Since this is what has been selected for the group build here are some relevant links.
The tonepad file with schematic and pcb can be found here.
A similar circuit can be found on this page, if you scroll down to "Calavera"
and here are some user reviews from harmony central (the most credible review source).
If anyone who wants posting permission on this site please email me firstname.lastname@example.org and i will add you