I recorded with the predecessor type KM 83 in the 1970s for a while--the same capsule paired with older, low-current electronics that use an output transformer. The capsule is very definitely a diffuse-field design; if you're already orienting the 0-degree axis of your microphones away from the sound source, you would certainly want to do so with the KM 183 as well. I didn't believe in doing that back in the day, and the results were unacceptably harsh and "metallic" sounding to my ears. I quickly started to look for alternatives. More recently I've applied some EQ to those recordings and they sound all right, but why not start with the response that you want?
The microphones are certainly up to the standard of quality that you would expect from Neumann. For several years it was a curious situation--the three lower-cost, unitary microphones (KM 183/4/5) had better dynamic range than their counterparts in the higher-priced, modular KM 100 series. With the introduction of the KM A that discrepancy no longer exists.
But the K 83 capsule design has its roots in the time before spaced-microphone stereo recording, when a single microphone would be used at considerable distance to pick up large sound sources (such as an entire orchestra) in a reverberant space for mono recording or broadcast. In general when using spaced omnis for stereo recording, people place the microphones at a distance where the reverberation is more evenly balanced with the direct sound, i.e. distinctly closer to the sound sources than in the 1940s and 50s with the single diffuse-field omni for mono pickup. And at such miking distances, most people prefer a response characteristic that on axis, has a few dB of treble emphasis, but certainly less than the ca. 6 - 8 dB of a fully diffuse-field-equalized type.
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