mentioned. If the input jack on the 2k is shown to be stereo I vote for the opinion that the "ring" will either be grounded (which would be best) or left hanging. It may be cheaper/easier from a mass production viewpoint for Yamaha to use stereo sockets everywhere even for mono inputs and outputs.

ps. Tom Cav: You don't get many balanced microphone leads with stereo jacks on the end, because the XLR connector is SOOOOOO much more robust; but I have seen it when using budget / small mixers without room for XLR sockets.