this post was submitted on 09 Dec 2023
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"Most computer microphones use the third segment to carry bias power for the microphone." https://en.wikipedia.org/wiki/Microphone_connector
I had thought it was to deliver sound to both sides (rather than left- or right-only) but here we are.
What is bias power?
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.
This is false. I suspect this myth came about because this is how magnetic audio tapes work (tape bias).
Dynamic microphones do not benefit from bias. They can tolerate a small amount but too much will burn them out (depending on their resistance & the voltage applied) or increase distortion (depending on the mechanical construction & how much the diaphragm is moved by the DC). Some dynamic mic units are built with capacitors in them to intentionally block bias voltages, preventing them from burning out.
I have never seen a datasheet or research paper showing improved dynamic mic performance due to DC offset. If it helped then a manufacturer would be recommending it in the datasheets (so they could claim better distortion & sensitivity specs).
Mics with in-built amplifier circuits require bias voltage to function. Many small "electret" modules contain jfet amps, you have to check the datasheet because they look identical to non-amplified versions on the outside. This is very common in small computer & headset mics. Some might work without bias, but they will sound poor because the amplifier circuit is not designed to work this way.
Condenser mics need some form of bias voltage to function at all. Electrets provide this themselves through some magic materials science that's similar to a battery that lasts for years/decades/centuries. The other types of condenser mic require you to apply an external bias voltage (aka "phantom power").
Magnetic audio tape suffers 'hysteresis' and nonlinearity which cause distortion of audio (especially quiet audio). Applying a bias voltage works around this problem. DC biases work, but high frequency AC ones are typically better.
I suspect the source of this myth is a confusion between the magnetics of tapes and the magnetics of dynamic mics. I think I recall a year 8/9 science class where I was taught that audio could be amplified slightly by putting a battery in series with a microphone and speaker. I failed to find any sources to support that at the time, but the teacher was adamant that this used to be a legitimate method. Perhaps if the coils were not glued properly in the speaker & mic? It was supposed to be a solution before the days of tube amplifiers but I think the true information turned into nonsense somewhere along the chain.
Resistance, capacitance and inductance are linear. They will affect all signals the same way, they will not only affect small signals.
To affect the small signals differently to the large signals you need nonlinear elements, like diodes and transistors. EDIT: there are also nonlinear capacitors and resistors, but they're from more exotic materials than what you find in standard headphone wires & mic designs.
Most likely the mic is simply powered by the voltage which also powers the headphones.
But there's also mics which don't need voltage to work at all (unlikely for headsets tho).
Thankyou for asking this question, I have no clue and you're making me think that a recent frontpanel audio TRRS jack board I designed might be wrong :D
There are two possible options I can see:
I cannot find any good references or info about mic bias and TRRS connectors :( Anyone else have any luck? Wikipedia says it's a standard referred to as "CTIA" or "AHJ" but those appear to be company names, not standard names.
My current headset uses a TRRS, but also provides an extension cable that splits into two 3.5mm TRS just like yours. I might probe it out and find out what it's doing (but that doesn't mean it's the right/universal solution).