25 & 70 Volt Sound Distribution Systems

There are some excellent reasons to consider using 25 and 70-volt line connections when desiging a distributed sound system. The foremost reason is the system impedance configuaration is actually very simple. Often calculating the impedance for a system with multiple speakers is difficult and if done incorrectly can damage the power amplifier.

Most audio amplifiers can handle a load of 4 to 8 ohms (certain amplifiers can handle loads as low as 2 ohms but they are less common. Most speakers are rated between 4 and 16 ohms. Therefore, when connecting the speakers to the amplifier the installer must be concerned with final impedance the amplifier will "see" connected to its output. (There are formulas for determining impedance, however we will limit our discussion here to the connection of speakers with transformers attached to them and avoid impedance calculations.) In a transformer-based system a transformer is attached to each speaker. The transformer increases the impedance of the speaker which allows connection of numerous speakers in parallel to the amplifier. The amplifier sends either 25 or 70 volts to each speaker. The additional voltage compensates for the increased impedance created by adding the transformer. (Note: not all amplifiers have 25 or 70 volt connections. This is a requirement for the proper functioning of this type of system.)

Once you have the correct amplifier and speakers with the mounted transformers the calculation is simple. Each transformer has several connection taps enabling the installer to select from as little as 1/2 watt up to 10 watts of power for each speaker. (Note: speaker transformer power taps vary depending on the transformer design.) The best way to determine what tap to use is to take the RMS power rating of the amplifier and divide by the number of speakers you intend to connect to it. Example: if your amplifier is 50 watts RMS and you have 10 speakers, divide 50/10, meaning that the maximum power tap connection for each speaker should be watts. This is assuming all of the speaker transformers use the same tap. If different values are required for different speakers, just make sure that when you add all of the tap values together you do not exceed the maximum RMS power of the amplifier. Also, remember that all transformers should be connected in parallel for correct operation.

Finally, volume controls, also called line attenuators can be connected to cntrol various speakers or speaker zones. For this purpose, autotransformers is recommended. The most important consideration when selecting an autotransformer volume control is to be sure the control's power rating is equivalent to or greater then the transformer tap chosen to power the speaker.

For more information of 25 & 70 Volt Sound Distribution Systems, visit our ceiling speakers page.