If the board was made with the original schematic, then it needs a dual supply. As in +9V -9V and ground. Looks like you need a dual supply if you want to use an AC adapter. Or you could just use 2 9 volt batteries. Connect the + of one battey and the - of the other to Ground. Then connect the - of the first battery to the - on the amp and the + of the second battery to the + on the amp.
not be be belabouring a point but I wanted to dbl check that I do have the wiring correct for this part of the amp since I initially figured the - was just grounded to any chunk of metal and the switch seems to be in an odd place. Thanks for dbl checking