Public Address (PA) Basics

We get many questions from customers regarding PA systems - what equipment do I need, how many loudspeakers do I require, how much power do I need, etc? For some people, setting up a Public Address or P.A. system for the first time can be a little daunting. To help matters we have put together this short tutorial to help explain the different components and their purpose. We end by suggesting some various systems to suit different events.

For more technical articles on various aspects take a look at our Tips'n'tricks page that contains various questions and answers.

1 - The Absolute Basics

A Public Address or PA system is composed of 3 main components - a sound source, an amplifier and a loudspeaker.

The sound source can be a number of things. It is normally a microphone or a playback device such as a cassette deck or a CD player. The sound source produces an electrical signal that represents the sound.

The amplifier is used to increase the level of the electrical signal from the sound source so that it can be heard at sufficient volume from the loudspeaker.

Finally, the loudspeaker is the device which converts the electrical impulses from the amplifier into vibrations in the air which our ear interprets as sounds.

Additionally, you will normally be dealing with multiple sound sources. In order to control these properly a Mixer is used to add the sounds together so that a single signal can be sent to the Amplifier. Mixers also have other roles. Some sound sources have very weak signals (microphones especially) which are not suitable for direct connection to an amplifier. The mixer provides what is known as a Pre-amplifier that will boost the input signal up to a suitable level. This is also referred to as Input Gain

Let's now take a look at each of these components in more detail.

2 - Sound Sources

Sound sources vary widely. They can be microphones for capturing sounds from voices or instruments. They can be from a recorded media such as CD or Minidisc or Cassette. Or, in broadcast situations, they may be from a radio transmission or the telephone. Whatever the source is we need to understand its properties so that we can connect it properly to our PA system.

Microphones produce a very weak signal. They are essentially the reverse of a loudspeaker where the vibrations in the air cause movements in a small diaphram in the microphone capsule. The resulting electrical signal is very weak and needs to be boosted before it can be sent to an amplifier. This is the job of a Pre-amplifier. These devices can be found in most mixers as mentioned above but they can also exist as separate units in some cases.

There are many microphones available and it can sometimes be confusing knowing which one to choose. We have created a separate page on Microphone Selection to help with this.

Line Level Sources refer to the group of devices whose output is at a suitable level to be connected straight into the amplifier, although it is still common to put them through a mixer first. Devices in this category include CD's, Cassettes, Minidiscs, keyboards and video recorders. These devices require no pre-amplification.

We must make another differentiation with certain line level sources - Professional vs Domestic. Professional audio equipment has a higher output voltage than its domestic equivalent. This is expressed in decibels. Professional equipment has an output of +4dB whereas domestic gear has a -10dB output. This is important to know when connecting audio equipment together. If the output and input gains are not matched it can lead to overloading problems. Some professional equipment has a switch (usually located on the rear panel) that allows you to switch the output down to -10dB to help in these situations.

3 - Amplifiers

As we said earlier, amplifiers take a line-level signal and boost it to the levels required to drive a loudspeaker. We refer to all these amplifiers as Power Amplifiers as they handle the high powers required to drive loudspeakers. There are many different makes and models to choose from and you need to ensure that the correct one is chosen. Here are some of the facts you need to understand.

Power

The main parameter you will pay attention to when selecting an amplifier is the Output Power. This is expressed in Watts - e.g. 650 Watts. Care must be taken here as the power delivered by an amplifier depends on the load being driven by the amplifier - in our case, the loudspeaker.

Load

The load in this case is the loudspeaker. Each loudspeaker has an electrical characteristic called the Impedance and is expressed in Ohms (W). It is a measure of the resistance offered by the loudspeaker.

Think about trying to pushing a loaded cart up a hill. You only have enough strength to push it at a certain speed. The heavier the cart, the slower you will be able to push it. In amplifier terms, the higher the Impedance of the loudspeaker the less power will be delivered by the amplifier.

Now, when reading the specification for an amplifier the power will be stated for a specific load - e.g. 650 Watts into 4W, 450 Watts into 8W. It is quite common to see more than one power/load figure quoted as there is not always a direct ratio between them - i.e. double the load does not always mean half the power. If only a single figure is quoted then assume this is for a 4 ohm load. Also bear in mind that these figures generally only apply to a single channel. As most amplifiers have two channels this power is attainable on both and is not a sum of the two.

Bridge Mode

Another feature built into most stereo amplifiers is Bridge Mode. When in Bridge Mode the two channels in the amplifier are electronically linked to form a single channel with increased output power. (Imagine two people pushing the one cart up the hill). Again, the amplifier specifications should indicate the output power/load for bridge mode operation. This is a useful feature when you need a bigger amplifier than you already have.

Protection Circuitry

Good quality amplifiers these days will have several levels of electronic and thermal protection built in. These are important as things can go wrong that could overload or destroy the amplifier - ooo, expensive!

Thermal Cutouts protect the amplifier from overheating. Some of you may have had experience with some of the cheaper mixer-amplifiers which had a tendency to cut out for 5 minutes in the middle of a gig to cool down. This was the thermal cutout in action. Of course, we don't really want the amplifier to cut-out in the middle of a show. Dedicated power amplifiers are a better choice here. They are better designed thermally and most incorporate a fan which operates when some cooling is required. Adequate ventilation is important though and care should be taken to ensure the intake and output vents are not obstructed. The thermal cut-out is still in place though - just in case!

Short Circuit Protection protects the amplifier from a damaged or badly made cable or speaker. If there is a short in either it can lower the Impedance presented to the amplifier. Too low an impedance can damage the amplifier and so the protection circuit can detect this and shut down the channel until the fault is corrected.

4 - Loudspeakers

The loudspeaker is the final link in the chain. In essence it is a very simple device but is takes careful design and construction to produce a unit that delivers clear, precise sound. A loudspeaker is essentially a box or cabinet containing one or more Drivers that produce the actual sound. Different combinations of drivers and cabinet construction methods produce speakers with different characteristics. The most common design is probably the two-way, bass reflex design. This speaker is capable of producing the full range of audible sounds. It is comprised of two drivers and a cabinet with a reflex port to enhance the low frequency response. Lets now take a look at some of the components.

The Cabinet

Some people may think that the cabinet is only there to mount the drivers. It actually plays a much more important part. The design of the cabinet can seriously affect the overall sound of the loudspeaker especially when it comes to the lower frequencies. Cabinet design is very scientific these days, with computer simulation being employed to calculate how sound would behave in a particular design.

Cabinets can be constructed from many different materials - wood being the most popular. One of the main goals when selecting a material is to choose one that doesn't start vibrating when the speaker is in use as this affects the sound. Cheaper speaker cabinets are made from chipboard but they do tend to sound quite nasty. Better ones use MDF. The best speakers use some form of Birch Plywood. This wood is not cheap but produces very little vibration and allows the speaker to produce a clean, uncoloured sound. Other materials in use include a range of plastics which help to reduce the weight of the cabinets while maintaining a good sound.

The way the cabinet is put together is also very important. The dimensions, volume and construction techniques all affect the sound. The low freqencies are most affected. Most cabinets will have a hole, known as a Port, that will allow the air to move freely in and out of the cabinet as the driver cones move. This extends the bass response of the cabinet. Cabinets that are designed to only handle the low frequencies, known as subwoofers or subs, use complex construction within the cabinets to emphasize these sounds. You may come across terms such as Folded Horn, Back-loaded Resonator, etc when dealing with such cabinets. These all refer to the way that the cabinet is constructed.

The Drivers

Drivers are the components that produce the sounds. They are generally comprised of a coil of wire sitting within a magnetic field. As electric current flows through the coil of wire it moves within the magnetic field. By attaching a diaphram to the coil, we can make a volume of air move at the same time and produce sound waves.

A single driver can be capable of producing the full range of audible sounds. In practice, however, multiple drivers are more common in a loudspeaker for efficiency reasons. A two-way system would have a low-to-mid range frequency driver and a high-frequency driver too. One of the reasons for separating these out is that high frequencies are more directional. High frequency drivers are normally mounted along with a horn in order to control the direction. If you take a look at the specification for a loudspeaker, you should find a Dispersion property. This indicates the angle at which the high frequencies leave the loudspeaker. Narrower dispersion angles (30° - 60°) are more suited to long-throw speaker applications whereas wider angles (90° - 100°) are better for short-throw.

Crossovers

When a loudspeaker has more than one driver we need a way to split the frequencies so that the low frequencies go to the low frequency driver and the high frequencies to the high frequency driver, etc. This is done using a Crossover.

A crossover splits an incoming audio signal into its component frequency bands. Most two-way or three-way loudspeakers have an inbuilt crossover network that does this job. This is known as a Passive Crossover. More advanced systems use an external crossover known as an Active Crossover. This works slightly differently as it is inserted before the amplifier. In this case, you will require additional amplifier channels to run each individual driver. This technique is referred to as Bi-Amping. Such systems tend to sound cleaner, more detailed and controlled with each driver being run by its own dedicated amplifier channel. The down side is the expense as more amplifiers are required as well as the active-crossover itself.

Impedance

We mentioned Impedance when we talked about amplifiers. This is a measure of the Load that the speaker represents to the amplifier. It is measured in Ohms (W). Most speakers have an Impedance of either 4W, 8W or 16W. This figure must be taken into account when choosing the correct amplifier to use.

But what happens when you want to connect two or more loudspeakers to the same amplifier channel? Does the impedance change? Well, yes it does, but not as you may expect. When speakers are connected together they are connected in parallel. When this happens the following maths applies:

To deal with more than two loads you just keep on adding terms to the right. So two 8W speakers would give a total of 4W and three 8W loads would give you a total load of 2.67W. Be careful not to make the load too small. Most amplifiers have a minimum load that they can drive, 2W being the common minimum. Always check the specifications of the equipment.

5 - The Mixer

If we want to combine the audio signals from more than one source the obvious thought would be to connect all the wires together. In practice this doesn't really work due to various electrical properties such as impedance, phase cancelation and noise. What we require is a device that combines the signals properly and gives us control over the balance between each input. This is the job of the Mixer.

In its simplest form a mixer has several inputs and a single output where the combined signal comes out. To give us a little more control, individual level (volume) controls are fitted to the inputs and a master level control to the output. This then allows us to not only control the overall output level from the unit but also the balance between the individual inputs. Most mixers have more functionality than this and we need these extra functions when setting up a PA system.

We will now describe the functions of a typical Mixing Desk (also referred to by some as a Console). The desk can be split into 3 main areas.

The Channel Strip - This contains the controls for each input. Multiple channel strips are to be found on a mixer, normally in multiples of 4 or 8 - e.g. 4,8,12,16,24,32. This simply corresponds to the number of inputs that the desk can support.

The Master Section - This containes the Output level controls and other controls that affect the overall output of the mixer.

The Connection Panel - This contains the various connectors for attaching the inputs and outputs and other auxiliary equipment.

We will now take a look at each of these in detail. It should be noted that no two mixers are the same and the one you are using may have more or less features than are described here. All the basics operating procedures, however, are the same.

The Channel Strip
As this is the most important to understand we'll go through this in detail. Lareg mixers can appear very complex with hundreds of controls. However, once you understand the basic channel strip, everything else is simply duplication of this.

	The PRE-AMPLIFIER allows you to boost the imcoming signal to a suitable level. This single control is very important and must be set carefully. Too low, and noise will be more apparent in the signal. Too high, and peaks in the signal may overload the mixer causing the signal to 'clip'. This leads to distortion.
	The EQ or Equaliser section allows control over the tone of the audio signal. Simple mixers may just have two controls - Bass and Treble. Slightly better ones have a Mid-band control too. The best ones have Parametric control that allow you to select the frequency on which the control is acting. The one shown in the diagram has 2 parametric bands allowing for good control over the signal. EQ is used not only to control how something 'sounds' but can also be used to help correct problems such as feedback and to create particular effects. If you use more than one brand of mixers you may hear differences between the EQ sections from different manufacturers. This is one of the reasons that some engineers favour a particular brand of mixing desk.
	The AUXiliary SEND controls allow you to create mixes that are separate from the main output from the desk. This is useful for numerous applications. Certain inputs (e.g. vocals) can be mixed and sent to an external effects processor to add reverb. Other Aux Sends can send a mix of channels to be used as a 'Monitor Mix' for the band on stage. Again, different desks may have different numbers of AUX's. They all operate the same way. Depending on the desk, some Auxiliaries can be set to PRE or POST. A PRE of PRE-FADE AUX is effective before the signal gets to the master fader (discussed further down) and is therefore independent of it. A POST-FADE AUX is implemented after the master fader and is therefore linked ot the level of the fader as well. This allows different levels of control depending on the application.
	The PAN control is only really effective when you are using a stereo output (i.e. left and right). It allows you to control the ratio of the signal between the left and right channels. Using this you can create a stereo-image between all the instuments and vocals in a band even though each one comes in on a mono channel. It is used more in recording work than live as it is more difficult to appreciate stereo effects in a live venue. Also remember that if you 'pan' sound to the speakers at the right, then the person standing at the left of the venue won't be able to hear it all that well.
	Finally, the FADER allows you to control the level of the particular vocal or instrument. The linear design of the control makes it easy to adjust as you work. During an actual performance most of the other controls should already be set and it is only necessary to adjust the balance between the individual sounds using the Faders.
	One final control to mention on the Channel Strip is the PRE-FADE LEVEL or PFL button. This button, when pressed, sends the ouput from this channel only to the headphone output on the desk allowing the engineer to monitor what is happening on this one channel. It also sends its output to the level meter on the desks master section, again allowing you to visually monitor the signal level for this one channel rather than for the whole desk. This control is Pre-Fade meaning that you can monitor the signal while the Fader is down - very useful for checking channels before bringing them into the mix.

The Master Section
The Master section of a mixing desk contains controls common to all function of the desk. The exact layout of the master section varies from desk to desk but the following features are common to all of them.

	The MONITOR SECTION contains the monitor level controls together with metering and a headphone socket. The headphone socket is normally located on the front of the mixing console but may sometimes be on the rear or even under the front lip of the console. The monitor section allows the operator to view the output levels of the desk and also the individual levels on channels and AUX buses using the PFL and AFL buttons located on the console.
	The AUXILIARY MASTERS control the overall mixed signal levels from the AUX controls. These can be used to set the overall levels for effects and monitor sub-mixes.
	The MASTER FADERS control the overall output from the desk. Sometimes there will be two of these - one for the left channel and one for the right. Some desks may combine these into a single control as it is uncommon to run these at different levels.

6 - Cables & Connections

Cables, although among the cheapest items in a PA, are very important. These carry the audio signals between components and should therefore be chosen carefully in order to minimise losses and interference which could degrade the resulting sound.

Cable Types

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