Most sound card microphone inputs require a minimum signal level of at least 10 millivolts, but some older 8-bit cards need as much as 100 millivolts. The typical impedance of the PC soundcard microphone input is in order of 1 to 20 kohms (can vary from card to card). The microphone type which works best with computer sound cards is the electret microphone.
Sound Blaster soundcards (SB16, SB32, AWE32, AWE64 or Live) from Creative Labs have a 3.5mm (1/8 inch) pink stereo jack for the microphone input, with the following pinout:
Note: Most soundcards will wire the positive DC bias voltage to the ring, but a small number of non-standard soundcards can have the bias voltage wired to the tip. A few cards have a jumper 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 mainboards with stereo microphone support will provide the bias voltage for both the tip and ring.
The approximate schematic of a Sound Blaster microphone input circuitry shows that the +5V voltage on the connector is heavily current limited. The card's voltage might not be exactly 5V, but it is usually something between 3 and 5 volts when no microphone is connected.
The electret microphone is the cheapest omnidirectional microphone you can buy. Very sensitive, durable, extremely compact in size, electret mics are used in many applications where a small and inexpensive microphone with reasonably good performance is needed. You can find them in almost every stereo equipment, in consumer video cameras, mobile phones and so on.
The electret is a modified version of the classic capacitor microphone, which exploits changes in capacitance due to mechanical vibrations to produce a small voltage proportional to sound waves. The electret does not need an applied (or phantom) voltage like the condenser microphone -- as it has a built-in charge -- but a few volts are still required to power the internal Field Effect Transistor (FET) buffer.
The bias is needed for the small built-in FET follower which converts the very high impedance 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, and a resistor value from 1k to 22k.
A simple modification which works with most soundcards is presented on the right. The circuit works because usually the power is fed to the microphone connector through a few kohm resistor and the DC bias on the tip is removed by the input capacitor inside the card.
Use a simple one conductor shielded cable: wire the shield to the connector's sleeve; connect the 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 the Andrea Superbeam Array stereo microphone -- make good use of this new feature, providing more accurate and reliable signals in noisy environments.
When the stereo mic input mode is selected, the bias voltage will be provided for both the tip and the ring. The wiring for a stereo microphone is simple -- see the schematic diagram on the left -- connect the shield of both microphones to the sleeve of the plug, the left mic to the tip and the right mic to the ring. For best performance, use unidirectional electret microphones.
Quality dynamic microphones usually do provide sufficient signal to drive a reasonably good computer sound card. All you must do is to wire the mic properly, and in some cases, turn on the mic preamplifier built into the sound card (called 'mic boost' on most PCs).
The connection is as simple as it gets: wire the microphone to the tip and sleeve of the sound card's 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 simple adaptor, wire the mic audio (XLR pin 2) to the sound card input connector's tip; wire the mic audio 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, new PCs with stereo microphone inputs will provide the bias voltage to both the tip and ring of the microphone input when the stereo mic input mode is selected. This situation needs special care -- the sound card's bias circuit is current limited, so your microphone may survive this small DC bias, but it will probably cause severe distortion. A simple solution is to insert a small 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 level required by your hardware -- you'll end up with a very poor sound with lots of noise, even when you turn on the sound card's internal preamp. An easy solution is to build a microphone preamplifier similar to this simple single transistor circuit below:
The amplification is small, but it's enough to make the signals compatible with the sound card's input. The circuit does not need any external power supply, it uses the bias voltage (around +5V) of the sound card.