Many public buildings need to have an audio distribution system, be it for PA or occasional music or sound reinforcement. Unlike domestic Hi-Fi systems, where the amplifiers are usually located in the same room as the loudspeakers, these commercial distribution systems need cabling that spans long distances. The low impedances that domestic systems use are unsuited to long cable runs, because the need to use thick cables to keep copper losses low means that long spans are expensive.
Another difference in distribution systems means that often several speakers are put in parallel across the line, either where a number of rooms are served such as in a school, or a number of ceiling speakers are being fed, such as in a factory or canteen.
Use 100V Audio Step Up Transformers To Keep Cable Losses Down
The way the problem is solved is to take a leaf from the power companies, who needed to span long distances. They operate their long distance runs at high voltages, stepping up the voltage to feed the power lines, and then stepping down the voltage to feed the domestic loads. The same can be done with an audio signal, using a transformer to step up the voltage from a regular PA amplifier. Amplifiers are also made specifically for use with 100V line systems, these can work at the line voltage directly without needing a step up transformer if suitably designed.
Since losses due to cable resistance are proportional to the square of the current, whereas power is the product of voltage times current, minimising the current my increasing the voltage reduced transmission losses.
There are limitations on how high a voltage can be used due to safety concerns - these vary from country to country, so the US often uses 70.7V line voltage for distribution systems (to keep the peak voltage below 100V above which extra safeguards have to be taken for the cable runs, such as running in conduit). In Europe audio distribution systems often use 100V.
At the destination, another transformer is used to bring the 100V line voltage down to the low impedance used for loudspeakers. This transformer may have multiple taps, to allow the installer some coarse adjustment of volume to suit the location. In some installations the taps are used to provide a coarse volume control available to the user.
Advantages and Disadvantages of 100V (70V US) Audio Distribution
Only one amplifier is needed to feed all the speakers in a 100V distribution system, and the speakers do not require local powering. This was particularly advantageous in the past, when amplifiers were more expensive and less reliable than now, however the benefits of not needing local powering are often valid now. The transformers usually provide galvanic isolation, preventing earth loops in the distributed system and any ancillary audio system.
Against that has to be set the cost of the transformers, and the fact that about a fifth of the power may be lost in the transformers themselves. Audio transformers have pathologies of their own - they can saturate easily if presented with high powers at low frequencies, and the frequency response of the transformers is often limited at high and low frequencies. This is not an issue for a lot of sound reinforcement and PA usage, but doesn't necessarily meet modern high-quality expectations without making the transformers expensive.
Feeding a balanced low level audio signal and using local amplifiers over short cables to regular low impedance speakers of 4 or 8 ohms is a common alternative. However, there are still many sites that 100V line distribution is particularly suited to, such as driving ceiling speakers, and large rooms where it would be undesirable to have lots of amplifiers that users could fiddle with such as churches and convention halls. 100V line systems are not generally used for live music reinforcement, as they are unsuited for the power requirements and the specialised nature of live PA needs.
Richard Mudhar - Originally from London, now enjoying a less hectic pace of life in Suffolk, Richard Mudhar is a keen wildlife sound recordist and ...
Join the Conversation