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this post was submitted on 09 Dec 2023
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Technology
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I have some headphones with a mic, but there is no bias voltage contact, only Left, Right, Mic(return?)and Ground. How does that work?
When you speak into a microphone, there is a membrane that picks up the vibration from the air, and either by moving a coil, compressing a piezoelectric crystal, or moving a MEMS system, it changes the voltage between ground and output.
Now, every time that membrane moves, it converts sound into electricity (same happens if you speak into a speaker, BTW), but... all real world materials have a resistance, capacitance, reactance and a resulting impedance, which need to be overcome for the signal to resemble the sound the membrane is picking up.
The difference is:
Basically, without power, the mic is "deaf" to part of the sound, which might be fine if you give it sound loud enough, but will always "sound" worse than a powered mic.
Soooo close; generally only condenser microphones require bias power, which is called phantom power in the professional audio world. Dynamic, ribbon and piezoelectric microphones do not need phantom power as the movent of the diaphragm directly creates the output voltage that is measured and applying phantom power does nothing, with the exception of ribbon mics which destroy themselves. But yes the 5v bias is the third connector to enable electret condensers microphones to work.
Phantom is 11-48V power over a balanced pair with the signal galvanically isolated, bias is direct 5-9V power over the 3rd cable. It's generally not a good idea to mix them up and hook up a bias powered mic to a phantom power source. Also better not to hook up a passive mic with balanced output, to any kind of power source.
As for the voltage generated by the movement of the diaphragm, the amount and power varies by multiple orders of magnitude depending on technology and size of the mic, but you will always have some part just converted to heat and lost from the output. The best way to minimize that, is to provide power beforehand, to a preamp specific to the particular mic, that will keep the diaphragm in its optimum operating range, then boost the signal if needed.