Microphones: Low & High Impedance

First: a little background. In any conductor of electricity, there is an opposition that resists the flow of the electric current through that conductor. If the electric current is moving in one direction, the current is called direct current, or DC. The opposition is called resistance and it is measured in ohms.

Another type of current is called alternating current, or AC. Here, the current moves in one direction and then moves in the opposite direction. This type of current is what appears at the electric power outlet your computer or sound system plugs into. AC also describes the type of current your microphone puts into the microphone cable and sends to the microphone mixer.

There is also an opposition to AC in a conductor. This opposition is called impedance and it is also measured in ohms. What makes impedance complicated is that it usually varies with frequency. That means an engineer must not only deal with the loss of signal in a cable, but the loss varies with the frequency of the signal.

High Impedance Microphone

A high impedance microphone has an impedance from 10,000 to 100,000 ohms, depending on the model. The most common values range between 10,000 and 50,000 ohms. About the only reason to purchase a high impedance microphone is for budget reasons. They generally cost less than a comparable low impedance microphone.

However, there is one severe limitation to using high impedance microphones: the cable length is critical. A 10,000 ohm microphone is already experiencing loss at the higher frequencies above 10,000 Hertz (Hz) with a cable length of only six feet. In fact, for good fidelity, the cable length should not exceed 18 feet. A 50,000 ohm microphone will have lost half of its high frequencies with only a ten foot cable.

Because of the problem of different losses dependent on the frequency, the high impedance microphone will lose much of the high frequency sounds if the cable length is too long. We often visit churches that are using high impedance microphones and the cable may be 50 to 75 feet long. As can be predicted, the microphone sounds muffled because of the loss of the important high frequencies.

Figure 1

High impedance microphones usually use unbalanced cable (Fig. 1). This means the very small signal from the microphone is carried in one inner wire and the outer cable shield. As the cable length increases, the unbalanced cable will have an increasing chance to pick up unwanted stray signals from dimmers, fluorescent lights and radio interference (RFI).

Low Impedance Microphones

Low impedance microphones have an impedance between 50 and 600 ohms. The most common specification is 150 to 250 ohms. A major advantage of low impedance microphones is their ability to be used with long cables. A microphone cable made with a popular cable like Belden 8412 could be run over 400 feet with a 150 ohm microphone. Even at that distance there would be only a slight loss (1 dB) of high frequency signal at 10,000 Hz.

Figure 2

Low impedance microphones usually have balanced outputs (Fig. 2). This means the microphone cable has two inner conductors (wires) to carry the tiny microphone signal. The two wires are surrounded by an outer conductor called the shield. The shield does not carry any signal. Its job is to provide an electrical shield to keep unwanted radio and other noise away from the inner two wires. A balanced cable will give much better immunity to electrical and radio noise than an unbalanced cable.

Which Is For You?

With the above facts in mind, we must recommend that only low impedance microphones be used in any sound system. These microphones, used with balanced microphone cables, will assure you a very low level of noise interference and a high degree of quality.

by Ron Huisinga © 1996 Internet Sound Institute (www.soundinstitute.com). This article is for personal use only. Any commercial reproduction is not permitted without permission. To obtain permission, contact ISI at hopi@soundinstitute.com

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