Midi controlled portable mixer

I am contemplating a project idea that is not yet full scoped and well thought out. Trying to collect my thoughts here to carve it out a little more.

Recently I have been picking up my old small synths that I never really used much, including the fabulous Axoloti, added a couple of Pocket Operators, A Novation Circuit, a Bastl Kastl and also a Koma Elektronik RH301 for syncing the clocks. Two friends of mine are also into making music so we started meeting and jamming just for fun.

This collection of smaller devices needs a mixer of course. The knobs of the older Behringer Xenyx Q1002USB I own are starting to gristle and scratch and also this part of my gear somehow does not really feel right for my setup. It is somehow doing too much while also doing too little. Hard to express my feeling towards this piece. Lets just say I don't love it any more.

Scavenging for small audio mixers I came across three interesting pieces of gear, Bastls Dude, Syntherjacks 4sum and Koma Elektroniks' Field Kit. The latter sports a nice little 4 channel mixer and a whole bunch of other lovely features, so it was added to my setup recently. But offering all the tinkering cables and mics and stuff, it still is not going to be enough mixing capability.

The remaining two mixers somehow do not offer what I am looking for. Flirting with ping-pong delays I would like the mixer be stereo. 3.5mm jacks would be prefered, since the POs and most of the other gear is on that jack format. I'm not 100% decided with mono vs stereo yet, since the synths are mostly mono as well. Maybe a combination of both? Switchable? Dunno.

The main feature I would like to have would be midi capability. I'd love to have some mixing presets and recall them with Midi program change messages. Also allowing Midi CC to control the output volumes could turn this into a musical component of the setup. I'm thinking about gating a melody playing channel with a Midi clock synced square wave to make for some interesting patterns as can be heard in some of Four Tets pieces.

The one mixer that really looks awesome with this regard is the Keith MCMillen K-Mix. But unfortunately this is out of my allocated budget range, aka 'too expensive for me'.

Looking at the excellent documentation of Syntherjacks 4sum I thought that building a device to my own specs should be doable. A challenge not doubt, but not an impossible mission. There is an ongoing discussion on crowdsourcing the design of a minimal mixer but I have the impression due to too many diverging requirements this thread is not going to be productive too soon and probably not arriving at a design that meets my requirements. Still worth skimming the discussion, since there are going to be information nuggets in there for sure.

Doubling the design of the 4mix for Stereo as a first step should not be too hard. The Potis would have to be operated by Midi though. I like endless encoders and thinking about nice rotary encoders with LEDs really got my imagination going. For Midi connectivity the mixer would need a microcontroller. The Teensy seems to be quite capable and popular for audio applications. There is an audio shield available, but this would not be enough for making a mixer, since it is lacking audio inputs. There is another interesting PCB, but I have no idea whether this board would be suitabe for the intended purpose. Maybe have to get in touch with the mentioned community.

I guess the next step would be to order double the BOM of the 4sum and to build it on a breadboard in stereo. Have to understand the amplification section that incudes the TL072 IC since I guess this would be a part that could be controlled with the GPIOs of the microcontroller.

Ordering a teensy and an audio board, some TRS Jacks for Midi in and out as well as some nice encoders and starting to tinker with the controller section would the following step.

This is a side project slowly condensing. Don't expect too much activity too soon. Feedback and input is very welcome and if you would like to join in, design and build this together with me, I'd be delighted. Open Hardware of course.