Have you tried the circuit with different capsules?
Did you test the circuit for linearity without capsule (with a small C as dummy capsule)?
It is quite easy, replace the capsule with a small capacitor. The original schematic says 36pF/50V is right for that circuit. Some pF more or less will work as well. Than insert a small test signal and check the output of your mic. This will test the frequency response (and THD if you want) of the circuit and transformer without the capsule signal. The small cap simulates for the circuit the capsule.I do not know the technique to simulate a capsule
I don't understand this part. "Grid on the surface"?I polarized with the capsule mounted, for the moment I only have a single capsule which has a grid on the surface,
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