With the collection of noisemakers growing, a mixer might prove useful. This chapter provides some completely passive circuit designs—they use no batteries, no chips, no circuit boards, no on/off switches. The simplest mixer of all is just a Y-cord tying two signals together. Audio taper pots are generally preferred for mixers, but linear ones will suffice if budget demands. David Tudor, one of the pioneers in the field of live electronic music, used matrices to combine relatively simple circuits into complex networks that produced sound of surprising richness and variation. Feedback matrices benefit greatly from the inclusion of some kind of equalization to aid in steering pitch response and nulling out unwanted shrieks—a simple graphic EQ effect pedal provides both the requisite level boost and useful frequency shaping. The basic photoresistor gating circuit can be expanded into a matrix as well.