Just completed an A12. It would have been a very simple, easy build if I had taken my time to do it right.
I had one issue where I didn't check the schematic and I ended up putting a capacitor in the wrong place. This wouldn't be a big deal, except I hadn't ever de-soldered something from a pcb, and I ended up ruining my solder pads, and almost ruining the board. So practice de-soldering before you try it on the A12 board, if you need to remove any components. Or better yet just put them in right the first time.
Also, the + end of the cap goes to the - terminal on the board. This seems kind of obvious, but I had to think about it when I was stuffing the board.
Get a tap for the XLR connectors. I tapped one with the screw, and it sucked. Got a tap, and it worked great, just remember to take out the metal tab. In one of the connectors there was a metal tab in the screw hole that got caught up in my tap and I ended up breaking the tap.
The DC offset was a bit of a pain. My multi-meter isn't really good enough to do the job properly. On the lowest voltage setting it jumped around and didn't give good readings, so I just kinda guessed on the DC offset. I don't think it mattered too much, because the unit worked fine.
The preamp is not nearly as "colored" as I thought it would be. I compared it to my Mackie Onyx pre, which is regarded as a "neutral" pre. I just did a quick acoustic guitar recording with identical mics placed side-by-side.
At low gain settings there was a very subtle difference in the midrange, with the A12 being a bit more forward, but honestly it was hard for me to tell the difference. According to my frequency analyzer plugin, there wasn't more than 1dB difference at any particular frequency, it was more a tonal difference in the midrange..
At higher gain settings, the A12 smoothed out a bit and sounded a tiny bit warmer. I never noticed this before, but the Onyx has a sort of tinny sound, slightly hollow at high gain. It was kind of like it picked up a little bit more reverb, in a bad way. The A12 sounded smooth and clear.
I was surprised by how it sounded. I expected a thicker transformer sound and a bigger midrange, but I didn't get that. It just sounded good, in a way that could be used on any source.