How do I increase the signal to noise ratio in a true condenser microphone?
If you're making your own capsule, try a plate spacing of 0.001" (0.025 mm) as a starting point.

Also, it would be normal to connect one plate to the +V supply (for example  60V DC and very well smoothed), and the other plate directly to the gate of a low-noise JFET (2N5459 for example). 

The gate should have a DC path to ground via a high resistance (usually about 1000 Mohm) Any lower than this, and the capsule capacitance (which will be in the region of 30pf with the gap suggested) will attenuate low frequencies. With a larger gap, capacitance will be lower, and attenuation considerably higher.

You can use the FET as a source follower, taking a low impedance output from the source (which is connected to ground by a few hundred ohms) into a decent mic amp or your sound card.







simulate this circuit &ndash; Schematic created using CircuitLab

(R3 is optional, but gives a complementary output to connect to a balanced input on a decent mic amp or mixing desk)

And I recommend surrounding the whole assembly with perforated aluminium sheet, or wire gauze, which has to be grounded, as an electrostatic screen.

Even with all these steps, my own home-made capsule was still about 15dB less sensitive than the commercial capsule I eventually bought...