Short answer
A 100 V line system is a connection method in which the PA amplifier carries the sound over a high-voltage line, and each speaker draws as much power as it needs from that line through its own transformer. This allows many speakers to be connected in parallel on a single line over long distances, and each speaker's volume is set separately with its transformer tap. It is the standard solution for large spaces such as schools, mosques, factories, hotels and shopping malls.
Why 100 V?
In home audio systems, speakers have an impedance of 4 or 8 ohms and are connected to the amplifier with short cables. This setup works well for a few speakers. But if dozens of speakers are to be connected along the corridors of a school, the halls of a factory or the minaret of a mosque, two problems arise. First, connecting many low-impedance speakers upsets the impedance the amplifier sees. Second, long cables carrying high current lose a lot of power.
The 100 V line system solves these problems. The amplifier converts the sound to a high voltage at its output. Carrying the same power at high voltage and low current reduces the loss in the cable. A small transformer inside or on each speaker steps the high line voltage down to a level the speaker can use.
In practice, this means lines hundreds of meters long can be run with cables of reasonable cross-section, as many speakers as needed can be added in parallel on the line, and each one can work at a different volume. Some countries apply the same principle with 70 V; in Türkiye and Europe, 100 V is common.
Speaker transformer and power tap
The transformer of every speaker on a 100 V line has several power taps, for example 1.5 W, 3 W and 6 W. The speaker draws as much power from the line as the tap it is connected to. A low tap is chosen for a quiet meeting room and a high tap for a noisy corridor.
This setting lets you balance the volume of the speakers on the same line one by one. If an area sounds too loud after installation, the tap of the speakers in that area is lowered instead of turning down the amplifier. That way the sound in other areas is not affected.
The tap selection also determines the amplifier's load. Adding up the selected tap powers of all the speakers on a line gives the total power the amplifier is loaded with.
Expert note: During installation, write down in a list which tap each speaker is connected to. Years later, when an added speaker overloads the amplifier, you will need this list to redo the calculation.
How is amplifier power calculated?
The calculation is simple. The tap powers the speakers on the line are connected to are added up. The amplifier's rated power is chosen above this total, and the difference is left as a margin. The margin covers cable losses, transformer efficiency and a few speakers that may be added later.
For example, if a school corridor has twenty speakers working on the 6 W tap, the total load is 120 W. A 120 W amplifier would run at its limit on this line, so a somewhat more powerful amplifier is chosen. Running an amplifier at the limit of its power causes distortion at high volume and makes the amplifier overheat.
In zoned systems, the load of each zone is calculated separately. If one amplifier feeds more than one zone, the total load when broadcasting to all zones at once in an emergency is also taken into account.
Cable cross-section and line length
Because the current on a 100 V line is low, thin cables can carry it over long distances, but this is not unlimited. The longer the line and the higher the total power on it, the greater the loss in the cable. The cross-section is calculated from the line length and the total power on the line.
Points to consider when choosing:
- The total length of the line up to the farthest speaker
- The total power on the line, that is, the sum of the tap powers
- A cross-section calculated for an acceptable loss
- Fire-resistant cable if there is a voice evacuation function
- UV- and moisture-resistant sheath for outdoor lines
- A route separate from power cables
Can an 8 ohm speaker be used on a 100 V line?
Connecting an 8 ohm speaker without a transformer directly to a 100 V line will quickly burn out the speaker and disrupt the line. Only speakers with transformers are used on a 100 V line. If a speaker without a transformer is to be used on this line, a separate suitable line transformer is fitted to it.
The reverse is also true. The transformer of a 100 V speaker does not work properly when connected to a low-impedance home amplifier. The two methods should not be mixed in one system.
Some speakers have a selector that bypasses the transformer and switches to low-impedance operation. This option can be used in places such as conference halls, where music quality matters and the lines are short. The mode it will work in is decided during installation and labeled.
Common installation mistakes
The first mistake is mixing up the polarity. If one of two speakers in the same area is wired in reverse, the sounds weaken each other, and the bass in particular is lost. The positive and negative wires are followed along the line by color code.
The second mistake is adding speakers without calculating. Over the years, a speaker is added to a classroom, a corridor or the yard, and the total load exceeds the amplifier's capacity. The amplifier goes into protection or distorts at high volume. The load calculation should be updated before every addition.
The third mistake is the joints in the lines. Joints made by wrapping tape in the ceiling loosen over time, take in moisture, and part of the line goes silent. Joints are made with proper terminal blocks and in junction boxes.
Measurement and maintenance
When the installation is finished, each line is measured with an impedance meter. The measured value is compared with the value calculated from the total power on the line. A result very different from what was expected points to a disconnected speaker, a short circuit or a wrong tap.
These measurements are written into the handover documents. The same measurement is repeated at annual maintenance, and if anything has changed, the fault is tracked down. In voice evacuation systems, a line monitoring device does this check continuously and reports faults to the panel. For outdoor speakers, the seals of the junction boxes, rainwater ingress and cracks in the housing are also checked. In the Şanlıurfa summer, plastic housings left in the sun can harden and crack over the years.
Frequently asked
How many speakers can be connected to a 100 V line?
The number is determined by the total of the tap powers the speakers are connected to. This total, with a margin left, must stay below the amplifier's rated power. Line length and cable cross-section are also part of the calculation.
Can I connect a home speaker to the PA line?
A 4 or 8 ohm speaker without a transformer must not be connected directly to a 100 V line; the speaker will be damaged. It can be used if a suitable line transformer is fitted.
The sound is too loud in one area. Should I turn down the amplifier?
Turning down the amplifier also lowers the sound in the other areas. Instead, the transformer tap of the speakers in the area where the sound is too loud is lowered.
Let's calculate your PA line correctly
Share the number of speakers and the areas; Desa Bilişim will work out the amplifier and cable calculation and prepare a written quote.
