This uses a very basic mixer design.

There is a basic bus (stripboard) with two audio signals (left and right), for the mixing part, then other two lines for the output. Finally there are three power lines: ground -V and +V. Input and output are joined in this prototype, but at the end I explain the reasoning behind. It makes a total of 7 lines. There are some decoupling capacitors in this bus to fix some buzzing and provide consistent power.

Some special attenuation would be needed on the monitoring input.. i guess. It needs some prof of concept.

Each input first passes to a potentiometer to control the input gain, then to DC bias removal and finally to an op amp used a buffer. This finally passes though a resistor to create the proper minimal resistance. Then it just goes to the input line on the bus.

The output goes to a DC offset remover, and then to an opamp in a non inverting configuration. This goes then a potentiometer and finally to the output jack.

And all this is done twice in parallel to allow for stereo.

In the input modules the jack allows a mode in which the right channel is not connected and it just copies the left signal, allowing for mono inputs.

At the output I tried to create a mode that if no right output jack is connected does a mixdown with mixed results. Were it working properly it could double as a 8 mono input mixer, but with per double chanel volume control.

For the power I decided to go USB C PD, 20 v. this gives a lot of headroom for internal voltage. But it introduces some noise. I'm waiting for a LM7815 to limit the voltage but with better noise.. I hope.

Problems encountered while developing:

Future improvements:

Input | Input | Input | Output | Buffer | Input | Output 
   |_____|____|_____|_____|________ |> ______|______|