Yamaha DTXpress/DTXplorer/DTXtreme

Here it is, please find the PiezoSwitch.jpg in the Files section. The 
only parts needed to make this circuit are the LM339 comparator IC and 
a few Schottky diodes. There are no resisters to adjust or transistors 
to match, and is very reliable. The only catch is, it needs a power 
supply. I am using a 5 volt adapter from a cellphone charger. It is 
possible to use the 12v power from the DTXpress if you want to further 
reduce the parts count. Just make a Y-splitter for the 12v DC power 
cord.

The beauty of this circuit is its simplicity and low cost - a LM339 
($1.29 from RadioShack) and 8 diodes will take care of 4 stereo inputs. 

I thank Keith for his original DTXpress circuit for the inspiration.