This may apply for other similar head amplifiers of microphones as well.
The human ear is a miracle of dynamic range handling (and not only). [Stevens]
One property it has is automatic gain control. One way this is done is by the 3 smallest bones inside the human body. They pivot inside the middle ear. They do impedance matching and effectively at large signals the pivot point changes perhaps in an analogous way to a transformer ratio changing or changing gear in an automobile transmission system as a crude example.
If for example unplugging the capsule of the Neumann U47 and applying a sinewave signal to where the capsule is connected the following happens. If we keep increasing the amplitude of the sinewave it does not get clipped. Why? Because after a threshold the amplitude stays the same by the gain of the amplifier being reduced. Why?
If a signal is high enough, its peak for example may drive the grid positive with respect to cathode. This means the grid will attract electrons. So it will become more negative even when the signal is released. This makes a reduction of bias and therefore transconductance ie gain. The 60MΩ or so grid resistor brings slowly back the grid to its normal potential. The release time of the compressor.
Further reading:
Older euroelectron posts describing this effect
Operating Features of the Audion - E. H. Armstrong
The other today's euroelectron post on Neumann U47
Sound and Hearing - S. S. Stevens, Fred Warshofsky - Life science series
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