How To Wire 70-volt Distributed Sound Systems

So you want to feed sound from your main sound system into more than one other room, and you want local volume controls in each room, right?  You’ve called ProAcoustics, and they’ve recommended to you a 70-volt Distributed sound system.  It all looks well and good, but there’s one thing standing in the way – how do I wire this up?

We’ve posted this article to help explain some key points of a 70-volt distributed system, from definitions of terms to wiring (diagram included!), and even some tips on selecting components.

Firstly, allow us to explain some of the very good reasons for using a distributed 70-volt system:

  1. Once you start connecting more than 2 speakers in series and parallel combinations, unless you really know what you are doing, you can mess up the impedance to your amplifier. This will result in uneven volume and it could possibly burn out your amplifier.
  2. The only reliable volume controls are those designed for a 70 volt system. “L” Pads and “T” Pads become scratchy and burn out quickly.
  3. You can connect many speakers and volume controls throughout the building using one power amplifier and one wire run. Each room”s volume can be controlled independently. The number of speakers is limited only to the power required from the amplifier.

*You may also want to read our Distributed Speaker Systems 101 article for even more information.

How a 70 Volt Distribution System Works

A 70 Volt Sound Distribution System is actually quite simple once you understand it. Here is the concept behind it.

Most audio amplifiers can handle an output impedance down to 4 ohms (sometimes 2). Most speakers have an impedance of 8 ohms (sometimes 4). Once you connect two 8 ohm speakers in parallel you have reached the 4 ohm limit of the amplifier. If you connect them in series, you get 16 ohms which is safe for the amplifier, but inefficient in terms of power transfer. You could connect any number of speakers in combinations of series and parallel to achieve various impedances, but the volume of sound from each speaker will vary significantly unless you have the design perfectly balanced. No matter what you do however, you cannot control speaker volumes independently.

In a 70 Volt distribution system, a transformer is added to each speaker which increases the speaker’s impedance significantly. This allows you to connect a large number of speakers in parallel to the amplifier. The amplifier sends a higher voltage than a regular 4 or 8 ohm amplifier (up to 70 volts) in order to compensate for the high speaker impedance. By connecting the speaker transformers at different wattage levels, or by adding 70 volt attenuators, each speaker can be set to the desired volume independent of the others.

In a 70 volt system, you can add as many speakers as you desire as long as the total power requirement of all the transformers added together does not exceed the power output capability of the distribution amplifier.

Installing a 70 Volt Sound Distribution System

Here is how you connect a 70 volt sound distribution system:

For a larger image, click here

  1. Connect your speaker wiring to the common and 70 volt output terminals on the distribution amplifier. All of the speakers need to connect in parallel. They can be either “home run” individually back to the amplifier, or “daisy chained” from one to another. You can of course use a combination of both. The important thing is that the inputs to the volume controls (or primary of each transformer if you are not using volume controls) are in parallel with each other.
  2. The Common and 70 Volt outputs of the amplifier connect to the Common and Input of each attenuator.
  3. The Common and Output of each attenuator connects to the Common and desired wattage wire on the Primary of the 70 volt transformer.
  4. If your speaker doesn’t have the transformer already soldered to the unit, the Common and 8 ohm secondary wires on the transformer connect to the speaker terminals. Connect the Common to the (-) speaker terminal and the 8 ohm to the (+) terminal.
  5. Now you need to connect from the Line or Auxiliary output from your main mixer to the Auxiliary input on the distribution amplifier.

Selecting the right Transformer taps

For a ceiling or wall mount speaker such as the ones described above, you shouldn’t require more than 10 watts to produce more than enough sound. If you are not using an attenuator, you can feed sound to the speaker and experiment using different 70 volt wires (10w, 5w, 2.5w, 1.25w) until you get the volume you desire. It is better of course to use an attenuator (volume control) and adjust the volume with that. If you do, you should connect the system as shown above, turn the attenuator to full volume, and choose the transformer tap which gives you the most volume you could want.

When we use an attenuator, we usually wire the transformer for the full 10 watts at 70 volts.

Selecting the right attenuator

The Attenuator you install needs to be rated for a power equal to or greater than the transformer settings. If the attenuator is controlling only one speaker with a transformer set at less than 10 watts, then the Atlas Sound AT10D is perfect. It could also control two speakers set to 5 watts each.

If you are controlling more speakers, such as four or six at 5 watts each, you will need to use the Atlas Sound AT35D, which can handle up to 35 watts.

Selecting the right Distribution Amplifier:

First of all, the amplifier you use needs to have a 70 volt output. Second, it must have an output power rating equal to or greater than the total wattage requirement of the system. For example, if you have two speakers at 10 watts each, plus two more at 5 watts each, your amplifier must be rated at 30 watts minimum. If you have six speakers at 10 watts each plus three at 5 watts, you need an amplifier rated at 75 watts or more.

Note that the power rating of the amplifier is determined by transformer settings, not attenuator ratings. It is a good idea to install an amplifier with more than enough power in case you want to add more speakers later.

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Image and Article from Alectra Systems, Incorporated

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