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25/70 Volt Distribution Systems
A constant voltage distribution system is a method of connecting loudspeakers to an amplifier where
the output voltage of the amplifier and the amount of power delivered to loudspeakers remain
constant.
A constant voltage distribution system is similar to a power generating station that delivers
electrical power to your home. Just as appliances can be turned on and off without affecting the
operation of other appliances connected to the electrical system, speakers can be added to, or
subtracted from, the distribution system without affecting its overall operation. However, the total
number of speakers connected to the amplifier is limited by the amplifier output power capability.
The most common voltages used in a constant voltage distribution are 25 volts and 70 volts. In most
areas of the USA, 70 volts is the maximum voltage that a pair of wires may carry without being
enclosed in metal conduit. An amplifier that is designed for use is such a system that must
develop 70 volts across the output terminal when driven to its full rated power. Actually, the
output of the amplifier swings from 0 to 70 volts in response to changes in its input signal.
In a constant voltage distribution system, "Pre-Packages" speakers with matching transformers
with several taps are normally utilized. (Each tap is designated in watts.) You may simply connect
the output terminal of the amplifier to the tap of the power rating desired. However, if speakers
without transformers are selected, the correct transformers must be carefully chosen to match the
speakers. Transformers are usually designed for a specific constant voltage level (i.e. 70 volts and 25
volts), and have primary winding taps rated in watts, and secondary winding taps rated in ohms.
They may have several primary and secondary winding taps.
Primary windings of the transformer are connected in parallel across the output terminals of the
amplifier, while secondary windings are connected across a loudspeaker or loudspeaker system.
Secondary winding taps are chosen to match the impedance of the loudspeaker. Primary winding
taps should be selected to match the power that a speaker receives. An amplifier's output voltage will
determine the power delivered to a speaker.
Transformers are connected in parallel across the output terminals of the amplifier.
Primary windings of certain transformers are rated in ohms instead of watts. To select a transformer
for a specific power level, the following formula can be used:
where Z is the impedance of the primary winding of the transformer (ohms)
For a 70 volt system, where 5 watts are desired at a particular speaker, the primary winding of the
transformer must have an impedance of:
2
Note: 70
is approximately 5000
All manuals and user guides at all-guides.com
E is the constant voltage level of the system
P is the desired power (watts)
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