"Real" condenser microphone circuit board

Schematic source: https://www.instructables.com/True-Condenser-OPA-Mics/

The preamplifier part is the same as the large diaphragm pure op amp microphone (48V) . I integrated the polarization plate into one board.

I used this board to verify the homemade capacitor capsule, so I directly used the maximum polarization voltage. When R11=0, the polarization voltage is the highest 80V. The most common imitation K67/87, CK12 and other capsules generally have a polarization voltage of 60V. Using a higher polarization voltage will increase the sensitivity, but the frequency response curve will also change. If you need to lower the polarization voltage, do not solder R11 first, connect a potentiometer to H1, power on and test the voltage from the Pol_T pad to ground, adjust it to the target voltage, and then connect a resistor with a value similar to the potentiometer's resistance to R11. U2 is a Schmitt trigger, 4584/40106, etc. can be used.

Validation results:

The sensitivity of an externally polarized pure condenser microphone is indeed much higher, and it is better than an electret with the same large diaphragm. However, whether I DIY a pure capacitor or an electret, it is very easy to get damp in a Guangdong room with a humidity of 80%. If it is not put in a box, it will become very noisy overnight, but if I put it on the dehumidifier and blow it for a few minutes, it will stop It's completely better, and commercial condenser mics won't change that much. I'm not sure whether it's the microphone problem or the circuit board problem. Yesterday I bought PCB insulation paint to paint the circuit board to see if it can improve the problem.

Directions for improvement:

1. I use 1206 for R9R10 just because I don’t have the resistance value of 0805. OCD can replace it.

2. A group of friends reminded me that it is not recommended to use ordinary MLCC for the C6C7 on the front board. Using C0G or CBB capacitors can reduce noise.

3. I have a problem with the shielding hole on the input pin of the op amp. It is recommended to encrypt the shielding hole and thicken the wiring.

I am an amateur, and I have not learned many basic circuit knowledge comprehensively. If you have any other questions or suggestions for improvements in the project, please feel free to submit them.