Determine the microphone’s impedance rating. Impedance is calculated in units called “ohms,” which are denoted by the Greek letter Omega. Many microphones have their impedance rating printed somewhere on the handle, often near the output connector at the bottom. If the impedance rating is not printed on the microphone, it can likely be found in the microphone’s documentation or on the manufacturer’s website.
Write down the microphone’s output impedance rating. Writing down the impedance ratings for each piece of equipment is useful for future reference.
Find out the input impedance ratings for all available preamplifiers. For example, many USB audio interfaces offer multiple preamplifiers with different impedance ratings. The impedance rating is often printed near the input, but you may need to consult the preamplifier’s documentation to determine the input impedance rating.
Write down the input impedance ratings for each available preamplifier. If necessary, draw a diagram indicating which preamplifier is which for easy reference.
Connect the microphone to a preamplifier whose input impedance is at least 10 times that of the microphone for the most flexible results. For example, a common microphone output impedance rating is 600 ohms. In this case, you need a preamplifier with an input impedance rating of at least 6,000 ohms. An impedance ratio of 1:10 ensures the widest possible frequency response range and reduces unwanted noise introduced by the microphone, particularly one with a very long cable.
Connect the microphone to a preamplifier with an equal or somewhat higher impedance rating for “darker” results with a flatter frequency response range. In audio production, “dark” refers to a signal that lacks harmonics, resulting in a recording in which the signal sounds slightly muffled and distant. Although this darkening may not always be desirable, it is entirely appropriate to many musical genres or when working with vocalists whose voices are too harmonically rich.
Pair the microphone with a preamplifier with a lower input impedance rating to strongly muffle the results. Although this approach is rare, the resulting signal attenuation is appropriate for some genres of music. Not only will the harmonics be removed, but the signal itself will have a very flat frequency range, resulting in an effect similar to the sound of an analog telephone or old radio receiver.