For me, though, this is a secondary consideration when selecting a FET for a mic, as most J-FETs will only vary the mic's output by a few dB. What complicates the test somewhat (though not as much as the specs seem to indicate) is that different J-Fets have different gain. Then move on to the next FET and do the same again. Increase the level of the speaker output until you can clearly record (or hear) the sine wave at the output of the mic collapsing into third harmonic distortion (while the speaker output remains undistorted) Measure the mic's output at onset of the audible distortion, and repeat the test with a sine wave fed directly into the input of the gate. Feed a clean sine wave into a high quality speaker, which you place 1 foot in front of your mic. The bottom line: you need to make simple noise and headroom tests, whereby ONLY the FET becomes the variable under test. I am not an electronics engineer and not even the kind of layman who would dedicate enough time necessary to be reaching deeply into this matter, but I can speculate that the enormous input impedance the gate sees in a single-FET microphone (in excess of ten thousand million Ohms) throws off the usual lab specs of the device, similarly to the impossibility to predict tube noise in this type of high impedance environment, no matter what the tube tester (or tube salesman) says. I tried a few of these "superior" FETs (as per spec sheet) and found that their headroom was so grossly inadequate that I was scratching my head where that discrepancy may stem from. If I were to rely on that information, many a FET would be preferable to the good old 2N3819 or other stand-bys which Neumann used in more microphone models for more years than any other FET. I have found that the two most relevant parameters of J-FETs, noise and headroom, cannot be gleaned from 1/F, max voltage or other information provided by the manufacturer. I am afraid the answer is not as straight-forward as throwing you a simple part number or referring to spec sheet for a given FET.
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