As the Internet has turned so many of us into content creators, we’ve seen the quality of webcams and microphones steadily increase to the point at which even a fairly modestly-equipped YouTuber now captures their wisdom at a quality far exceeding that you might have found in some broadcast studios not so long ago. Still, decent quality costs money, and for that reason [Spirit532] has built his own high quality condenser microphone for less expenditure.
The capsule and body are off-the-shelf items — what he’s produced is the bias voltage supply and preamplifier. In both cases these are the interesting parts of a condenser microphone, so their circuit bears a second look.
The condenser microphone takes a diaphragm and turns it into one side of a capacitor. If you apply a charge to this capacitor, the voltage over it changes minutely with the capacitance as the diaphragm vibrates. Thus to have a usable audio signal level a high-voltage bias supply is required to provide the charge, and a very high impedance preamplifier circuit to catch the signal without draining the capacitor.
His bias supply is a charge pump using a string of diodes and capacitors fed by a chain of CMOS inverters, with an RC filter and resistor chain to provide that super-high impedance. The preamplifier meanwhile is a unity gain high-impedance op-amp with an inverting stage to provide a balanced connection. For good measure the circuit also includes a phantom power supply.
This is an interesting project for anyone with an interest in audio. if you’re further interested in condenser microphones, how about also looking at electret microphones?