Computer microphonesInterfacing Microphones to Computer Sound CardsMost sound card microphone inputs require a minimum signal level of at least 10millivolts, but some older 8-bit cards need as much as 100 millivolts. The typical impedanceof the PC soundcard microphone input is in order of 1 to 20 kohms (can vary from card tocard). The microphone type which works best with computer sound cards is the electretmicrophone.Sound Blaster soundcards (SB16, SB32, AWE32, AWE64 or Live) from Creative Labs havea 3.5mm (1/8 inch) pink stereo jack for the microphone input, with the following pinout: 1. Signal input (tip) 2. +5V bias (ring) 3. Ground (sleeve)Note: Most soundcards will wire the positive DC bias voltage to the ring, but a small numberof non-standard soundcards can have the bias voltage wired to the tip. A few cards have ajumper which enables or disables the power to the microphone jack. If the jumper is put on,the bias voltage ( +5V through a few kiloohm resistor) is wired to the tip. Newer mainboardswith stereo microphone support will provide the bias voltage for both the tip and ring. The approximate schematic of aSound Blaster microphone input circuitry shows that the +5V voltage on the connector isheavily current limited. The cards voltage might not be exactly 5V, but it is usuallysomething between 3 and 5 volts when no microphone is connected.Electret microphonesThe electret microphone is the cheapest omnidirectional microphone you can buy. Verysensitive, durable, extremely compact in size, electret mics are used in many applicationswhere a small and inexpensive microphone with reasonably good performance is needed. Youcan find them in almost every stereo equipment, in consumer video cameras, mobile phonesand so on.
The electret is a modified version of the classic capacitor microphone, which exploitschanges in capacitance due to mechanical vibrations to produce a small voltage proportionalto sound waves. The electret does not need an applied (or phantom) voltage like the condensermicrophone -- as it has a built-in charge -- but a few volts are still required to power theinternal Field Effect Transistor (FET) buffer.The bias is needed for the small built-in FET follower which converts the very highimpedance of the electret element (tens of megohms) to an acceptable level (several kohms).The circuit on the left shows a safe way to connect electret microphone capsules to old, non-standard soundcards. Build this circuit only if the simple schematic below does not work.The component values are not critical; you can use any capacitor between 1uF and 22uF, anda resistor value from 1k to 22k. A simple modification which works with most soundcards ispresented on the right. The circuit works because usually the power is fed to the microphoneconnector through a few kohm resistor and the DC bias on the tip is removed by the inputcapacitor inside the card.Use a simple one conductor shielded cable: wire the shield to the connectors sleeve; connectthe ring and tip to the central conductor.Note: A few, recently manufactured PCs have implemented true stereo microphone inputs.High performance speech recognition and advanced noise canceling applications -- see theAndrea Superbeam Array stereo microphone -- make good use of this new feature, providingmore accurate and reliable signals in noisy environments.When the stereo mic input mode is selected, the bias voltage will be provided for both the tipand the ring. The wiring for a stereo microphone is simple -- see the schematic diagram on theleft -- connect the shield of both microphones to the sleeve of the plug, the left mic to the tipand the right mic to the ring. For best performance, use unidirectional electret microphones.
Connecting dynamic microphones Quality dynamic microphones usually do provide sufficientsignal to drive a reasonably good computer sound card. All you must do is to wire the micproperly, and in some cases, turn on the mic preamplifier built into the sound card (called micboost on most PCs).The connection is as simple as it gets: wire the microphone to the tip and sleeve of the soundcards microphone input. Leave the ring (bias) pin open, do not connect it to anything.Most professional mics will be fitted with the standard XLR connector. To make a simpleadaptor, wire the mic audio (XLR pin 2) to the sound card input connectors tip; wire the micaudio return (XLR pin 3) and the shield (XLR pin 1) to the sleeve.Note: Some non-standard soundcards will have the bias voltage wired to the tip. Also, newPCs with stereo microphone inputs will provide the bias voltage to both the tip and ring of themicrophone input when the stereo mic input mode is selected. This situation needs specialcare -- the sound cards bias circuit is current limited, so your microphone may survive thissmall DC bias, but it will probably cause severe distortion. A simple solution is to insert asmall capacitor between the mic audio output and the mic input to cut the DC current.There are a few cases when your dynamic microphone does not provide the signal levelrequired by your hardware -- youll end up with a very poor sound with lots of noise, evenwhen you turn on the sound cards internal preamp. An easy solution is to build a microphonepreamplifier similar to this simple single transistor circuit below:The amplification is small, but its enough to make the signals compatible with the soundcards input. The circuit does not need any external power supply, it uses the bias voltage(around +5V) of the sound card.