I done some more playing with the LTspice simulation of Dxvideo`s preamp-with-DC-Servo circuit. I have been using the .four command to have spice..

The design is based loosely on some ideas from K8CU and AG6K. However, I have simplified the circuit somewhat and used different components. The mechanical encapsulation is also different. I used LTspice to verify the tester before building it. The schematic is from the LTspice simulation (see below). I have also posted the simulation files so you