AUDIO MIXING 

Also: not-so-simple audio mixing

Passive (resistor network-based) audio mixing:

• DIY Passive Summing Box — DIY Recording Equipment — December 8, 2011

With active components (op-amps &c.):

• How to build an audio mixer — CircuitLib

Why we don't use passive resistor nets (alone):

• Audio Signal Mixing — Rod Elliott — Elliott Sound Products — 27 September 2010
• Audio Signal Mixing - old web host older ESP server URI
• Audio Signal Mixing - WaybackMachine archive.org copy of the above URI