Not all microphone capsules are flat. Sometimes they need electronic equalization.

First let’s look at the circuit, leaving aside capsule bias supply and other non-audio bits. We have a simple common source FET amplifier driving a transformer. The feedback circuit consists of C11, C10, R10, C12, and R11. R12 is a test point for injecting the test oscillator signal. We see that the feedback and test signal are injected into the “ground” side of the capsule which then couples them to the gate of the FET.

C10  and R11 form a high pass filter for negative feedback. C11 is big enough it doesn’t affect audio frequencies. C12 is small enough that very low bass won’t pass thru.

R10 limits the depth of HF feedback. NFB in the highs means high frequency cut for the amplifier as a whole.

So we should expect a top end fall off due to the feedback from C10, C11, and R10 and maybe some bass fall off due to C12 and R11.

To disable feedback, we simply disconnect C11.

Lo and behold, that’s just what we find. Feeding in white noise, with NFB removed (yellow), the top end is flat. Green is NFB enabled. So the effect of the NFB network is surprisingly small, 3 or 4 dB at 20KHz and 3dB at 50 Hz. Blue and magenta are lo cut without and with NFB respectively.

Considering that some of the capsules used in low-cost mics have sharp peaks of 8-10dB around 10 KHz, this EQ circuit won’t tame them by more than 1 dB.