So I did a test fit and discovered that it is impossible to populate the PCA board with 12.5mm wide caps on both input and output, and still have room for two 9v batteries in the tin. I was crossing my fingers and everything, but no dice.
This means that if I want to use a big fat output cap like Funch did, I have two options. One is to use a smaller 10mm wide power rail cap like he did. But I already soldered in a 12.5mm wide Panny on the power rails, and I like it. The second option is to make the batteries smaller!
Popped one open, and was surprised at the size reduction. The actual cell is small enough that I could probably fit four of them in the tin...but using two, I'll have plenty of room for big caps to hang out over the edge of the PCB.
Another advantage is that I ought to be able to glue the micro USB ports flush with the tin surface now, which was impossible before. I had to cut large messy holes in the side of my first tin to accommodate the full diameter of the male USB plug housing. Now I can probably just keep it neat with a hole punch.
But of course this begs the question, isn't there a better way entirely? It would be awfully nice to be able to charge my amp with a single USB cable instead of two. And even nicer if the battery capacity was larger. It is now quite obvious to me that a much larger cell could fit in the tin, probably doubling runtime, at least. X, do you have any insights or recommendations for adding a purpose-made battery and charging circuit to this amp? I really like the full 18V power, by the way