Frequently Asked Questions


Q. "What is a Tannoy System?"
A. For background music and paging over large areas, 100 volt line loudspeaker systems are an efficient method of installing many speakers over long distances. Even now, some people still refer to these commercial sound installations as ‘Tannoy’ systems. In the same way that a vacuum cleaner is often called a ‘Hoover’, the Tannoy name was synonymous with this type of equipment, but these days, many other world famous manufacturers have huge product ranges in this sector.


Q. "What is 100 Volt Line?"
A. A 100 volt line system is often called a ‘constant voltage system’.

This voltage is more stable over long distances. Think of our electricity supply - the National Grid transports electricity across the country at around 400,000 volts and then local substations ‘step down’ the voltage to 230 volts.

In the same way, a step up transformer in the amplifier increases the output voltage to 100 volts (it’s usually 30 volts) and then each loudspeaker in the system has its own step down transformer matched to the impedance of the speaker (typically 8 ohms). The primary side of the transformer usually has several connections known as ‘tappings’ and marked in watts. This allows the maximum volume level of the speaker to be set by selecting an appropriate wattage. The total primary wattage should not exceed 90% of the amplifiers output.

The speakers in a 100 volt line public address or background music system are wired in Parallel whereas a low impedance (4 or 8 ohms) system is wired in Series Parallel. It is also important to use the correct cable for a 100 volt line music system, so if you are in any doubt, please ask for advice.


Q. "How do I know whether to use 100 volt line equipment or low impedance?"
A. If the loudspeakers are spread over a wide area or they are a long distance from the amplifier, it would be best to use 100 volt line. (See our Technical Information Page for more details.) However if the cable runs from an amplifier are reasonably short then you may wish to consider using low impedance loudspeakers which tend to be superior in performance to that of a 100volt line loudspeaker (see technical information)


Q. "How many speakers can I use with a 150 watt 100volt line public address mixer amplifier?"
A. You need to add up the total wattage of the speakers and keep it below 90% of the amplifiers rated output.


Q. "The PA amplifier I have bought has both 8 ohm and 100 volt line outputs on it. Can I use both kinds of speakers on this amp at the same time?"
A. No. You must decide whether to use low impedance or 100 volt line and use the appropriate speakers. The different outputs make the product more versatile, but they cannot be used simultaneously.


Q. "I have found a speaker which would fit into my low impedance project perfectly but the product description says it is 100 volt line. Can I still use it?"
A. Not as it is. However, the transformer can usually be bypassed – call us for advice.


Q. "What is AFILS?"
A. AFILS is an acronym for Audio Frequency Induction Loop System.

An induction loop system, sometimes referred to as a ‘Deaf Loop’, enables the hearing impaired to hear more clearly in conjunction with the ‘T’ or ‘MT’ switch on their hearing aid. A microphone picks up the required sound which is then amplified and fed through a closed loop of cable. This generates a magnetic field which is picked up by the coil in the hearing aid.

Induction loop amplifiers come in all shapes and sizes to suit different rooms and requirements. However, please bear in mind that the resulting magnetic field means that the sound can be heard by anyone with a listening device within the coverage of the loop, so it’s not suitable where confidentiality is important. In these cases, an infra-red hearing system may be better – please call for advice.

An induction loop system or hearing assistance system is now a legal requirement in many places, especially where the general public access services and care. There are many regulations which govern ‘help with hearing’, including the Disability Discrimination Act (DDA) which has now been absorbed into the Equality Act 2010 as well as British Standard BS8300 (2009), the Human Rights Act 1998 and the Care Standards Act 2000.

One of our suppliers, Signet, has an excellent guide which outlines everything you need to know about AFILS. Click here to download.


Q. "I don’t wear a hearing aid, so how can I tell if the induction loop system I’ve installed is working correctly?"
A. You can buy a simple ‘Loop Listening Device’ which enables you to test the loop with a standard pair of headphones. See RxTi2 in the ‘Induction Loop Accessories’ department of our ‘Help with Hearing’ section.


Q. "What if I need to install induction loops in adjacent rooms. Will they conflict?"
A. Yes they will. In order to reduce or negate any ‘overspill’ between loop systems we can specify or design alternative systems. Firstly we can offer infra-red as a broadcast method. This method ‘contains’ any signals with a room without overspill but requires the use of neck worn receivers. Alternatively we can design a low spill or phase shift system using special loop cabling patterns within a room thus containing the loop field within a given area. Please ask for details.


Q. "What kind of induction loop amplifier can I use for hearing assistance in a hospital consulting room where the conversation needs to be kept confidential?"
A. Some small induction loop amplifiers have a very localised coverage, which would just cover the area around a desk. However, for complete confidentiality an infra red hearing system can be used. The hearing impaired person would wear a special receiver around their neck which generates a small induction loop signal for their hearing aid to pick up. The receiver can only work if it is in line of sight of the transmitter, so this is much more private than an induction loop system.


Q. "How far will my radio microphone transmit?"

A. Normally 75 to 100m, line of sight. This range is reduced considerably if the transmitter and receiver are separated by a wall or if the room is full of people or other obstructions.

Some systems have a high power switch which will increase the RF output, but will shorten the battery life.

If a receiver can only be located in an adjacent room, you will need to use remote antenna, cable and, possibly, antenna boosters.


Q. "When should I use an antenna distribution amplifier?"

A. If you are operating several radio microphone systems in the same area, it is strongly advisable to use an antenna distribution amplifier. This minimizes the number of receiving antennae in the system improving system performance and appearance. It can also distribute power to each receiver from one mains point.


Q. "My radio microphone systems interfere with each other when multiple systems are switched on together"

A. This often happens when trying to run multiple systems with transmitting frequency and channel numbers too close together. It may also be a result of one system producing a harmonic which then interferes with another system.

Careful consideration must be made to radio microphone system planning and how many systems you are expecting to use together. 

More expensive systems often incorporate more sophisticated RF filters to allow more systems to be run together.

System performance can also be affected if the wireless transmitter is too close to the receiver. It is advisable to leave at least 5m distance between the two components.


Q. "I’ve heard a lot about changes to radio microphone licensing and use within the UK. Does this affect me?"

A. There have been many changes to the frequencies that you can operate wireless microphones on in the UK.

If you do not own a licence and operate a radio microphone then you should only be using the licensed free frequencies of 863 to 865MHz for UHF systems, 173.8 to 175.0MHz for VHF and 2.4GHz for digital systems.  Use of frequencies outside this band is illegal unless you have the correct licence. Further details of radio microphone licensing can be found from


Q. "Where do I get a licence for my radio mic system?"
A. You will need to contact JFMG for all the up to date information.