I see where you come from, and I agree with many of your statements from technical point of view, but I think at least some of your proposals might prove to be overkill or may even introduce other issues. There are plenty of reasonably RF-immune mics around without some of your, imho rather extreme RFI countermeasures included. Let me review your statements and allow me to give my point of view. Admittedly, I'm not an analog RF wizzard and don't want to pretend I'm on the same level wrt RF knowledge and experience as you are, but I do have a background in electronics and 40+ years of hardware design. I apologize in advance if you consider my comments to be Too Long To Read, but some things are not easily explained in a few words (at least, not by me).
"Confirmation bias is real, so divide and conquer. Exclude other causes, use another mike, pre amp, power supply, another mike cable etc etc.": Agree with that. When reading all the experiments OP already and described in this thread and another one, he is definitely open minded and not afraid to experiment.
"The mike should be a Faraday cage, with a continuous metallic contact around any susceptible point. Any opening should be a "waveguide beyond cutoff". Any metallic weave or screen should be cut from a flat sheet and expanded, then bonded to the mike cavity for continuous peripheral contact.": Agree with the Faraday cage statements. As I already said in another post, the weak point is the XLR for its opening and for the almost inevitable inductance between XLR cable shield (for what it's worth as a shield at cell phone frequencies), via XLR pin 1 to the mic body. Whether your metallic weave proposals are practically feasible and attractive...? Perhaps you have a clear view of how to implement this in a practical way while at the same time not being too visually disturbing. A round PCB with ground plane connected to pin 1 and the mic body by as many points as possible, like shown in one of the earlier posts, seems more practical to me, to be honest. But for a DIY already difficult enough to build, especially the connections to the mic body.
"Add a 2k - 10k resistor in the gate lead, wirewound is OK, max 1mm from FET.": I would not do that, at least not above 2k as it would add noise. A ferrite bead having the same RF resistance at the frequencies of interest will not add noise in the audio band.
"The leads from the XLR to the PCB could be (wirewound) resistors, 50-ish Ohms should be tried, followed by caps max ~100pF to case, with a low inductance connection, like BeCu fingerstock soldered to the PCB then circumferentially connected to the conductive mike tube's inside wall.": From technical point of view, I agree. I actually made similar statements here in this and other threads: large caps on the output won't help against UHF cell phone RFI. It's the inductance to the shield/Faraday cage/mic housing which should be kept as low as possible. The question is only how to do that in a practical way. The BeCu fingers you mention are effective and commonly used in the RF arena, but I have yet to find those fingers small enough to be of practical use in an SDC
and do not get stuck when inserting the PCBA with XLR into the tube. There are some XLRs having a proper ground contact to the mic tube, but I cannot find them as a commodity part. Takstar CM-63 has one. Where to get those...?
I have just ordered 4 different PCBs with the KM84 circuit, KM84 + CMOS oscillator for capsule bias voltage and two other circuits I don't want to share (yet). They were designed for the Takstar CM-60 and 63. All SMT designs, except for some parts like the elcaps. Will do some experiments with my RF jammer to see what helps and what doesn't. One of the designs has below RF filter with X2Y cap, Common Mode filter and ferrite bead in the ground path. Inspired by the Takstar CM-63 RF filter, which proved to be very effective. But as said, that may be attributed to their good, low inductance pin 1 to mic body connection, rather than the CM filter. Anyway, whether it is effective or not, such a filter cannot be applied to OP's PCBAs.
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