Low Impedance and 100V Line Systems
16 July 2010
Low Impedance and 100v Line Systems
Constant impedance lines (low impedance) are used normally in the case of a low number of loudspeakers, placed at a minimum distance from the amplifier (<20m).
The constant voltage connection system (e.g. 100V line) offers a host of advantages making it ideal for sound systems of all sizes, above all in the case of long distance speaker runs. This connection system requires each loudspeaker to be equipped with its own line transformer, which adapts the impedance of the loudspeaker (usually very low) to the much higher level of the line itself. In proportion to power transmitted, the current circulating on a line at 100V is considerably lower than that circulating on wires of a constant impedance system and, consequently, drops along the line are less frequent; therefore the cable gauge can be lower.
The function of the amplifier device is
to raise the signal of a sound source, such as a microphone, CD player, cassette recorder or an AM/FM tuner to a sufficient level to pilot the transducer LOUDSPEAKER. The amplifier has several inputs, to which the various sound sources are connected.
The sound sources have to be mixed (mixer) and acoustically modified with tone control and equalisers to optimise listening in relation to the environment’s characteristics. A terminal board is also fitted on output indicating impedance and voltage to which the loudspeaker line is connected.
A few basic rules must always be observed:
1. When the load is on ‘constant impedance’; must be connected to the relative socket.
For example, a load of 4 ohm
must be connected to a socket marked 4W, and so on. The power that the load can withstand must always be greater than the amplifier’s rated power.For example in the case of a load of 100W, the amplifier must have a power equal to or less than 100W.
2. When the load is at ‘constant voltage’; must be connected to the sockets indicating output at constant voltage - 50, 70 or 100v; the power that the load can withstand must always be less than or equal to the amplifier rated power. For example, with a load of 100W, the amplifier must have a power equal to or greater than 100W.
To calculate the required value of a 100v line amplifier power, simply add together the power of each loudspeaker. (See Below).
However, on a constant impedance system, the load must have an equivalent impedance (series/parallel) equal to or greater than the minimum value indicated on the amplifier output (See Below).
The loudspeaker power supply lines, with no connected attenuator, must be with 2 sheathed wires; cables with 3 sheathed wires are required in the case of lines using attenuators which can be disabled if an emergency is activated. Adequate wire sections are required:
1.5mm² for lines up to 500W power, 2.5mm² for lines with power over 500W. The following general standards must also be observed:
• Never use a single cable (lead) for loudspeaker connections: this would cause disturbance producing a loud ‘HUM’
• Never use shielded cables for the loudspeaker connections: this would cause serious damage to the power units.
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