The Mellotron Group

I'm building my own mono-summing box as I'm tired of "smoking" on the signal of my clonewheel.

http://www.rane.com/note109.html

Simple, straightforward, except...
Scroll down to the "stereo-to-mono summing box."
What on earth is that 20Kohm resistor doing bridging hot and signal ground? I just don't understand why I'm to put that there. To my (limited) knowledge, the box should work as expected without it.

-Sean