Assembling custom eurorack audio shield for teensy 4.1

Part 1 of 2: building the audio pcb (“mainboard“) from start to finish. a lot has happened in one year upgrading teensy 4.0 to 4.1 - the extra pins allowed us to removed the extra samd microcontroller. but i wasn’t satisfied that the dac and the adc were not integrated into the audio library. reduced to only two pcboards instead of three pcbs removed samd microcontroller removed rgb encoders removed noisy smd bypass caps added extra 4 cv analog outputs integrated cv input and output with audio library cleaned up schematics, symbols, footprints moved to entirely open source tool chain version 2 is much easier to build. all dacs/adc/codec are accessible via the audio library. the dacs and adc now share a reference voltage. previously the adc was using its own internal voltage reference hopefully next revision of breakout board will resolve the last tiny issues. lots of really exciting new details in v2.0. i also plan on one more revision to see if i can route the breakout board using two layers.