Hi Andre,
The two diodes between the + and - input on the 2520 op-amp are to protect the inputs from extreme waveforms step, (so not the output will latch up to either voltage supply) and possibly destroy the 2520 amp.
This can be a usal small signal switch diode type 1N4148 or 1N914,
The two other diodes that is conected in series from the DI-input circuit to ground is a Zener diodes 12 volt 0,5 watt, and are to protect the 2520 amp from very high levels if the DI input happen to be connectet to guitar amp speaker output, this can also destroy the 2520 amp.
The 1 Mohm resistor that is connected from the + input to ground are a ground/0 volt reference resistor for the amp, in orginal when the mike transformer always is connected to the input, the amp have the 0 volt reference through the transformers secondary to ground.
But when we have add this switch between mike and DI, and when the DI input have a capacitor in series after the input, we must have this 1 Mohm resistor to get 0 volt reference for the amp in the DI mode.
Good luck with the project
--Bo
