Mic/Pre Impedance "Matching"- What Are The Rules?
David Satz posted on the Neumann forum a really clear and informative summary of microphone impedance strapping found through the 1980s on Neunann mics like U87, KM84 etc. I am reprinring it here with his permission.
There are actually three possible settings for the output impedance of the KM 84 and most other Neumann "fet 80" microphones: 50, 150 or 200 Ohms. In Europe, 200 Ohms was the de facto norm, with 50 Ohms as an alternative if you needed to drive long microphone cables; the lower source impedance helps avoid high-frequency losses and other problems that can occur due to the distributed capacitance of the cable. The 50 Ohm setting also has 6 dB lower output levels for any given SPL, which can help prevent overload in the input stage of a marginal preamp, mixer or recorder.
Most fet 80 microphones sold via Gotham Audio Corporation (the long-time U.S. distributor) were set to the third option, 150 Ohms, although many have been rewired in the years since then. The two secondary windings of the output transformer were paralleled as in the 50 Ohm setting, and a pair of ca. 50-Ohm series resistors, normally bypassed with solder bridges, were "un-bypassed" by opening (unsoldering) the bridges.That setting has the lowest output levels of the three--more like those of a dynamic microphone.
When the KM 84 was introduced in the 1960s, some U.S. broadcast stations still had old mixing desks with microphone inputs designed for dynamic microphones. Mr. Temmer, the head of Gotham Audio, was concerned about overloading those inputs, and also about some high-frequency response variation that occurs in certain preamps when the output impedance of the microphone isn't what the preamp's input circuit was designed for. He didn't want Neumann microphones to be blamed for any possible adverse interactions with other equipment. As a result, if you were a U.S. customer, you had to special order the microphones if you wanted them set for 50 or 200 Ohms--and then (as I experienced myself) someone from Gotham would call to quiz you about what kind of equipment you intended to use the microphones with.
But nowadays, if you're using a good modern preamp or recorder, it should be perfectly safe to use the 50 or 200 Ohm settings. Either one is preferable to 150 Ohms for avoiding interference in the microphone cable; if resistive pads are needed to prevent overload, they should always be placed immediately before the part of the circuit that is threatened with overload, rather than at the output of the microphone.