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Multimedia microphones

Nowadays multimedia microphones which come with Sound Blaster soundcards or are built into a computer monitor are becoming more and more common items. Those microphones are typically small electret micropone capsules, which are powered by soundcard through then 3.5 mm stere plug installed into microphone cord. Typical characteristics of Sound Blaster microphone input:

     Input Type: Unbalanced Low Impedance
     Input Sensitivity: Approx. -20dBV (100mV or 0.1Volt)
     Input Impedance: 600 to 1500. (Ohms)
     Input Connector: 3.5mm Miniplug (Stereo Jack)
     Input Wiring: Audio on Tip, Ground on Sleeve, 5Volts DC Bias on Ring
Typical characteristics of microphones designed for Sound Blasters:
 Frequency response      50 - 12500 Hz
 Output Impedance        2 Kohn
 Equivalent noise level  34 dB
 Operating voltage       1 -12 volt.
 3.5 mm minature male standard plug

Electret microphones need some operating power to work. Without power there is no sound output. You might have noticed this if you have tried to use them with any other equipment than the soundcard they are designed for. The trick is the wiring scheme used in those microphone connectors:

TIP:    Sound signal from microphone
RING:   Power to microphone (+5V) 
SLEEVE: Ground
                                   

     +------+                                                / \
     |      |-------------- output--------------------------|   |
     |      |                                                \ /
     | mike |                                               +===+
     |      |--------------- bias +5V ----------------------|   | 3.5mm plug
     |      |                                               |===| to soundcard
     |      |                                               |   |
     |      |---------------- ground -----------------------|   |
     +------+                                               |   |
                                                          +=======+
                                                          |       |
The power feeding to microphone is typically +5V feed through 2.2 kohm resistor (this approach is used in SB16 soundcard). You can use this same powering method if you want to use those microphone in your own projects.

If you want to use a two wire electret microphone using soundcard bias voltage you have to use tle following wiring:

                                       10 uF
     +------+                          +||                   / \
     |      |---------------+-----------||--- audio --------|   |
     |      |               |           ||                   \ /
     | mike |               |                               +===+
     |      |               +--RESISTOR---- bias +5V -------|   | 3.5mm plug
     |      |                  2.2 kohm                     |===| to soundcard
     |      |                                               |   |
     |      |---------------- ground -----------------------|   |
     +------+                                               |   |
                                                          +=======+
                                                          |       |

Rember that the microphone connections in PC soundcards are not standardized, so there are also other wirings in some soundcards on the market. One modification is that the TIP of the microphone connection holds both the bias voltage feeding (+5V through 2.2 kohm resistor) to microphone and carries the audio signal from microphone.

Quite many soundcards do not supply any kind of bias voltage to micrphone connector. Those soundcards are designed to be used with dynamic microphones and the input is more sensitive. Typical parameters for this type input.

     Input Type: Unbalanced Low Impedance
     Input Sensitivity: Approx. -50dBV (3mV or 0.003Volt)
     Input Impedance: <600. (Ohms)
     Input Connector: 3.5mm Miniplug (Mono or Stereo Jack)
     Input Wiring: Audio on the Tip, Ground on the Sleeve
You can use the microphoens which need bias voltage with this type of circuits using the following wiring to supply external +5V power.
 To soundcard or other circuit                         Electret microphone
 without bias voltage feeding
                  
                           10 uF
        / \                 ||+                              / \
       |   |----------------||------------------------------|   |
        \ /                 ||                               \ /
       +===+                                                +===+
       |   |         +5V ----------------Resistor-----------|   | 3.5mm plug
       |   |         power               2.2 kohm           |===| to microphone
       |   |                                                |   |
       |   |----------------- ground -----------------------|   |
       |   |                                                |   |
     +=======+                                            +=======+
     |       |                                            |       |

For more information about microphone connections, read the technical document available at Shure WWW server at http://www.shure.com/app-soundcard.html.