Impedance and Balance
Impedance - Is a measure of a microphone’s resistance to the electric current flowing from its output. Measured in ohms. High Impedance (high Z) > 10,000 or Low Impedance (low Z) < 600 ohms.
Named after its creator (Georg Ohm), the ohm is a measurement of electrical resistance or impedance in a circuit or object. It is symbolized by the Greek letter Omega (Ω).
High-impedance microphones can be built very inexpensively. However, their response deteriorates rapidly as cable lengths increase, with 18-20 feet being the limit to avoid seriously compromised signal.
Low impedance microphones can transmit signals for hundreds of feet without appreciable changes in their signal. That's one reason most sound systems are built around low impedance mics.
The audio signal that travels through a cable from the microphone to the mixer is a low level electrical signal that is very susceptible to outside interference (Hum).
Unwanted noise and hum added to the audio signal is derived from radio frequencies that travel through the air, fluorescent lights, electric motors, and AC power cables.
A shielded microphone cable is used to counteract this unwanted noise.
Consists of an inner copper conductor wire which covered by a flexible plastic insulator.
The insulation is then surrounded by tightly braided fine strands of wire forming a “shield” around the center of the conductor.
There are two basic cable designs that carry the microphone signal to its destination: unbalanced and balanced.
Unbalanced cable - Single center conductor, has maximum effective cable length of 25 feet.
Beyond 25ft. is noticeable deterioration of high frequencies and an increase in interference in the form of noise and hum.
Cables use 1/4” plugs.
Balanced cable - Refers to a “three-legged” type of electrical signal that has two legs independent of ground.
Longer cable lengths (several hundred feet), reduced interference and increased fidelity.
Balanced microphones are low impedance and most often use a three-pin XLR cannon connector.