Figure 6-4. - Matching four speakers connected in series parallel.
When you use more than one speaker in a sound system installation, phase the speakers to reduce the cancellation effect, as shown in figure 6-5. Speakers out of phase will lose up to one half of their normal volume and operate with degraded tone quality.
For speakers facing in the same general direction, they are in phase when their respective diaphragms move in the same direction. This is achieved by connecting the speakers + to + and - to -. For speakers facing each other, they are in phase when their respective diaphragms move in opposite Directions. This is achieved by connecting the speakers + to - and - to +.
Efficient transfer of power from the amplifier to the speakers is the prime consideration in sound system connections. Basically, there are two methods of connection. One connection runs from the amplifier directly to the speaker voice coils and the other connection runs from the amplifier to the speaker voice coils through a transformer. You should use the first method with short runs (not over 200 feet) of wire and a simple speaker arrangement with low impedances. Use the second method whenever a 15-percent power loss in the transmission lines is noted or when wire runs are more than 200 feet, or there is a complex speaker arrangement. Constant voltage transformers are most commonly used for this purpose although impedance- matching transformers may be used. For an in-depth look, refer to NEETS, Module 8, Introduction to Amplifiers.
Cable installations are just as important as the other component installations. The cable used should be recommended by the manufacturer and in compliance with the NEC. For the best results in sound, a two- conductor shielded cable should be used.
In complex systems where the input lines are run in close proximity to the speaker lines for long distances, currents in the speaker lines may be picked up by the input lines. When these stray currents are fed back to the amplifier, cross talk and hum can be heard, or the amplifier may oscillate. Because of this, balanced line connections are recommended when long input and speaker lines are run close together. A balanced line is achieved by ungrounding the common terminal, leaving the outputs floating. Any current that develops on one side of the line and is offset by an equal and opposite current on the other side is called a BALANCED LINE. This reduces the possibility of creating stray currents in nearby input tines.
If hum is encountered with a balanced line, it may be necessary to run a shielded two-conductor cable to the speakers and ground the cable at the amplifier.
Figure 6-5. - Phasing speakers facing in the same and opposite direction.Continue Reading