How do I increase the signal to noise ratio in a true condenser microphone?

Question

I made a simple condenser mic as a test project for something more functional. However, when I connect my microphone I only hear the mains hum. Using a factory-made condenser I hear the mains hum except I am also getting a decent signal. I know for a fact then that my laptop has some grounding issues (the metal case always shocks me). I want to know why my circuit does not provide any audio signal what so ever. Go easy on me please, I know my way around audio equipment, but building it is a totally new field for me. Here is the schematic (adapted from John Salmon's version on YouTube):

Note: The Al foil is conductive as well as the back plate. The capacitor, resistor and power supply are functional.

Answer 1

Condenser microphones generate notoriously low output voltages. at 5 mm spacing your output voltage will be very very small.

It is as good as impossible to obtain good S/N ratios without:

• Putting a low noise preamplifier in the mike itself. (Eg. a low-noise FET amplifier). Ever seen those old, large studio microphones? They had a special amplifier in the lower part.

• Running the connection over a balanced feed (I.e. a shielded, twisted pair of conductors, with a transformer at each end) to avoid unbalanced coupling of AC fields.

• I'm note sure what the + and - mean at the output, there's only AC at that point... Do connect the front (outer) plate to the shield.

Answer 2

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 – 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...