active amplified transformer isolated signal splitter that enables hum-free connection of one guitar to more than one amp as well as having a direct output
http://www.geofex.com/FX_images/splitter.gif
can mix four separate audio inputs to one mono or stereo output signal.
http://home.maine.rr.com/randylinscott/may20.htm
micropower and low-voltage op amps allow you to build high-performance analog-signal processors that require no batteries or wall transformers, this simple mixer is powered using one large capacitor such as a Supercap or Dynacap for some time.
http://www.edn.com/archives/1997/031497/06DI_04.htm
Includes Active Direct Box circuit.
http://www.ethanwiner.com/gadgets.html
full featured modular mixer design.
http://sound.westhost.com/project30.htm
mix two line level sources to one signal, includes separate volume controls for both signals, passive circuit
http://www.tkk.fi/Misc/Electronics/circuits/linemixer.html
control voltage mixer
http://www.musicfromouterspace.com/analogsynth/dcmixer.html
designed for three dynamic microphones.
http://www.aaroncake.net/circuits/mixer2.htm