Looking at the schematic, the "output" jack seems to be AC coupled to the input. Not sure why that would ever be useful, and seems a bit of a safety issue to me. Probing AC, the full voltage will be on that jack with nothing plugged into it, just waiting for something to fall in, at an impedance of about 56K for 60Hz mains, giving a couple of milliamps. Personally I would probably leave out C1 to disable the jack.