[sdiy] hiss in a simple mixer

[creekree at gmx.de]

hi all,

today i built a simple mixer (four mono channels) based on a design that 
can be found on generalguitargadgets.com
the thing is working fine except for one detail:

i added a second (mono) out by simply wiring a second jack to the 
existing one so i can plug it into my dj mixer and have a signal on both 
channels.
oddly, when i insert a plug into the second jack, it instantly produces 
a hissing noise. it doesnt matter which  one i pull -  one plug 
inserted, i get a clean (as in: free of noise)  signal, second plug - hiss.
i guess i overlooked something very basic here, but basic is my 
knowledge too...
so, please tell me how to get rid of that noise!


thanks, christian