Here is a layout of an early version of D-120 OD. It is basically a J FET conversion of a Matamp GT120 with some interesting tweaks like diodes connected to the sources for some of the transistors. It also have a clean blend /summing amp circuit. The original had fixed resistors across drain and V for Q1 through Q3 but I replaced them with 50k trimpots for easier bias setup.
Note that the more recent revisions is a completely different beast circuit wise with a rotary switch, MOSFET transistors and a quad op amp.
