PC microphone phantom powering improvements

Nowadays the most commonly used microphone types that need power from microphone connector are PC sound card microphones (3.5 mm plug) and Phantom powered professional microphones (XLR connector). Those two microphones are not compatible with each other, but there are cases where you might want to interchange them. In this posting I will describe how to connect PC microphone to professional mixer phantom powered XLR mic input. This is an application I have made few times and I have got questions several times on my mailbox.

Phantom power feeds 12-48V power to both signal wires on the connector as describes in this picture from Belden document Phantom Powered Microphones:

PC microphone feeds lower voltage (typically 3-5V) to ring contact of 3.5 mm jack (similar to plug-in-power PiP). Here is the operation principles of PC sound card microphone interface from my Microphone powering document:

On my Microphone powering document I had provided a “simplest circuit”: very simple method of connecting an electret microphone capsule to the a balanced phantom powered (48V) mixer XLR input.

                     _____
   HOT (2) ---------|_____|------+
                    47 kohm      |+
                                MIC
  COLD (3) ----+              CAPSULE
               |                 |-
GROUND (1) ----+-----------------+

Note that this is a very simple “hack” to interface an electret microphone capsule to a mixer. This circuit works but has it’s downsides like sensitivity to noise in phantom power, unblanced signal transmission (prone to interference) and high output impedance (can’t properly drive long cables). This circuit can be used to test electret capsules connected to mixer using very short cable. The circuit has proven to work with some modern electret capsules and some mixers, but has had some problems. For example some microphones fail to work with this circuit properly (if they take very little current the voltage over them can get too high for them to work).

So something better was needed. The balanced circuits from Microphone powering document are out of question for PC microphone adaptation, because in PC microphones minus side of electret capsule is connected to cable shield, while my balanced adapter circuit needed the electret capsule to be floating (T-power like connection).

Trying to just wire in PC mic with cable to such circuit will cause the circuit to pick up lots of noise! “Better electret microphone phantom powering circuit” and similar designs from Microphone powering document are not suitable for PC microphones or headsets as they are normally wired.

So I needed a “different better approach” to power PC microphones from “P48” or “phantom power 48 volts”.

Some years ago I made a better trick that works with PC microphones which are wired so that tip and ring contacts are shorted together on the mic (quite many microphones have I have are two wire electret capsules wired in this way). I took apart an old adapter I built years ago and had not documented so I can document it now:

I have wired 27 kohm resistor from XLR pin 2 to 3.5 mm plug tip.
I have wired XLR pin 1 to 3.5 mm plug shield/ground.
I have wired 2.2 kohm resistor between 3.5 mm plug tip and shield/ground.

In this hack I had not connected anything connected to XLR pin 3, so maybe it would be a good idea to have there 26 kohm resistor to pin 1 ground.

I added that and ended with the following circuit design:

 

schemeit-project

 Drawn with Scheme-it 
 R1 = 27 kohms
 R2 = 2.2 kohms
 R3 = 27 kohms
 J1 = 3 pin XLR male
 J2 = 3.5 mm stereo jack

This circuit seems to work well. The voltage divider formed by R1+R2 make sure that the voltage the microphone sees is 4 volts or less (depending on load caused my microphone). The source impedance of the power source is also similar to the circuits in PC sound cards. R1 passes the signal to mixer + pin somewhat attenuated. R3 tries to provide approximately same load to – side of XLR as the + side gets (around same amount of current taken and similar AC impedance to take care of balance).

This circuit has been proven to work pretty well when the adapter was plugged directly to mixer XLR input or though short cable. Due high output impedance it might not be a good idea to use long cable. This circuit has limitation that it only works with PC microphones that have tip+ring directly wired together to two wire electret capsule as shown in Computer microphones web page:

Most of the PC microphones and PC headsets are wired in this way, but maybe not all. All the PC microphones and headsets I had access were wired in this way. You can check with a multimeter if your microphone is wired in the ways: just measure the resistance between tip and ring, it should be zero or almost zero. If that is the case my circuit works with it well. If the resistance is not zero or almost zero, there is no guarantee that things will work out as expected. The circuit can be built so small that it fits inside XLR connector:

pcmic2xlr

Here is the circuit in use:

mic2xlrmix

14 Comments

  1. paolo gramigna says:

    Hi,
    I have tried the above circuit, and it works!
    But there is a problem: when i connect the mike using this circuit, i get a loud “POP” on the mixer’s output.
    I supposed that it comes from the mike when powered.
    But when i disconnect the mike, i get another loud “POP” and that’s second POP cannot come from the mike…
    Those POP are really annoying.
    How can i get rid of them?
    thanks for any advice,

    Paolo from Italy

    Reply
    • Tomi Engdahl says:

      Thank you or your feedback on the circuit.

      I don’t have any ready solution for POP problem.

      Reply
    • lasershark says:

      hi. it is standard practice to mute channels/pull down faders/switch off amp and speakers before connecting or disconnecting a phantom powered microphone to a mixer.

      Reply
      • Tomi Engdahl says:

        Yes it is.
        And it is a recommended practice to turn off the phantom power (or whole mixer) when plugging/unplugging phantom powered microphones because some sensitive (not well protected older devices) can be damaged if you plug/unplug with power on.

        Reply
  2. Raziel says:

    Greetings,

    I am interested in using a high quality gaming headset for DJing. There seems to be better headsets in gaming than pro-audio.
    I would like to know if you have a Digi-Key part number for the resistors you used for the last circuit with 3 resistors.
    Your insight is greatly appreciated. Thanks in advance.

    Cheers.

    Raz

    Reply
    • Tomi Engdahl says:

      Thank you for your feedback.

      Gaming headset market has evolved while it seems that pro audio headset market has not changed that much.
      Depending on the different needs I have faced needs to adapt some pro headsets to gaming and
      gaming type headsets to pro audio devices.

      Unfortunately I don’t have Digi-Key part number for the resistors in the circuit, because I have not bought
      those resistors from there and I don’t use Digi-Key to get parts for my circuits (I know Digi-Key, but
      I typically source most my parts I don’t have from local components suppliers or directly from china…)

      Reply
      • Raziel says:

        Sounds good.

        Can you provide a bit more detail about the resistors?
        27K Ohm (1W? 1/4W?)
        2.2K Ohm (Watts)

        I just want to be sure to order the right ones. I am really planning to make a few circuits for my pro-audio setup. Any help you can provide would be most appreciated. Thanks.

        Cheers.

        Raz

        Reply
  3. Chris says:

    Hi Tomi, thank you for these explanations. I want to use a homebrew gooseneck mic w/ XLR connector on my Alesis iO26 interface to talk-back to the musician and realised the circuit with the voltage divider R1-R3 as you described here. It works nearly perfect, but the level and/or gain is too low, the signal is too weak, I have to yell into the mic to get heard. So, my idea is, to build up a little FET preamp, using a BS170 due to it’s high SD-voltage of 60V. But – at the moment – I have no clue how to build it up with the symetrical XLR connector. Do you have any suggestions for this project? Any hint is appreciated!

    Thank you very much!
    Chris

    Reply
    • Tomi Engdahl says:

      For stronger signal with my circuit, you could add a suitable capacitors in parallel with R1 and R3.
      The capacitor will pass the AC signal while still blocking the phantom power DC on normal operation.
      What still needs to be considered is power spike when phantom power is turned on.
      For getting more signal to mixer I would consider something like instead of just capacitor, use capacitor + suitable resistor circuit.
      I would try something like 10 uF electrolytic capacitor (60V or higher voltage rating, + side to XLR connector) in
      series with 2.2 kohms resistor… This should give much stronger signal, depending on mixer designs I would expect to get something
      like 10 dB more signal level…

      You can also find some more complicated microphone interfacing circuits at
      http://www.epanorama.net/circuits/microphone_powering.html

      There are several circuit ideas at “Balanced electret microphone circuit”.
      Those give stronger signal and well balanced mic output – at expense of more complicated circuit and electret capsule is floating.

      Reply
      • Chris says:

        Great, works very good, seems more than 10dB! I forgot to mention, all componets must fit into the XLR plug and/or the head of the mic, so (your) other sugestions I was still aware of, but I’m not a SMD Pro ;-) Next days I’ll see whether the level is high enough for my guitar player, but I think, it’s gone be ok, otherwise the punishment well be SMD as shown here:

        http://www.naiant.com/images/MSH_Kit_Mic.GIF

        Thank you!

        Reply
  4. Jon Blackstone says:

    Tom -

    I’ve got a solution for this that’s very simple, and is working for me.

    I connect the ECM capsule (cheesy computer mic) directly to pins 2 and 3, and just pull pin 3 towards ground (pin 1) with a 47K resistor to create a 6V differential across the ECM.


    (2)---------------+
    MIC
    (3)----+----------+
    47K
    (1)----+
    (both mic wires insulated from any metal casing)

    This avoids attenuating the signal as the 3-resistor scheme does. And I may be mistaken, but I think it’s even roughly “impedance balanced”, as the ECM floats much the way a transformer in a dynamic mic would (the 47K additional loading on one side being negligible).

    Am I missing something?

    Reply
  5. Jon Blackstone says:

    (Oops. My little diagram got its spaces stripped out.)

    Reply
  6. Tomi Engdahl says:

    That stripping down of spaces is a feature of this blog system commenting.

    I still understood the drawing anyway.

    And I think that your idea is a really good idea indeed.
    I have to test it.

    Reply
  7. Tomi Engdahl says:

    I tested the circuit and got it working… So the idea works in practice.

    Reply

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