very nice metalwork, I'm impressed! It's good to see something different than the ever popular tube mic clone project.
There's nothing too special about the AKG C414 EB circuit(s). The silver 414 EB has a fairly simple darlington style circuit - whatever magic it possesses is due to its brass CK-12 capsule. The later black C414 EB P48 has a slightly more sophisticated circuit but omits the DC converter which also means that the circuit must do with very little current to keep the polarization voltage above 40 volts. The optimal C414 EB would be a combination of the two, i.e. the EB P48 circuit combined with the original EB's DC converter for 60 volts capsule polarization.
There's nothing too special about the AKG C414 EB circuit(s). The silver 414 EB has a fairly simple darlington style circuit - whatever magic it possesses is due to its brass CK-12 capsule. The later black C414 EB P48 has a slightly more sophisticated circuit but omits the DC converter which also means that the circuit must do with very little current to keep the polarization voltage above 40 volts. The optimal C414 EB would be a combination of the two, i.e. the EB P48 circuit combined with the original EB's DC converter for 60 volts capsule polarization.