I built a hub cap banjo. It is a bit short on volume. I was thinking about adding a disk piezo under the bridge. Big disk, little disk? Multiples? I'm not sure what sound I'm even looking for. I'd rather not cut up the top.
Any advice appreciated.
Piezo (single or double) can be done easily. But you could also try one of the flat style of pickups made by other CBN members. The Thinbuckers by Dan Sleep, or the Flatcat pickups from Wayfinder mount on the surface under strings, and wire goes through very small hole...no need to cutting a big rectangle hole in soundboard.
Id' either go 27mm single piezo or dual 20's... duals are nice, glue them directly under the corners of the bridge, and cover them with hot glue, silicone, or foam tape.