The first boards came back from OSHPark. There were a couple of bugs and improvements in the firmware that came out of it, but at the moment there are two hardware bugs that I'm going to have to fix.

The first is that I reversed the differential inputs on the LM4871 audio amplifier input. I was able to work around this on the prototype by lifting the pins and air-wiring them. The v1.1 board design has already been fixed for this.

The other issue is that the audio output is just way too quiet. You can hear it if you hold the speaker right up to your ear, but it's not nearly loud enough to actually use. I'm not sure if it's because the speaker is too puny or if the input levels aren't high enough. What you can hear, however, is definitely good enough in terms of audio quality. The tick/beep output has just a little bit of shrillness (the low-pass filter isn't quite sharp enough to really make it a pure sine output), but that's not out of place. Even increasing the feedback resistor (and thus increasing the amplifier's gain) from 20k to 56k wasn't a significant increase in output volume.

Another correction v1.1 will have is that I've decided to put a MOSFET in to properly level-shift the audio !SHUTDOWN pin. The current design has a 5v pull-up resistor, but that still winds up feeding some current into the output pin of the controller when it's off (since the off level is 3.3 volts). It's not fatal, but it's not ideal.

I'm not going to order the next version of the board, though, until I decide what I need to to do get more audio out.