Hi there I have an electric violin and it has a Mono stereo jack plug, I would like to remove the mono jack plug and install a stereo jack plug. I am not sure how to wire it as the mono jack has two pins and the stereo has 3.

My stereo jack plug looks like this: http://www.effectsconnection.com/oscommerce/images/LongJohnnystereoopen 2.jpg

My electric violin has a positive and negative wire, do I need a third and where do I solder them? Thank you! =)
The link was broken here is a working one. http://www.effectsconnection.com/oscommerce/images/LongJohnnystereoopen%202.jpg