Constant Voltage Speaker Systems

November 3, 2013 in AV Design Tips, The Basics by Sam Davisson

overhead-speakerWhile this might be a great reminder of how much technology has changed, I’m not sure why when I searched for an interesting image of overhead speakers this image came up. Can I assume that in Google’s eyes this instructor is teaching something that is way over the heads of the students?
Or, perhaps even, that should something catastrophic happen and we had to revert back to "old tech" people would be completely lost? But this has nothing to do with furthering today’s topic, Constant Voltage Speaker Systems.

Electrical power companies figure out a long time ago that the best, most efficient way to distribute power was to step up the voltage at the power station and then to step down at your house. The audio industry took this model for distributing audio to large numbers of speakers or speakers no where close to the amplifier which gives us the constant voltage system we currently use or over use but I’ll probably discuss that at some later date.

The term “constant-voltage” is somewhat misleading causing some confusion. In electronics, two terms exist to describe two very different power sources: “constant-current” and “constant-voltage.” Constant-current is a power source that supplies a fixed amount of current regardless of the load; so the output voltage varies, but the current remains constant.

Constant-voltage is just the opposite: the voltage stays constant regardless of the load; so the output current varies but not the voltage. Applied to distributed sound systems, the term is used to describe the action of the system at full power only, an important point in understanding. At full power the voltage on the system is constant and does not vary as a function of the number of loudspeakers driven. As long as you do not exceed the maximum power limit of the amplifier you may add any number of speakers and the voltage will remain constant.

The other thing that is “constant” is the amplifier’s output voltage at rated power – and it is the same voltage for all power ratings. Several voltages are used, but the most common in the U.S. is 70.7 volts rms (100 volts is common outside the US). The standard specifies that all power amplifiers put out 70.7 volts at their rated power. So, whether it is a 100 watt, or 500 watt or 10 watt power amplifier, the maximum output voltage of each must be the same 70.7 volts.

Advantages of 70V Systems

A 70V line reduces power loss due to cable heating. That’s because the loudspeaker cable carries the audio signal as a low current. Consequently you can use smaller-gauge loudspeaker cable, or very long cable runs, without losing excessive power.

Another advantage of 70V operation is that you can easily provide the amplifier with a matching load if you’re connecting hundred of loudspeakers. With a single 8-ohm amplifier output it can be difficult to wire the loudspeakers in a series-parallel combination having a total impedance of 8 ohms and if one loudspeaker fails, all of the loudspeakers in series are lost. This changes the load impedance as seen by the power amplifier.

Conversely, with a 70V system you can hang hundreds of loudspeakers in parallel on a single amplifier output. In addition, a 70V distributed system is relatively easy to design and allows flexibility in power settings meaning different speakers can be set to have differing volume settings.

Disadvantages

Simply, your audio quality suffers, especially in the low frequency range, transformers can degrade the frequency response and add distortion. Also, designers have gotten lazy. Ceiling speakers are thought of only in a 70V configuration and used to fill a room with noise rather than doing a proper LCR (left, center right) 8 ohm sound system with 8 ohm ceiling speakers for speech reinforcement.

Yes it may be cheaper but isn’t the goal natural sound.

Opinion

This is my strictly my opinion. Constant voltage systems are great when used as intended, as a means of connecting a large number of speakers where needed or in remoting speakers a long way from the amplifier.

But this technology is being over used because of simplicity. A design engineer doesn’t need to understand how to fill a room with sound. Clients and the industry suffers because of this.

I’ll leave you with this little chart on cable requirements I borrowed from Belden Cable’s article on this 70V systems:
Speaker Cable Distance Chart

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