Recently I have built a midibox sid step B. It is not quite finished yet, but in the proces of finishing it I would like to built a headphone out which combines the four independent outs I have on one headphone Jack.
What do i need to do to achieve this?
Thanks in advance,
Patrick
maybe this could help you:
http://www.midibox.org/forum/index.php?topic=5967.msg36766#msg36766
Maybe this too?
Small kit, or you can look at their nice circuit diagram 
From what I read of thefRont’s link, that’s a line level mixer. It’ll need amplifying to drive headphones.
I use the PAIA one here. I built it by the schematic. If you use two channels in parallel the sound quality is good, one alone is to bad in my “ears” (it´s going into distortion to early then).
Greetz.