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100V Line Loudspeakers

100V Line speaker systems are sometimes referred to as "constant-voltage system" it is the most economical way to install a multi-speaker sound system. The term 100V system relates to the maximum output voltage of the amplifier. To generate this high voltage the amplifier is equipped with a step-up transformer, which increases the normal output voltage of around 30 Volts, up to 100 Volts

Series Parallel of low impedance speakers

Series Parallel connection of low impedance speakers


The main difference between a low-impedance speaker system (4 or 8 Ohms) is the way the individual loudspeakers are connected to the amplifier, with the conventional low-impedance system special care has to be taken to maintain the correct impedance for the amplifier.

100V Line Speakers

Parallel connection of 100V Line Speakers


In a 100V line system, a large number of single loudspeakers each equipped with a step-down transformer can be connected to one single speaker cable. With the 100V line installation, each speaker or speaker cabinet is equipped with a step-down transformer


Step-Down Transformer


A step-down transformer has a relatively high impedance at the primary side and the secondary side of the transformer matches the speaker impedance (typically 8 Ohms). The primary side of the transformer normally has several tapings, these are marked in watts and are used to enable each speaker to be set at different volume levels. The total primary wattage should not exceed 90% of the amplifiers output. A step-down transformer will be required for each loudspeaker cabinet connected to the amplifier.


Step-Down Transformer

100v line step-down transformer

100V Line Speaker Cable

When installing a 100V line system, it is important that the correct size of cable is used. The following table gives an indication of the distance a particular cable can deliver the full power of the amplifier.

 Cable Size
30 Watt 800m1066m1600m2666m4266m6400m
60 Watt400m533m800m1333m2133m3200m
120 Watt200m266m400m666m1066m1600m
240 Watt100m133m200m333m533m800m


As the voltages can approach 100 volts, a double insulated cable should be used at all times. The cable should be adequately secured and cable runs should be kept away from any potential source of interference. These include other cables: 3 phase mains, data, telecom cables etc.


Loudspeaker switching

When switching 100 volt line loudspeakers "on" or "off", it is necessary to use the circuit indicated below. This circuit will ensure that the loudspeaker is not effected by inductive pick-up in the disconnected cable, this can happen when speakers are switched at the amplifier and leave long open ended cables. The loudspeaker is not only isolated from the line but also shorted to itself in the "off" position this ensures that there is no possibility of breakthrough.


Speaker Isolation Circuit

Speaker isolation switch


Volume Controls


There are two ways to adjust the volume of individual 100V line loudspeakers. Some cabinets can be fitted with a wire wound potentiometer which absorbs power so reducing the volume of the speaker. This is a very inefficient way of reducing the volume and only works with low wattage cabinets.


Potentiometer Control

10 Watt Potentiometer Volume Control


The preferred method use controls which are specially designed for 100V line operation (attenuators) and simply wired between the amplifier and the loudspeaker. The maximum power ratting of the volume control must be observed and not exceeded, otherwise damage to both control attenuator and amplifier can occur.

Inductive Attenuator Control

Inductive Attenuator Control

L Pad attenuators are designed to be used with low impedance (8Ω) loudspeakers. The L Pad is fitted after the amplifier and before the speaker to be attenuated. The L Pad will always give a constant load of (8Ω) to the amplifier.

L-Pad Attenuator

L Pad Attenuator


ESR Electronic Components Ltd Cullercoats Tyne and Wear NE30 4PQ