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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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    \$\begingroup\$ 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. \$\endgroup\$
    – AndrejaKo
    Jan 18, 2012 at 8:01
  • \$\begingroup\$ @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. \$\endgroup\$ Sep 12, 2015 at 6:02

7 Answers 7

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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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    \$\begingroup\$ 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 The pages you link to are somewhat old/outdated (20-years old 'puters).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. \$\endgroup\$ Sep 12, 2015 at 5:44
  • \$\begingroup\$ 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. \$\endgroup\$ Sep 12, 2015 at 5:53
  • \$\begingroup\$ 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. \$\endgroup\$ Sep 12, 2015 at 6:49
  • \$\begingroup\$ 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.) \$\endgroup\$ Sep 12, 2015 at 7:34
  • \$\begingroup\$ 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. \$\endgroup\$
    – Oli Glaser
    Sep 13, 2015 at 1:56
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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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  • \$\begingroup\$ What you mean of current capacity? \$\endgroup\$ Mar 27, 2022 at 13:07
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They need a supply as they have a built-in FET buffer.

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Electret microphone always requires "bias voltage" (typically 1-5 V with 2-10K resistor) that's different from "phantom voltage" (48V with 6.81K resistor) used for condencer microphones. The bias voltage is also called "plug-in power" in some US publications.

The sad fact is that most electret microphone manufacturers do not say this in marketing material but instead say that a microphone is compatible with e.g. old iPhone only with 3.5 mm connector.

It's unfortunately somewhat random if PC soundcards can output the full 5 V bias voltage so electret microphones may or may not work when plugged to PC without a simple adapter with bias voltage in between. Note that some electret microphones might work even with 1 V bias voltage but some require full 5 V bias voltage to turn on. Most smartphones and some camcorders output bias voltage which causes the whole issue about microphone being supported with selected devices only.

In theory is should be very simple to create a device that has one USB connector to use as 5V power source with 3.5 mm TRS in and out to function as bias voltage adapter for a PC. I've yet to found a cheap pre-made device to do that so it may be time to grab a soldering iron. If you don't yet have one, I'd suggest getting TS100.

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  • \$\begingroup\$ Nope, sorry. Most every PC provides the bias voltage required for typical electret microphones. That won't power a professional electrect microphone that requires 48V phantom power, but then you should connect professional microphones to a professional sound card - and professional sound cards do usually provide the required 48V phantom power. \$\endgroup\$
    – JRE
    May 25, 2021 at 16:43
  • \$\begingroup\$ Perhaps I've been unlucky but none of the desktop computers I've used have been compatible with electret microphones and even some laptops have failed, too. \$\endgroup\$ May 26, 2021 at 6:53
  • \$\begingroup\$ Very unlucky indeed. Since PCs have had sound cards, I've never seen one that didn't have a bias voltage for an electret microphone. I've been fiddling with that kind of stuff since the mid 1990s, and every PC and laptop soundcard I've ever used had the bias voltage available. \$\endgroup\$
    – JRE
    May 26, 2021 at 7:24
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    \$\begingroup\$ Are you sure you are using the microphone input rather than the line in? Microphone is pink, line in is blue. Line in will not have bias voltage. Line in is also stereo. Microphone in is mono and has the bias voltage. \$\endgroup\$
    – JRE
    May 26, 2021 at 7:36
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    \$\begingroup\$ It is not "rare" or "random" for a PC sound card to have the bias voltage for an electret microphone. Every sound card on the planet has it. I have never found one without it in over 25 years. They may exist, but you'll have to go looking for one specifically made that way. You will not find a typical sound card (internal, add on, USB) with out the microphone bias voltage. \$\endgroup\$
    – JRE
    May 26, 2021 at 10:52
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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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Check your (PC's) mic jack is rigged to bias an electret's internal FET by exposing its conductors (eg. 3.5mm extension cable) and holding any skuzy old red LED against them (LED cathode [usually large part of the light element] to ground obviously). If it glows steadily it can only indicate bias power: no standard high impedance capacitative ac amp' input could do that. (Luv+hugs, mc)

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Check the electret is wired correctly in terms of polarity +/- if it's the type that has a built in preamp in the module. This is usually indicate by the colour of the lead,red to +ve/tip, black to -ve/earth. If it isn't one with a built in preamp then you need to connect the connection that is also connected to the case to earth. You can check which it is by using a test meter from the case to the two connections in turn. The one that give 0ohms is the one to connect to earth.

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