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I was under the assumption that electret microphone capsules do not require any phantom power an can be connected in the same way dynamic microphone capsules can be, due to the fact that they contain a small permanent static charge built in. I am trying to build a really simple mic to hook up to my computer via the 3.5mm mic jack, and after wiring everything up, it doesn't seem to be working. Is this because there is in fact supposed to be some sort of phantom power circuit, or should I just double and triple check my wiring again?

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Be sure to check everything again on the computer side. Many dumb computer microphones are just an electret microphone connected to the microphone input jack and they do work correctly. If I remember correctly, computer should already provide power on the microphone jack for electrets. –  AndrejaKo Jan 18 '12 at 8:01
@AndrejaKo: Yes for "mic in", color-coded as the pink connector on Intel-standard motherboards. No for "line in", color coded as the blue connector on said boards. Cheap laptops often have only "mic in", lacking line in, which in this use case (electret) is a blessing, but otherwise not. If you accidentally plug a preamp's output into the mic in jack, interesting stuff can happen, ranging from popped output cap to oscillation. –  Respawned Fluff Sep 12 at 6:02

4 Answers 4

Electrets don't need phantom power for the diaphragm, but the small capsules usually have a FET buffer inside which needs bias voltage. This can of course be derived from a phantom power source if necessary.

Here is a typical electret circuit:

Electret circuit

The value of the resistor is usually between 2k and 10k (cap say 10uF or higher) The datasheet for your capsule will probably give recommended operating conditions.

Here's a good link on simple electret circuits.
Another page with some more advanced ideas.

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The (AFAIK) non-standardized bias voltage (around 2.5-3.3V) supplied through the 3.5mm microphone jack in most consumer equipment is sometimes called "plug-in power" or PiP. This can (and usually is) used to bias an electret through a simple resistor, often a 4.7 KOhm one. The pages you link to are somewhat old/outdated (20-years old 'puters). My experience with equipment built in the last ten years is that they often provide PiP on both tip and ring on the mic jack. I only have one piece of hardware that gives PiP only on the tip. –  Respawned Fluff Sep 12 at 5:44
The voltage for nowadays equipment is also lower (2.5V to 3.3V like I said). I have not seen a single piece of hardware made in the last 10 years that gives the full 5V claimed on that epanorama web page. –  Respawned Fluff Sep 12 at 5:53
Perusing various Realtek ALC datasheets (which often provide the DAC/analog portion of Intel [HD] Audio solutions), explains these voltage choices, e.g. "Software selectable 2.5V/3.2V/4.2V VREFOUT as bias voltage for analog microphone input". I have not seen the 4.2V option implemented though. –  Respawned Fluff Sep 12 at 6:49
Also, the WM8731 used in some iPods has a MICBIAS of 0.75*AVDD; the latter is normally 3.3V, yielding 2.475V = ~2.5V MICBIAS. I have yet to see why the iPhones typically only dish 1.5V on their mic output. That causes some off-the-shelf electrets not to work with the iPhone (more sales for proprietary Apple stuff though.) –  Respawned Fluff Sep 12 at 7:34
Thanks for the info, I was mainly just giving the "big" picture on electrets. Both pages state a voltage between 1V and 9V or higher. There are advantages to using a higher voltage, and I was thinking the OP may want to supply his own whether the PC supplies a bias voltage or not. The first link (and site) has some excellent info on building quality mics from cheap electrets. Of course if the PC supplies a suitable bias and a "standalone" mic is not necessary (e.g. to work with other equipment that may have no bias voltage), then bu all means use it - the circuit is the same. –  Oli Glaser Sep 13 at 1:56

They need a supply as they have a built-in FET buffer.

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"Phantom power" is not the same thing as "DC bias" for a condenser mic. The former is like power over ethernet, or USB, it's riding on the data or analog lines for the purpose of operating circuits, like an LED or a circuit to power a USB extender, or to power a preamplifier for a 'condenser mic', for example.

DC bias for a bare condenser mic is a completely different thing, as the capsule has a capacitor with a moving plate and some of that DC voltage (note I did NOT mention 'power', because it's a voltage device, not a current device) is there to keep a charge on the capacitor, so when the moving plate vibrates, electrons are forced off the plate or more must be added to keep the physics of the static charge and the distance solved. That's how the mic creates the voltage signal. The capsule also has a FET buffer, and it converts the high impedance capacitor to a low impedance output across the FET terminals (Drain and Source).

Phantom power is anything but "phantom", as it's generally 40+ volts and has considerable current capacity to drive circuits and amplifiers. Whereas, DC bias is generally under 10V, mostly around 2.5-3.0 VDC, has little or no current capacity.

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I have done my final year project using this electret condenser mic. The gain of the output voltage the mic is so low and so the output is only around 5mV. So you need to implement a mic pre-amp circuit.

The pre-amp circuit should raise the value of the voltage to a certain level such as between 2.7V and 5V. The range of the voltage that you have to set for pre-amp circuit depends on the voltage range of the ADC of the computer.

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