Distributed 70V Loudspeaker Systems Rick Kamlet, Sr. Director for Commercial Sound at JBL Professional
Distributed speaker systems are often used for the background music and paging systems in applications such as retail stores, restaurants, hotels, office buildings and other large business locations. In the US, these systems are often called 70-Volt Systems (usually printed as “70V”).
This article is a brief introduction to why 70V distributed systems are used for these applications and what the advantages are.
What is a 70V distributed system? In a 70V system, the voltage sent down the loudspeaker line from the power amplifier to the loudspeakers is high (as high as 70V on peaks) and then a transformer is used at every speaker to bring the voltage back down to what the speaker needs. Figure 1 shows a typical hookup diagram for a 70V distributed speaker system.
Why use a 70V distributed system? Distributed systems allow driving many speakers from a single amplifier channel (which saves money), allow the use of smaller cable sizes (which also saves money), allow for each speaker to be set at a different volume without affecting the other speakers, and makes the calculations for choosing the amplifier size simple and straightforward.
6 FIGURE 1. Basic 70V Distributed Speaker System
In 70V systems, amplifiers are designed so their full power output voltage can reach as high as 70 volts RMS, which is a fairly high voltage. Then the power that each speaker takes from the distributed speaker line is set via a tap setting on the speaker.
Connecting Many Speakers on an Amplifier Channel With low impedance speakers (4-ohm or 8-ohm speakers), you cannot parallel very many speakers before the power amplifier gets loaded with too low of an impedance. In distributed systems, it is not uncommon to have dozens or even hundreds of speakers driven from a single 70V power amplifier.
Setting Speakers at Different Volumes By changing the tap setting of the 70V speaker, the volume of the speaker can be adjusted upward or downward, depending on what volume is needed from that speaker.
Wire Gauge In commercial-sound projects, speaker cables are frequently quite long, often hundreds of feet. By running the distributed speaker signal at the higher voltage, the current going through the wire is lower so you can use smaller gauge cables. Smaller gauge cables can save the installer and customer a substantial amount of money.
How powerful of an amp to use – computing the watts All the loudspeakers in a distributed system get connected across the two wires (plus and minus) coming from the power amplifier, and each speaker gets tapped as needed. To determine the amplifier size, all the designer has to do is count Watts, adding up the sum of all the tap settings on all the speakers. When the total wattage drawn by the speakers equals the power output rating of the amplifier, then the maximum number of speakers has been attached to the amplifier’s output. In practice, it is best to keep the total sum of the speaker taps below 80% of the power amplifier rating, giving some headroom and reducing the strain on the power amplifier.
Just make sure that the amp is a 70V commercial amplifier, in other words that it has been designed to be utilized in a 70V distributed speaker system.
Do 70V systems always use low power speakers? Many speaker systems do indeed utilize a large number of low power speakers. However, 70V systems can be used to drive fairly high power speakers, too. For example, JBL makes 70V speakers with taps as high as 150 Watts, both in surface-mount (on-wall) and in-ceiling speakers. These higher power speakers are used in places such as theme parks, ballrooms, convention facilities, music cafes, stadium concourses, and many other applications. You still get the benefits of being able to drive many speakers, using smaller gauge cable, and being able to adjust the volume of speakers without affecting the other speakers. 70V commercial power amplifiers are made in high power ranges, too.
To summarize, utilizing a 70V distributed speaker system allows for fewer power amplifier channels, uses smaller gauge (ie, less expensive) speaker cable, and allows for volume adjustments without affecting other speakers. Distributed systems can save the contractor and customer a lot of money and make a system that is straightforward to design and install.
Rick Kamlet is Senior Director for Commercial Installed Sound at JBL Professional.
www.adiglobal.com
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