Professional (and most semi-professional) equipment has balanced, low-impedance microphone inputs using 3-pin XLR-type connectors.
Less sophisticated musical instruments, consumer electronic products, computers and many portable recording devices typically have unbalanced, high-impedance microphone inputs using 1/4-inch phone jacks or 1/8-inch mini-phone jacks.
A few mixers offer both types of connectors for each input channel. Simple adapters may be used to mate different types of connectors if no configuration change (high/low impedance or balanced/unbalanced signal) is necessary. Use only high-quality connectors and adapters.
Phantom Power & Bias Voltage
In a condenser microphone, one function of the circuitry is to convert the very high impedance of the condenser element to a lower impedance.
For an electret condenser (the most common type), this is done by a single transistor. Some condenser designs, such as lavalier types or miniature hanging types, have their electronics separate from the microphone element.
In these models, the impedance converting transistor is built in to the microphone element itself. The main part of the circuitry is contained in a separate module or pack usually connected to the element by a thin shielded cable.
The main electronics of such designs operate on phantom power supplied through the microphone cable or by means of a battery in the pack itself.
However, the impedance-converting transistor in the microphone element also requires power in a form known as “bias” voltage. This is a DC voltage, typically between 1.5 and 5 volts. It is carried on a single conductor in the miniature connecting cable, unlike phantom power, which is carried on two conductors in the main microphone cable.
In addition, the audio signal in the miniature cable is unbalanced while the signal in the main cable is balanced.
This distinction between phantom power and bias voltage is important for two reasons. The first concerns the use of wireless transmitters. Body-pack transmitters which operate on 9 volt (or smaller) batteries cannot provide phantom power (12-48 volts DC). This prevents their use with phantom-powered condenser microphones.
However, the body-pack transmitter can provide bias voltage (1.5-5 volts DC). This allows a condenser microphone element with an integrated impedance-converting transistor to be used directly with a body-pack transmitter.
Miniature condenser lavalier types as well as other designs which have separate electronics can be operated with wireless systems in this way.
Picking Up Noise
The second reason concerns the wired installation of condenser microphones with separate electronic assemblies such as miniature hanging microphones for choir, congregation, or other “area” applications.
Since the audio signal in the cable between the microphone element and the electronics is unbalanced, it is more susceptible to pickup of electronic noise. This is particularly true for radio frequency noise because the cable itself can act as an antenna, especially for a nearby AM radio station.
For this reason it is strongly recommended to keep the length of this part of the cable as short as possible, preferably less than 35 feet. It is a much better practice to extend the length of the balanced cable between the electronics assembly and the mixer input.
Optimum microphone performance depends on the associated connectors and cables. In addition to quality connectors of the types described above, it is equally important to use high-quality cables.
Beyond the basic specification of balanced (two conductors plus shield) or unbalanced (one conductor plus shield), there are several other factors that go into the construction of good cables.
The conductors: carry the actual audio signal (and phantom voltage for condensers), usually stranded wire. They should be of sufficient size (gauge) to carry the signal and provide adequate strength and flexibility; use stranded conductors for most applications, solid conductors only for stationary connections.
The shield: protects the conductors from electrical noise, may be braided or spiral wrapped wire, or metal foil. It should provide good electrical coverage and be flexible enough for the intended use: braid or spiral for movable use, foil only for fixed use such as in conduit.
The outer jacket: protects the shield and conductors from physical damage, may be rubber or plastic. It should be flexible, durable, and abrasion resistant. Depending on the location it may need to be chemical or fire resistant.
Different color jackets are available and can be used to identify certain microphone channels or cables.
A large percentage of microphone problems are actually due to defective or improper microphone cables.
Microphone cables should be handled and maintained carefully for long life:
• Position them away from AC (electricity) lines and other sources of electrical interference to prevent hum;
• Allow them to lie flat when in use to avoid snagging;
• Use additional cable(s) if necessary to avoid stretching;
• Do not tie knots in cables;
• Coil loosely and store them when not in use;
• Periodically check cables visually and with a cable tester.