J.W. Davis & Company

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Distributed Sound Systems - Part 2 of 2

Simplicity of installation and modification are major benefits of 25/70 volt overhead sound systems. Several options are available to control the volume level of a single speaker or a group of speakers, either at the speaker or on a wall.

JWD transformers provide various taps for setting power consumption and speaker volume control (For instance, the JWD TR-705 5 watt transformer has 5, 2.5, 1.25, .625 and .31 watt taps). Connecting to taps of lower value reduces the highest possible sound level from the speaker. Due to its inflexibility, this method is primarily used for system-wide control.

In speaker systems that include individual speaker sound level controls, J. W. Davis uses L-Pad volume controls ("pads") exclusively. Unlike less expensive potentiometers ("pots"), L-Pads contain dual offsetting elements that provide a constant impedance to the system. In addition, smooth linearity is afforded throughout the volume range and reliability is superior. However, care should be taken to (1) match the impedance of pads to loudspeaker impedance ( An 8W loudspeaker should be wired to the 8W secondary taps of the line transformer.) and (2) be sure that the wattage of the L-pad is equal to or greater than the wattage of the loudspeaker.

If wall-mounted volume controls are desired, autotransformers should be used. These attenuators will control the volume level of groups of speakers as well as a single speaker. Although autotransformers have a "transparent" load, and should not be counted when determining the amplifier load for the system, the maximum power that they will pass through is limited by their wattage rating. Therefore, assuming that 3 speakers with 10 watt line transformers are to be controlled, the autotransformer must have a rated capacity of at least 30 watts.

To provide the necessary power ranges, JWD 20 watt (AT-20), 40 watt (AT-40) and 100 watt (AT-100) autotransformers are available with either stainless steel or white plastic face plates.

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