Electronic – How to increase the signal to noise ratio in a true condenser microphone

condenser-microphonegroundingnoise

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

Best Answer

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.

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