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