Adding a transformer to the loudspeaker increases the impedance of the unit and makes it possible to operate multiple units in parallel. The power output of PA amplifiers is high, constant voltage (typically 100 V), and the output current is low. Therefore, with standard cross-section cables, below 4 mm2, it is possible to connect a large number of loudspeakers over long distances.
In PA installations, the usual regulations for transmitting electrical signals apply, with the following considerations:
- Use a cross-section suitable for a maximum loss of 10%.
- Use multi-strand copper cable.
- Avoid electromagnetic interference from power line cables.
- Avoid possible crosstalk from sharing a channel with other music signals.
The cable cross-section calculations consider the load at the end of the line. In a real-life scenario, where the speakers are distributed along the cable, the distance can be increased. Power loss is also limited to 10% to avoid excessive sound pressure loss, although this can be compensated, to a certain extent, by increasing the output volume of the amplifier.
To avoid electromagnetic interference, it is best to use separate cable trays and keep the power cables at least 40 cm apart. To avoid crosstalk with other audio signals, whether from the PA system itself or from other sources, it is recommended to keep them separate from each other. If it is not possible to physically separate the cables, shielding and functional testing will be necessary.
Graphs for calculating the cross-section of a 100 V line cable
The following look-up tables are commonly used to determine the required cross-section of the 100 V line. They relate the distance and power (or impedance) parameters to the cross-section:
Section-distance-impedance graph
Exercise
A line consisting of 10 speakers with an impedance of 1000 ohms each. We'll use 1.5 mm2 cable. What is the maximum recommended distance, according to the table?
SOLUTION
- Line impedance: 1000/10 = 100 ohms
- Maximum distance: Approximately 200 m
Section-distance-power graph
Exercise
A line consisting of 10 speakers, each with a power of 10 W. We'll use 2.5 mm2 cable. What is the maximum recommended distance, according to the table?
SOLUTION
- Line power: 10 x 10 = 100 W
- Maximum distance: Approximately 360 m
Based on the above graphs and to facilitate the final decision on which cable to use, the following table summarizes the maximum distances (m) recommended for each cable section and amplification power, with a maximum power loss of 10%.
| Power/Section | 0,75 mm2 | 1 mm2 | 1,5 mm2 | 2,5 mm2 | 4 mm2 | ||||
|---|---|---|---|---|---|---|---|---|---|
| 60 W | 178 | 237 | 345 | 577 | 931 | ||||
| 120 W | 89 | 119 | 173 | 289 | 466 | ||||
| 240 W | 44 | 59 | 86 | 144 | 232 | ||||
| 360 W | 30 | 40 | 58 | 96 | 155 | ||||
