Signal Level
Professional microphones put out a very weak signal — less than 1/1000th of a volt, or 1 millivolt. Audio inputs on computers, even though they may be labeled “Mic In” or be identified by a small microphone-shaped icon, often are not designed to accept such a low signal level.

Most computer inputs require a minimum signal level of at least 1/100th of a volt (10 millivolts). This discrepancy means that if a typical professional microphone is connected to the mic input of a computer, the user will have to shout into the microphone or hold it just an inch or so away (or both) in order to produce a strong enough signal for the sound card to “hear.”

There are two possible solutions. One option is to increase the gain of the computer’s mic input, so that it can more easily detect the signal from the microphone. Software options supplied with some computers allow the user to increase the sensitivity or “gain” of the input, either with a click-and-drag input level control or a set of check boxes that “double” or “boost” the sensitivity. These options will typically be found under the advanced options on the Windows volume control mixer. (Note: Increasing the sensitivity of the input will always add some noise, so use only as much additional gain as necessary.)

If increasing the gain does not produce enough level, or produces an unacceptable amount of noise, the only other option is to amplify the microphone signal before it goes into the computer mic or line input. This can be done by running the microphone signal through a device called a mic preamplifier or mic-to-line amplifier.

A microphone mixer can also be used, as they typically have an output that will provide adequate signal level to the computer audio input. (In this case, the mixer is being used only for its pre-amplification function and not its mixing capability.) Either way, you will have to know the typical output level of the microphone (found on the microphone’s specification sheet) and the sensitivity of the computer audio input in order to know how much amplification is needed, and whether a particular mic preamp or mixer will do the job. When looking at the specs, keep in mind that a difference in level of up to 15 dB (decibels) between the output of the mic and the input of the computer is usually acceptable.

Impedance
Impedance is an electrical characteristic similar to resistance. It is important because the relationship between the impedance of a microphone and the impedance of the computer to which it is connected can have a significant effect on how much of the microphone’s signal is actually transferred to the computer. For acceptable results, the output impedance of the microphone must be less than the input impedance of the computer audio input. If the impedance of the microphone is the same or higher than the input impedance of the computer, some or all of the microphone’s signal strength will be lost (an effect called ‘loading’.) The higher the microphone’s impedance is compared to that of the computer, the more signal will be lost.

Connecting a high impedance (also called ‘High Z’) microphone to a computer with an input impedance of 600 ohms (‘Low Z’) will result in so much signal loss that the talker’s voice will be inaudible. Professional microphones typically have an output impedance of less than 600 ohms and most computers have a mic input impedance of 600 to 2,000 ohms, so impedance is not usually a problem. However, you should make certain that any microphones you purchase are ‘Low Z’, as both are available.

Connector & Wiring Scheme
The most visible problem encountered when connecting a professional microphone to a computer is that different connectors are used. Because of limited space, built-in computer audio inputs can only accommodate very small connectors. The 3.5 mm (1/8-in) “miniplug” used on most Walkman-type personal stereos is the most popular type. The standard 1/4-in and XLR connectors used on professional microphones are far too big to fit into a single card slot.

From left: XLR male, XLR female, 1/4-in male, RCA male, Tip Ring Sleeve (Stereo) 3.5 mm male