Assembled rev 3 over the course of the past three days - this one makes some pretty major changes:

• dropped the AA batteries in favor of a lipo & charge circuit
• samd10 on the frontboard to manage the buttons & LEDs. this is now fast enough to support PWMing the LEDs
• fixed the issues with the speaker & amp, actually sounds nice now
• sd card on sdio (yay, fast, 4bits at a time)
• line in detection works, headphone still isn't (I swear I worked this out on the breadboard)
• I put the wrong footprints in for the mic, so that won't work. I might be able to fix if I had hot air rework, but I don't.

This time I bit the bullet and actually soldered stuff in, rather than putting female headers everywhere so I could rescue the expensive bits. This let me mostly assemble it, with two execeptions. I stacked up the two ffc connectors that are underneath the teensy, rather than offsetting them. Also the ffc connector doesn't actually fit underneath the battery, so I can't fit it inside the sandwich.

I think both of these issues are fixable.

The larger question I'm up against is if I want to take a hard turn away from teensy, and just make this an stm32 board. It would majorly lower the complexity in some ways, and definitely lower the cost. I could potentially also make it a single board, rather than this sandwich stackup.

I think in need to grind on that question for a few more days, while I'm working on the firmware. Luckily the work that I'm doing on the firmware is transferable to whatever arm chip I end up slotting into the thing, so that isn't a huge issue.