The mixer circuit below has 3 line inputs and 3 mic inputs. The mic inputs are suitable for low impedance 200-1000R dynamic microphones.
http://www.mitedu.freeserve.co.uk/Circuits/Audio/6ipmix.htm
This circuit is useful for removing ground loop hum on a remote line level audio signal line. It can be used to to connect a computer sound card to a stereo amplifier line input.
http://www.solorb.com/elect/musiccirc/stereoiso/index.html
Great for checking the SWR of antennas.
http://www.wenzel.com/documents/circtran.htm
http://www.aaroncake.net/circuits/mixer1.htm
Send signal from one microphone to two mixers.
http://www.procosound.com/images/MS2.PDF
guaranteed parameters of the SA602A make this device particularly well suited for cellular radio applications.
http://www.electronics-tutorials.com/devices/602.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
convert consumer equipment stereo signal to mono or combine audio signal from two different equipments to one input
http://www.tkk.fi/Misc/Electronics/circuits/linesum.html
One of my most ambitious projects, this mixer was made for my friends Tony and Wilfred of Mixed Fruit Jam as a practice amp. It is a relatively straightforward design, and will probably not be the best way of doing this. However, it works fairly well, and is relatively inexpensive to build.
http://wiredworld.tripod.com/tronics/mixer.html
The dilemma is that solving noise problems is an art within itself. Since it does not come up every day, we all have limited practical experience. This has spawned an industry for those who are now specialize in solving noise problems.
http://www.epanorama.net/documents/groundloop/index.html