What's all this about High & Low Impedance, Balanced & Unbalanced, Mic & Line Level?
The Short Answer
High Impedance, Unbalanced Line Level:
High Impedance, Unbalanced Line Levels are used for musical instruments such as keyboards, guitars, etc. The connectors for these applications are usually the 1/4 inch Phone Type shown below. High Impedance, Unbalanced Line Levels are also used for consumer electronics such as VCRs, Tape Decks, CD Players, etc.
High Impedance, Unbalanced is effective for short distances only, up to 15 or 20 feet. Beyond this you are asking for trouble.
Low Impedance, Balanced Mic. Level:
Low Impedance, Balanced Mic Level is used for all professional microphones. They always use 3-Pin XLR Type connectors shown below. These signals can run hundreds of feet without problems, as long as you use good quality cables.
Balanced Line Levels are often used for professional electronics such as Mixer Outputs, Equalizers, Feedback Exterminators, Effects Processors, Amplifiers, etc and they use 3-pin connectors, usually XLR Type, sometimes 3-pin 1/4 inch Phone Type connectors. Unlike the 1/4 inch phone plug shown below, there is a third connection called a "Ring". The common industry term for these balanced 1/4 connectors is "TRS" standing for Tip, Ring, Sleeve.
Because they are still balanced, you can run these signals hundreds of feet if required. For any Church sound system, you must make sure that your microphones, mixer, and microphone wiring is all balanced.
Differentiating Between the Two:
If you are using a 3 wire system, it is balanced. If it is a 2 wire system, it is unbalanced. You can usually tell this by checking the microphone connectors at the mixer end. A balanced system uses a 3 pin XLR type of connector (see below) while an unbalanced system usually uses a 2 conductor 1/4 inch phone connector. If you have the XLR connector, you can go a step further by opening the connector to make sure that none of the 3 wires are connected together.
1/4 inch Type
If you find that the system is unbalanced, you will need to replace it with a proper balanced system. An unbalanced system is only useful in home type applications, where the microphone wires are no more than 15 to 20 feet long. Even here it is still prone to interference and noise problems. There are no professional unbalanced sound systems. All professional systems are balanced. If your system is unbalanced, you will need to replace the wiring and equipment.
The More Complete Answer
High Impedance versus Low Impedance:
Every wire is an antenna which is susceptible to picking up electrical and radio interference. A high impedance system uses an impedance of about 10k ohms (10,000 ohms). A low impedance system usually has an impedance around 200 ohms. The low impedance is inherently less susceptible to EMI (electromagnetic interference) and RFI (radio frequency interference) problems.
Simply using Low Impedance instead of High Impedance technology helps to reduce unwanted interference to a certain extent.
Mic Level versus Line Level:
This refers to the voltage level of the signal. Line level is typically 1.5 volts maximum while Mic level is significantly lower at a few milli-volts (1/1000 of a volt). The electrical and radio interference picked up by a wire can also be in the millivolts range. The longer the cable is, the higher the interference voltage.
If you are sending line level through an unbalanced cable, the signal voltage is usually significantly higher than the interference noise if the cable is not too long, and the interference is not too high. Therefore you can generally get away with this for Tape Decks, CD players, etc., if they are located close to the mixer. For longer runs however, over 15 feet, it is much safer to use Balanced instead of Unbalanced.
Balanced versus Unbalanced:
This explanation is a little more complicated, and requires some simple physics to understand.
An unbalanced system is very simple. It consists of a signal source with a single output referenced to ground (see "Unbalanced" figure below). The signal comes from the output of the source, through the center conductor of the wire to the input of the Receiving Amplifier. The foil or braid shield around the wire provides the return path for the signal and also protects the center conductor to some extent from electrical and radio interference.
Although the source signal might be "clean", it picks up interference "noise" along the wire. This is because the wire is acting as an antenna. By the time the signal reaches the amplifier, the noise is added to it. The resulting output from the amplifier consists of the signal plus the noise. This noise might be so small that it can't be heard above the signal. It could also be so large that you can't hear the signal because the noise drowns it out. Often it is somewhere in between.
A balanced system is a little more complicated. Instead of a single conductor plus a shield between the signal source and amplifier, there are two conductors plus a shield. One conductor carries the main signal, while the other conductor carries the inverse of the signal (180 degrees out of phase). The two together are referred to as a "differential" signal.
As this differential signal is passed through the wires, they pick up the same noise as the unbalanced signal does. By the time it reaches the amplifier, both the positive and negative signals have the same noise added to them. Keep in mind however that although the two signals are 180 degrees out of phase with each other, the noise is in phase.
These wires are connected to a Differential Amplifier which subtracts the out of phase signal from the in phase signal. In the diagram below, these signals are referred to as "-A" and "A" respectively. When they are processed by the amplifier, the output is equal to A - (-A) which equals 2A. Because the noise on the two lines is in phase, it is subtracted from itself and thus cancels itself. The equation for this is as follows:
Amplifier Output = (A + noise) - (-A + noise) = A + noise + A - noise = 2A
This means that the signal has doubled and the noise has been canceled out to zero.