July 19, 2016

Understanding 100V Line Distributed Speaker Systems.


Commercial audio installations often require large numbers of speakers to be powered from a single amplifier. These systems often called “100V line” or less common in Australia “70V line” are found in most large buildings including shopping centres, schools, offices etc. These audio systems are typically used for general announcing, public address, background music and vitally important fire/evacuation messaging.

We find there is a lot of confusion amongst installers and trades about how these systems are wired up. Largely this is caused by the fact that these speakers are not wired in the traditional way low impedance or home hi-fi speakers are. In fact, it may seem completely counter intuitive!

If you look at the traditional home hi-fi system, you have a single output connected to a single speaker. In some cases your amplifier might even be able to power two pairs of speakers from a single output, but any more and most amplifiers cannot handle the impedance load. This is because as more pairs of speakers are added to a circuit, the impedance drops, the further it drops, the more load the amplifier is under to power the speaker.

To get around this problem, the 100V line system is wired up completely differently and each speaker is fitted with a transformer on the back of it, this isolates the speakers impedance load from the amplifier. The transformer can be set to a wattage rating, allowing for independent volume level for each speaker. The net result is a 100V line system allows for long ‘strings’ of speakers to hang off a single amplifier output, each speaker is connected to the next one in the circuit by daisy chaining the wiring. See figure below.




Advantages of 100V line systems

  • Connect many speakers to an amplifier.
  • As needs change, additional speakers can easily be added or subtracted from a circuit.
  • Simple load calculation. Simply add together the wattage of each speaker in the string, and ensure it is below the rated wattage of the amplifier.
  • Individual transformer “tap” settings allow tailored volume level for each speaker.
  • Suitable for use with long runs of relatively small gauge, economical cabling.

Disadvantages of 100V line systems

  • Sound quality. Most 100V line speakers do not perform brilliantly at low frequencies, however for general announcing this is not generally a concern.
  • Additional cost per speaker due to fitment of transformer, line monitoring capacitor and regulatory requirements (such as speaker, transformer enclosures and cable restraint plates).
  • Usage limited to paging, voice and low level background music applications.

How it works?

A 100V line speaker is operated by applying 100V RMS from the amplifier to the speaker step down transformer via suitable figure 8 cabling. This stepdown transformer then converts the 100 Volts down to a standard speaker level to output audio.

In the case of fire & evacuation speakers, these have an additional requirement of “line monitoring” which is provided by the addition of a bipolar capacitor in series with each speaker transformer. This capacitor allows a line monitoring unit to measure the speaker circuit for ‘normal’, ‘open’ or ‘short circuit’ conditions. For more information, see our guide on ‘What is Line Monitoring?’.

Connecting Multiple Speakers or ‘How much load is on my 100V circuit?’

This part is relatively straight forward. Each speaker string should be wired up in parallel and the wattage of each speaker tap is added together to find the net result of wattage load on the amplifier. So for instance, a 100W Redback amplifier is capable of powering 100 speakers when each speaker is set to a 1W tap. Or 50 speakers when each speaker is tapped at 2W.

Sometimes of course, you may turn up at an installation that was done years ago, and you have no idea how many speakers are installed or what power taps they have been set to. In this case you will need to resort to measuring the AC impedance of the speaker string.

Importantly, this differs from DC resistance that most multimeters offer as a standard range setting. To measure the AC impedance you will need a dedicated impedance meter such as the Redback Q 2001.

Using the table below you can easily work out the system load for a measured 100V impedance.

Screen Shot 2016-08-19 at 3.58.04 pm

Why do some speakers have a two way terminal block and others have four way terminal blocks?

The connection of two way or four way terminal blocks is essentially identical. Simply positive and negative input from the amplifier. However a four way terminal block is a requirement of the Australian fire & evacuation regulation. Essentially it allows for easier loop in, loop out daisy chain wiring without having to put two cable conductors into the one cable entry.

Tips for 100V line Speaker Installations

  • Keep all the speakers in the same polarity. Redback amplifiers are marked with (+) and (-) on the 100V output terminals and speakers are marked with red and black on the terminal blocks. Maintain this polarity throughout your speaker circuit.
  • Be conservative with your ratings. Always design a system with 20% spare capacity, so if you have 80W of speakers, buy a 100W amplifier. This allows for expansion of the circuit later or changes to individual power taps for areas that might need extra volume.
  • Take advantage of higher wattage speakers where possible. If it means you need to buy and install less speakers overall the system will work out considerably cheaper.
  • Take into account local factors that may require different speaker solutions, such as horns, pendants or high wattage ceiling types. Factors such as elevated ceilings, exposure to elements, other background noise sources can drastically effect system components and price.
  • Avoid star wiring or multiple feed configurations, especially in fire & evacuation installations. These are problematic from a regulatory point of view and are not suitable for zone line monitoring.
  • Cater for local attenuation (or volume level) control. More often than not people like to adjust the level of sound in their work environment, especially with background music. Allowing for local level control makes it easy to set a comfortable volume. Ensure your attentuator has an override relay if used in a fire/evac system.
  • Fixing or expanding an existing install? Use an impedance meter! It’s an essential tool for a PA installer to ensure the speaker load stays below the amplifier rating.
  • Do not EVER mix 100V line speakers with traditional low impedance speakers within the same circuit. Redback amplifiers are also not designed for powering low impedance speakers and 100V line speakers at the same time. If this is required, we suggest using two completely separate amplifiers.