Originally Posted: Sun, 09 December 2007
I presume the great interest this subject has generated on this forum is the desire to hear certain vintage mics without some of the character-changing circuit alterations that were engineered into them.
These changes were either added because of engineering principles that evolved with time (some may say, devolved) or because buyers of large quantities of a certain model requested these changes (f.ex. maximum mic gains at the German broadcast system.)
The underlying principle and associated audible issue with Negative Feedback (NFB) are this:
In order to control gain and distortion of an amplifier, a small portion of the output of the amp is fed back out of phase (negative) to the input of the amp. This, in theory, has no side effect: the gain is reduced, the undistorted headroom of the amp is increased, and the amp becomes quieter.
However, an audible, though hardly measurable, side effect is created when feeding back from the output to the input of the amp: the negatively fed back portion of the audio takes a tiny bit of time to get back to the input of the amp, and thus introduces a time delay.
So what's the big deal? Phase shift: the audio wave of the audio signal fed back does not fully, completely overlap the original signal, because of the time it took for the electrical signal to get back to the input. I.e. the cancellation of the two wave forms is not 100%.
What does this sound like?
It depends on the amount and application of feedback, but in general, good ears will hear negative feedback as a smearing of the midrange, a thickening, a slight "falsification" of the original audio content.
This effect in certain mics (U67, U87) defines to a large degree the overall personality of the mic, for better or worse.
In true gain amplification (guitar amps, some Hi-fi equipment) the desired stabilization of the signal made possible by the NFB outweighs the undesired side effects (sonic artifacts)
It is debatable whether the application of NFB beyond a certain minimal degree is ever desirable in microphones, particularly when some of the most priced and revered models (Telefunken ELA M251, AKG C12, Neumann U47) do not employ it at all.