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Sound Equipment - 100V Line Systems
The "100volt line" loudspeaker system is used to overcome certain difficulties which would be encountered using conventional low impedance (4 - 16ohm) speakers in a public address (distributed loudspeaker) system.
The two principle difficulties with such a configuration are:
- Loss of signal on long cable runs;
- Use of many individual speakers on one amplifier.
Stemming from these difficulties, and a need to minimise cost, maximise efficiency, and simplify the design of complex audio systems, was born the constant-voltage system. The key to the solution came from understanding how an electric company distributes power accross the country: they elegantly solved the same distribution problems by understanding that what they were distributing was power, not voltage.
Just as importantly, they knew that power was voltage times current, and that power was conserved. This meant that you could change the mix of voltage and current so long as you maintained the same ratio: 100 watts was 100 watts – whether you received it by having 10 volts and 10 amps, or 100 volts and 1 amp. By stepping-up the voltage, you stepped-down the current, and vice-versa.
Therefore to distribute 1 megawatt of power from the generator to the user, the power company steps the voltage up to 200,000 volts, runs just 5 amps through relatively small wire, and then steps it back down again at, say, 1000 different customer sites, giving each 1 kilowatt. In this manner large gauge cable is only necessary for the short direct run to each house. Very clever.
Applied to audio, this means using a transformer to step-up the power amplifier’s output voltage (gaining the corresponding decrease in output current), use this higher voltage to drive the (now smaller gauge wire due to smaller current) long lines to the loudspeakers, and then using another transformer to step-down the voltage at each loudspeaker. Nothing to it.
It is common, although not universal, to find power (think loudness) taps at each speaker driver. These are used to allow different loudness levels in different coverage zones.
In practice...
Once you have chosen the type of speaker you wish to utilise, the cabling and connection of these units is very straightforward. Volume controls can also be incorporated either at the amplifier or the speaker end of the circuit.
Important rules to observe
1. Polarity must be observed throughout the cable runs.
This is particularly important when using volume controls in the system. At the rear of the amplifier, the connections are as follows: 'Common' = negative and '100 Volt' = positive.
2. The total wattage of the system, that is the sum of all the speaker watt tappings used, must not exceed the rating of the amplifier. Speaker wattage tappings can be varied to achieve different volume levels on each speaker. The greater the wattage tapped, the higher the volume. Use the common wire (negative) and the wattage desired (positive) on each speaker as desired.
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| SPEAKER 10watts |
SPEAKER 20watts |
SPEAKER 15watts |
SPEAKER 5watts |
AMPLIFIER REQUIRED 50watts or greater |
The above illustration also serves to demonstrate the simple way to establish the speaker circuit - simply connect twin flex from one speaker to next nearest speaker. This will finish with just one twin flex back at the amplifier location. A separate speaker circuit need only be installed if different levels of sound or controllability are required in separate areas of the installation. Adequate cable type for the environment involved must be used.
It is worth noting that, in the real world, a 100volt system will encounter several losses that will reduce the power available. These include insertion loss caused by the transformers (they are not perfect) and line loss (resistance in the connecting cable). {This lost power often is referred to as I2R losses, since power (in Watts) is current-squared (abbreviated I2), multiplied by the wire resistance (R). These losses occur physically as heat.}
You can go to a lot of trouble to calculate and/or measure each of these losses to determine exactly how much power is required. However, calculation turns out to be extremely difficult and unreliable due to the lack of published insertion loss information; measurement is the only truly reliable source of data, but requires the system to be installed.
The alternative is to apply a very seasoned rule of thumb: use 1.5 times the value found by summing all of the loudspeaker powers. Thus, in the example above, 1.5 times 50 watts tells us we need an amplifier of at least 75 watts power.
Why 100 volts?
Actually, there are a number of common voltages used in constant-voltage PA systems. The most common in Europe is 100 volts, although 70.7 volt systems are also seen - primarily from their development in the US.
The particular number of 70.7 volts originally came about from the second way that constant-voltage distribution reduced costs: back in the late 1940s, the UL safety code specified that all voltages above 100 volts peak (max open-circuit value) created a 'shock hazard', and subsequently must be placed in conduit – this is expensive = bad.
Therefore working backward from a maximum of 100 volts peak (conduit not required), you get a maximum RMS value of 70.7 volts (Vrms = 0.707 x Vpeak). It is common to see "70.7 volts" shortened to just "70 volts" – it’s sloppy; it’s wrong; but it’s common.
In Europe, and now in the US, 100 volts RMS is popular: this allows use of even smaller wire. Remember: the higher the voltage, the lower the current, the smaller the cable, the longer the line. [For the very astute reader: the wire-gauge benefits of a reduction in current exceeds the power loss increases due to the higher impedance caused by the smaller wire, due to the current-squared nature of power.]
See also:
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