SynthMaker module developer Stefan Hållén has released a free VST plug-in called Stereo Reducer.
Stereo Reducer is a simple VST effect plug-in that lets you reduce the amount of stereo in a sound and gives you the option to choose how to build the mono sound instead of just mixing down the left and right signal.
You can download Stereo Reducer here (.dll), and a schematic to be used with SynthMaker can be found here.
