4:1 mixer with discrete/op-amp plugins

ARC’s Modular Audio Mixer is a 4:1 AC mixer with discrete/op-amp plugins to change the circuit and audio character. The module ships with five postage-stamp sized plugins; two of which are preassembled, and three for user experimentation.

Preassembled plugins:
* Discrete design based on the amplifier found in the Studer A80 mastering reel-to-reel machine from Switzerland.
* Metal can 741 op-amp design as found in early 1970s west coast synthesizers

Experimental plugins:
* Single channel op-amp board with DIP socket for μA741 style chips such as the OPA627, NE5534, TL071, etc
* Dual channel op-amp board with DIP socket for TL072 style chips such as the NE5532, OPA2134, etc
* Perf-board for user designs

Some additional notes:
* The discrete op-amp plugin uses Alfa metal can matched transistor pair, Elna Silmic II cap and hand picked resistors
* Module mA draw is given for the discrete op-amp, this can change based on other plugins
* The base design is an inverting mixer, so signals are phase inverted - the dual-channel experiment plugin only uses one channel of the op-amp, so is also phase inverted
* Plugin orientation is notated by a circle at pin 1 on the main PCB and the plugin cards. All pins are labeled as well
* No additional op-amps are included. The experimental plugins offers access to the module's power rails, please don't goof with them if you aren't confident or do not have an op-amp's datasheet handy