four VCA circuits run into a summing mixer

(from http://www.bartonmusicalcircuits.com/)
This module is a combination of four simple VCA circuits run into a summing mixer. It was designed primarily to work with audio signals, being the final module in a system before going to an amplifier or recording interface. It was designed to be built by beginners, be as simple to use as possible and to use inexpensive parts. Each VCA his it's own individual output which is wired to be taken out of the mix when used.

http://www.bartonmusicalcircuits.com/vcamix/