Mixers (not the sort for cake making) but for adding together audio signals, from different sources.
I can not find the one I made back in the 1980's I think my brother gave it away to one of my school friend that helped the mobile disco with us, when I left home to move in with my girl friend. (oh that was why I stopped all this circuit making better thing as she put it to do ( no not that) more like rebuilding a small terist house in Grays Essex.
here is some new ideas that I've been working on.
You need one of these circuits for every strip (input channel).