100v line Distributed Audio and Paging Systems

Cables and Connectivity

CIE’s HowToAV.tv examines what 100v line audio is...

In this section:

  • What is 100V Line Audio?
  • How Can I Measure A 100 Volt Line Loudspeaker System?
  • Can I Use My Standard Electrical Multimeter To Test The Speaker Circuit?
  • How Can I Eliminate Feedback On My Loudspeakers?
  • How Do I Choose The Right Loudspeakers For A Public Address System?
  • Can I Make Paging Announcements From A Telephone To A PA System?
  • What Is The Difference Between An Unbalanced & Balanced Audio Signal?
  • What Is An Induction Loop System?
  • What Is Phantom Power?
  • 100v Line Distributed Audio Products
Large PA systems are now a common feature of many public places and businesses, allowing information to be distributed quickly and effectively to anyone in the area.

What is 100V Line Audio?

100 volt line audio is most commonly used for public address and low level background music systems. It is designed to simplify the installation of multiple speakers and allow the signal to travel further distances without any loss of sound quality. This is done with a smaller gauge cable in comparison to a low impedance music amplifier which is used in a HI-FI or live performance music system.

100V line audio systems are used in a wide variety of public and private conversations, including airports, train stations, sports stadiums and large offices. Some are used regularly for general information, whereas some are principally designed to provide information in emergency situations.

Do I Need Special Kit?

For an effective 100V audio system you will need to buy a suitable public address amplifier and speakers which have a transformer fitted. Once you have this equipment installed and set up, you can connect a standard microphone or CD/MP3 player which will play out the system without any modifications or extra equipment.

What Amplifier Should I Use?

The main decision you will need to make when choosing an amplifier is how many audio inputs you require for your system. Each audio source, such as an MP3 player or a TV screen will each require its own input.

Next you will decide how many speakers you require. Speakers often have a selection of power settings to pick from. Known as tappings or taps, it’s important to choose which ever is the most appropriate for your requirements. The higher the tap, the louder the sound is from the speaker.

How Do I Wire The Speakers?

The speakers can be installed and wired in parallel and ‘daisy chained’ together without worrying about the impedance of the system. This means you will not have worry about series and parallel wiring to match amplifier impedances on a 100 volt line system. You can go speaker to speaker or even star wire from a central point. Just ensure that the combined speaker wattage of all the speakers in the system doesn’t go over the output wattage of the amplifier.

What Type Of Cable Can I Use?

As the system is operating at a high voltage and impedance, the electrical current flowing in the speaker cable is very low. This means that large distances of up to 130m can be achieved through standard twin speaker cable.

How Can I Measure A 100 Volt Line Loudspeaker System?

Measuring the speaker load of a 100V PA system is needed for a variety of different installation and testing purposes, and can be extremely helpful for certain applications. These include:

  • Adding new speakers to an existing system – Often it’s difficult to know whether your amplifier can take the addition load of new speakers without going over what it can handle and potentially damaging parts of your system. Even if you count the speakers manually, they could have multiple power taps, plus you might not be able to reach each speaker easily to check what it is set to.
  • Checking the speaker line – If you are installing a new speaker system then it’s important to check the speaker line to make sure it’s safe to connect to your amplifier.
  • Checking for faults – If you have a fault and want to check that the amplifier isn't overloaded or short circuit on the cabling. Overloaded and short circuit speaker lines can result in expensive repair bills which most manufactures warranties will not cover.
  • One of the most effective tools for an installer of 100V speaker systems is an impendance meter which will quickly give you the speaker load measurements you require. To use it you simply disconnect the speaker line you want to check from the speaker terminals on the amplifier, and then connect the impedance meter to the twin speaker cable you have just disconnected. Press the test button on the meter and an impedance reading in ohms will appear.
  • This reading can then be used to work out the speaker load by either cross referencing your reading to an impedance-to-watts chart, working out a simple math calculation, or by checking the reading against the minimum impedance which is found on the back of the PA amplifier.


Can I Use My Standard Electrical Multimeter To Test The Speaker Circuit?

This is something we are asked frequently, and unfortunately the answer is no. The fact is you simply can’t use the resistance range on a standard electrical multimeter, as this will only provide the DC resistance of the speaker line.

A speaker line is driven by AC voltage from the amplifier. An impedance meter will test the line using an AC waveform normally at a frequency of 1KHz. A standard electrical multimeter will not be able to perform this test.


How Can I Eliminate Feedback On My Loudspeakers?

Feedback is something which is incredibly annoying when using sound equipment or musical instruments, unless you’re a rock guitarist. It can ruin a good performance, and even damage your sound system if not addressed properly.

Feedback is created when the sound from your speakers is picked up by a microphone, re-amplified and picked up again from the speakers. This creates a constant loop which produces the unpleasant sound. The sound can vary between a low frequency rumbling, but is most commonly a piercing screech or howl.

Feedback can come from a variety of factors, including the placement of the microphone, the layout of your speakers or the acoustics of the room. To prevent it the best thing to do is ensure all microphones are kept away from the speakers, particularly if you are performing in front of a crowd. For these instances, keep the microphones well behind the main speakers.

If your microphones are too far from the sound source, turning up the volume on the microphone could increase feedback. This means you must place the microphone as close to the sound source as you can. Using a directional microphone, such as a cardioid or a supercardioid will increase the amount of gain on the system before feedback occurs.

Before starting a performance, try setting up a graphic equalizer, which will prevent feedback by ‘ringing out’ the microphones. This allows the sound engineer to effectively cut out the frequencies at which feedback is occurring. Another option is a parametric equaliser, which will do a similar job to an equaliser only with more precise frequency control.

Other simple ways to reduce the chance of feedback include removing hard and reflective surfaces and replacing them with softer furnishing which will absorb the sound. Also ensure the microphone is close to the user’s mouth and ensure they stay in a position where feedback is least likely to occur.

 How Do I Choose The Right Loudspeakers For A Public Address System

Choosing the right loudspeakers is important to achieve the sound you are looking for, and takes various considerations to make the right decision. Firstly the type of system you want, whether it’s a PA system for speech or just background music in a shop, will influence the size and type of speakers you need to purchase

The location of the system is also important, whether it’s indoors or outdoors, one large room or lots of smaller rooms, and the amount of people who the system will be used for. Here are some of the main factors to consider when choosing loudspeakers:

  • Environment – What are the acoustic levels and reverberation times, and are there reflective surfaces which need to be addressed.
  • Ambient Noise – Is the environment relatively quiet or surrounded by noisy machinery, vehicle noise or crowds?
  • Area of Coverage – Are you installing the speakers in a small classroom or office, or a huge warehouse or car park?
  • Quality & Clarity of Sound – if you’re providing messaging or critical voice evacuation then clarity is very important, but if it is for background music or audio marketing, then the quality of sound will be one of the main factors.
  • Aesthetics – Do you care how the loudspeakers look, or are you choosing them primarily for their functions regardless of appearance.
  • Directionality – Is it important for the sound to be dispersed evenly or directed to specific places in the area you are installing them.
  • Environmental Factors – If it is an outdoor installation, then make sure you choose speakers which can handle the weather conditions properly.


100v line Distributed Audio and Paging Systems


Can I Make Paging Announcements From A Telephone To A PA System?

Almost all PA systems use dedicated microphones for making voice paging announcements, and if looked after properly can last for many years without any problems. Despite this, it has become much more common for people to use a telephone system to make paging announcements instead of a dedicated microphone. The benefits of this include:

  • Any location which has a telephone can be used to make announcements, removing the need to add extra microphones and instead using existing phone infrastructure. This prevents costly wiring installations and other changes which would have to be made to accommodate more PA microphones.
  • Telephones make it possible to make announcements from locations where it would be impossible to install a traditional PA microphone. It also means that large distances can be covered easily, allowing announcements to be made from another building or even different country.
  • Telephone interface devices have the option to either make the paging announcement ‘live’ or in ‘store and forward’ mode. ‘Store and forward’ mode can help eliminate feedback and provides the person making the announcement with the ability to preview the paging call before deciding to send or re-record the message before being sent out. Most microphones available are only able to make a ‘live’ announcement.
There are various ways to connect a telephone system into your paging system. The one you choose will depend on the telephone system you have and the facilities you need. The most cost effective method is to use the ‘paging port’ output on your telephone switch if possible.

The other way of interfacing your telephone to a PA system is with a telephone paging extension interface. This is an innovative device which plugs into your telephone extension which then connects to your PA system. The user has to dial the extension number designated for the paging interface, then wait for the call to be picked up by the device before making the announcement.


What Is The Difference Between An Unbalanced & Balanced Audio Signal?

Knowing the difference between unbalanced and balanced audio signals is important if you are to get the most out of your sound system. Unbalanced signals are generally cheaper and are normally used for connecting audio equipment which is close to each other, usually no more than about 3 metres. Balanced audio is used in professional audio systems and use more rugged connectors such as the XLR and 1/4" jack plugs, ensuring the best quality of sound over longer distances.

Unbalanced audio signals often use a single core coaxial type cable, using the ‘phono’ or ‘RCA’ type connector to connect the audio equipment together and also use ‘jack’ or ‘phone’ plugs. Unbalanced audio connectors are sometimes used on a variety of domestic audio equipment such as CD players, MP3 players and HiFi systems.

The centre conductor in the cable is called the positive or ‘hot’ line and the screen of the cable is the ‘earth’ line. The audio signal is carried on the screen of the cable as well as the centre conductor. Any interference is easily picked up and will be audible. Interference can be anything from an annoying hum picked up from local electrical sources to radio stations and taxi company radios.

Balanced audio uses a cable which comprises two conductors that are twisted together and an overall screen. This is a positive line (hot), negative line (cold) and earth (screen). The audio signal is transmitted on both the hot and cold lines but the signal on the cold line is inverted, meaning the signal is effectively flipped upside down. All of this ultimately means that balanced signals can be run over both short and long distances.

What Is An Induction Loop System?

If you have ever noticed a blue logo with a white image of an ear inside and wondered what it was, the answer is an induction loop system. These are commonplace in supermarkets, banks, reception areas and many other areas, and their purpose is to support improved hearing and communication for people with a hearing impairment.

These systems became even more common in recent years after the introduction of the Disability Discrimination Act and the Equality Act, which were put in place to provide non-discriminatory services in every public access area. This means that anyone with a hearing aid can use all of their local services clearly without extra help.

An Induction Loop System is made from an array of looped cables which are installed around a designated area of coverage, such as a room, building or reception desk, along with an induction loop amplifier and an audio source.

By connecting an audio source such as a microphone, music player or TV to the induction loop amplifier, the audio signal is transmitted via the loop cable and received as a wireless signal by the hearing aid or loop receiver device.

The ‘loop’ of cable creates a magnetic field signal throughout the area of coverage, transmitting the audio signal which can be received by the telecoil of hearing aids, cochlear implants and specific loop receiver devices. To use the induction loop system, the hearing aid user simply switches their hearing aid to the ‘T’ position.


What Is Phantom Power?

Phantom Power is an industry-standard way of transmitting DC voltage over an audio cable to provide power to professional audio equipment. The name ‘phantom’ comes from the fact that the power is transmitted through the same cable as the audio signal, meaning you can both power and play a microphone over a single cable.

Phantom power is generally used on devices and equipment which only need a small amount of power to operate, including DI boxes and preamplifiers. The power supplies to use this equipment through phantom power are usually built into most microphone preamplifiers, commercial mixer amplifiers and professional mixing desks. External phantom power supplies are also available if it isn’t included in your audio equipment.

Professional microphones that use phantom power will display their required voltage range within the product’s specification in order to ensure your phantom power supply is compatible. Some microphones are able to work through phantom power or batteries, if using phantom power it’s advised to remove the batteries to avoid damaging them.



Got a question for the HowToAV Team?

Thank you for reading our guide on 100V line audio. We’ve looked at PA systems and issues related to loudspeakers, including feedback and telephone announcements. We’ve also explored other technologies such as induction loop systems and phantom power. If you have any further questions regarding PA systems or 100V line audio, then please don’t hesitate to get in touch with our team of AV experts.

Subscribe to our YouTube channel now at howtoav.tv for all the latest video casts or send us your questions to findout@howtoav.tv

Related Products

back to top
  • UVDB Registered
  • PLASA Member
  • Member of BESA
  • BSI 9001
  • ISCE supporting member