Why a “70-volt” system?

A good question, and one that causes a lot of confusion among audio practitioners.

Some common misconceptions about 70-volt (actually 70.7-volt) distribution systems:
• There is 70.7 volts on the line at all times.
• The 70.7 volts is a “carrier” that the audio is riding on.
• A voltmeter across the line should measure 70.7 volts during the operation of the sound system.

And there are many more.

With that in mind, let’s take a look at the basics of this type of distribution system.

Part of the confusion is due to the choice of terms.

This is a constant-voltage distribution system, meaning that the voltage at the output of a component is not affected by the load impedance.

So whatever measured voltage exists (i.e., 1 volt, 10 volts, 70.7 volts, and so on) from any type of signal (sine wave, square wave, pink noise, music – it doesn’t matter), that voltage will be unaffected by changing the load on the amplifier.

In other words, it doesn’t matter if the amplifier is driving one loudspeaker or 100, the line voltage will be unaffected by the number of loudspeakers on the line.

Virtually all interfaces in a sound system are constant-voltage - it is unfortunate that this term has become associated uniquely with a means of distributing signals to zones of loudspeakers.

Figure 1: Power distribution at voice coil impedance.

Figure 1 shows how a typical 100-watt amplifier might be interfaced to a loudspeaker.

For simplicity, the loudspeaker is represented as a fixed-value resistance.

The very low impedance amplifier output drives the much higher impedance loudspeaker.

This is a constant-voltage interface, because the voltage across the amplifier’s output terminals does not depend on whether the loudspeaker is connected or not.

A 70-volt system example. Click to enlarge.

A drawback of this interconnect method is that due to the very low loop impedance (the amplifier plus the loudspeaker), the voltage across the load is affected by the wire resistance.

If the wire must be long, the increased resistance affects the signal transfer between amplifier and loudspeaker.

It works just fine for hooking one or two loudspeakers to a single amplifier through several feet of cable.

Your local power company has the same problem - the need to provide a waveform to numerous loads (homes and businesses) while reducing the cable losses.

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