Public Address System
When a large gathering of people is to be addressed, the sound must be amplified so that people away from the stage can listen to it comfortably. This type of system is called as Public Address system or P.A. system.
FUNCTIONS OF DIFFERENT BLOCKS Microphone: It converts sound to an equivalent electrical signal. Generally two or three microphones can be connected plus one auxiliary input for tape is also provided.
Mixer: The output of microphones is fed to the mixer stage. The mixer stage is used to isolate different channels from each other before they are fed to the amplifier. Voltage amplifier and Processing circuits: The voltage amplifier is used to amplify the mixer output further. The processing circuits block consists of the “master gain control” and the “tone control circuits”. The tone control circuit consists of the bass and treble control circuits. The bass control circuit will amplify or cut the low frequency signals and the treble control will amplify or cut the high frequency signals.
Driver and Power amplifier: The driver amplifier drives the power amplifier to give more power. It is basically a voltage amplifier. The power amplifier gives the desired power amplification to its input signal. The push pull type of amplifier is generally used because this type eliminates the even harmonics from the output of the amplifier and avoids the core saturation of the output transformer. The power amplifier drives the loud speakers. Matching transformers are used between them to match the low speaker impedance to the output impedance of the power amplifier.
REQUIREMENTS OF PUBLIC ADDRESS SYSTEM:
1. It must avoid the acoustic feedback 2. Distribute the sound intensity uniformly. 3. Reduce reverberations. 4. It must use proper speaker orientation. 5. Select proper microphones and loud speaker. 6. It should create a sense of direction. 7. Loud speaker impedances should be matched properly. 8. Proper grounding should be provided. 9. Use closed ring connection for loud speakers.
FACTORS TO BE CONSIDERED WHILE SELECTING A P.A. SYSTEM.
1. The output power of the P.A. system should be adequate for a giving application. 2. Number of microphones that can be connected at the input. 3. Provision to connect a tape recorder or CD player at its input. 4. Provision of a tone control/graphic equalizer circuit. 5. Facility of operating the P.A. system on the DC batteries in the event of power failure. 6. Separate amplitude control for each input microphone. 7. Number of speakers that can be driven by the P.A. system. 8. Facility to use the wireless microphones. 9. Size, weight and cost. 10. Guarantee for reliable operation and after sale service. PA Systems come in many different shapes and sizes, ranging from the very elaborate systems used in large stadiums all the way down to a simple microphone patched into your home stereo. Listed below are several of the most common setups.
Example 1: The Bare Bones The above example is about as basic a system as you can get. Hooking it together is relatively simple. The most important thing to remember when hooking up any size PA is the direction of the signal. This is indicated above by the arrows. The signal starts with your mouth (or drum, or horn, or whatever), then goes through the microphone into the system, then routes its way through the amp, and finally into the speaker where it leaves the system as a much louder sound. A good rule of thumb is to remember that when plugging something in (like an amp), whatever you plug into the input should be coming from the direction of your mouth while whatever you plug into the output should be heading toward the speaker. An easier way to think of it might be to think of it as a river, the microphone being upstream, the speaker being downstream, and the amp being a reservoir in between. As the water (the signal) flows from upstream (the Microphone), it must enter the reservoir (the amp) through an input, and then exit the reservoir through an output until finally, it reaches the downstream side (the speaker). So, that’s the theory (complete with a picturesque metaphor). Now here is the reality. In the example above, you would plug things up in this order.
1)Plug the mic cord into microphone (There is only one place to plug it in. Technically it’s an “output”). 2) Plug the other end of the mic cord into the “input” of the amplifier (remember, input is coming from the microphone). 3)Plug the speaker cord into the speaker “output” of the amplifier (the signal is flowing out of the amp toward the speaker). 4)Plug the other end of the speaker cord into the “input” on the speaker (the signal is coming from the microphone through the amp to the speaker. And there you have it. You have successfully hooked up your first basic PA system. Of course, although it will amplify the sound, this particular system won’t be of much practical use to you in any real life playing situation. It still lacks three essential ingredients.
Example 2: The Essentials With the addition of a mixer (soundboard), an equalizer (EQ), and a set of full range speaker cabinets, we can create a small PA system that can be used both for rehearsal and for some gigs. The principle of signal direction stays consistent. As the arrows indicate, the signal again starts at the microphone passing through each component in turn until it reaches the speakers where it exits the system as an audible, much louder sound. It is important to note the order in which the components are hooked up. No matter how many more components (such as effects or compressors) are added, these basic building blocks should always line up in this order relative to each other. The EQ should always be connected somewhere between the output of the mixer and the input of the power amp. The microphone should always be on the input side of the mixer, and the speakers should always follow the amplifier. Keeping in mind signal direction, the system shown in example 2 should be hooked up like this: 1)Plug the mic cord into microphone (only one end of the cord will fit). 2) Plug the other end of the mic cord into any “input” channel of the mixer (input comes from the microphone). 3)Plug a high Z cable (patch cable) into the “main out” of the mixer (the signal is flowing out of the board toward the speakers). 4)Plug the other end of this cord into the “input” of the equalizer (the signal is flowing from the microphone). 5)Plug one end of a high Z cord into the “output” of the equalizer (the signal is flowing out of the EQ toward the speakers). 6)Plug the other end of this cord into the “input” of the power amp (the signal is flowing from the microphone). 7)Plug two speaker cords into two speaker “outputs” on the power amp (the signal is flowing through the amp toward the speakers). 8)Plug the other ends of these cords into the “inputs” of the speakers (the signal is coming from the microphone to the speaker). Example 2a: The Powered Mixer
The powered mixer (pictured in the center of Example 2a) is very handy in that it combines the amplifier, the equalizer, the mixing board, and some limited effects into one relatively compact unit. These can be very convenient in some situations. They are especially useful for smaller acts (like acoustic bands), and great for rehearsal. In general, they are smaller and easier to transport, and easier and faster to hook up. Also, in many cases, buying one combined unit like a powered mixer can be considerably less expensive than buying the pieces separately. On the down side, they generally have less power than you can get using separate amps, and they often lack some controls on the EQ and Mixer, so if you are in a big loud band in a big loud room, you probably don’t want to use one of these.
Stereo to Mono You may have noticed that you have more than one channel in some (or all) of the components of your system. These are usually labeled in one of three ways; A and B, 1 and 2, or left and right. Each or the two channels will have its own inputs and outputs. What this means is that the component is stereo. Basically that means that it contains two completely separate pathways for two completely separate signals to flow through. The system examples so far, for simplicity’s sake, have been mono systems, but the principles are the same for stereo. A stereo PA can easily be used as a mono PA by simply running through only one channel in each component. For instance, If you wanted to use only channel “B” in your EQ, you would use the input and the output for channel “B”, and leave the channel “A” inputs and outputs open. Of course, when you actually use the EQ, you have to remember which channel you hooked up so you can know which one you will need to adjust to change the sound of the PA. Moving the channel “A” knobs won’t do anything if only channel “B” is hooked up. The sound board is a little different in that you need only be concerned with the outputs. There are many inputs in a soundboard coming in from the many microphones on the stage, but a stereo board will have two separate “main” output channels. You need only choose one of these to use it in mono. Just remember which one you choose and adjust the knobs on the board accordingly. The real draw back to running your stereo PA system mono is that you end up using only half of that expensive piece of equipment you bought. That isn’t that big of a deal with an EQ or a crossover, but with a power amp, only using one channel means you only use half of its power. Only using half of a 500 watt amp results in only getting 250 watts of power, and that just plain sucks. This problem can be fixed by the use of “bridging”, but it’s a variable and sometimes complicated setup that can be hazardous to the health of your equipment. If done right, it’s great. If done wrong, it’s disaster. Public Addressing System- PC Based
PA system is a Public Address or any sound reinforcement system. Also, it is an Audio System for an auditorium or a room. Indosoft Corporation has portable systems that can be used almost anywhere. Public address system is an electronic amplification system used as a communication system in public areas a simple PA (public address) system consists of a microphone, an amplifier and one or more speakers. Large Venue PA system. For larger venues, such as popular music concerts, a larger more complicated PA System is used to provide live sound reproduction. In a concert setting, there are typically two complete PA systems: the “main” system and the “monitor” system. Each system consists of microphones, a mixing board, sound processing equipment, amplifiers, and speakers.
For schools, we offer customized solution PC Based-PA System, wherein all daily/ occasional announcements can be programmed for a year. Also, for informing the period’s starts, over ringing of the bell (different tones) can be programmed into PC. Public Addressing System is used for playing light music & making an announcement at Airports, Hospitals, Factories, Show Rooms & Colleges etc. Basic Terminology
Amplifier- The part of the system that amplifies the sound. May be purchased independently (with a separate mixer) or can be purchased as an integrated unit (a powered mixer).
Channels – ‘Input’ channels usually relate to those items coming into the amplifier/mixer (microphones, or other equipment which needs to be amplified or mixed). ‘Output’ channels usually relate to those items leaving the amplifier/mixer (speakers/monitors etc.)
Delay – An electronic circuit or effects unit – purpose being to delay the audio signal for a specific period of time.
Equaliser – Equipment used to alter specific frequencies of the sound, thus having a precise overall effect on the sound heard from the speakers. This equipment is commonly integrated into an amplifier or mixer, and is now seldom used as a stand-alone unit.
Fader – Another name for an audio level control. Usually refers to a straight-line slider rather than a rotary control.
Jack – Commonly used term to refer to an ‘input/output’ socket.
Level – Another word for signal voltage, (volume, strength or power.)
Line-In (Input/Return) – This is where a signal enters the amplifier/mixer.
Line-Out (Output/Send) – This is where a signal leaves the amplifier/mixer.
Master Volume – Microphone volumes and backing track levels can be controlled independently via the input channels, however the master volume is used to increase or decrease the sound of the overall performance (microphone and instruments simultaneously.)
Mixer – This is the piece of equipment which enables you to control various settings such as the volume of individual microphones/instruments, pan, bass, midrange and treble. There may also be onboard effects such as reverb, chorus, delay, echo etc. Many mixers are referred to as having 6, 8, 12, channels etc. This relates to the number of different microphones or instruments that can be connected to the mixer. E.g. three connected microphones would use 3 channels (or lines) of the available 8 on an eight channel mixer. A powered mixer is an integrated unit that can combine amplifier, equaliser, mixing deck and effects.
Monitors – Additional speakers, commonly placed in front of the vocalist/instrumentalist, enabling them to clearly hear their own sound/performance.
Pan – This refers to controls on the mixer used to adjust the amount of volume sent between left and right speakers. Although very useful when sound from a left or right speaker may be hindered or obtrusive, many people usually leave the panning knobs central.
Phantom Power – A system providing power for condenser microphones from the mixer. Most quality microphones are designed to use +48 VDC phantom power.
Return – A mixer line input dedicated to the task of returning sound from effects devices such as reverb units, echo units etc.
Phono Plug/Jack – Commonly found on consumer audio equipment. One of the most inexpensive connection types – use alternatives if available on your equipment.
XLR Connector – Three-pin connector used in audio for transmitting a balanced signal (microphones etc.) – also referred to as a Cannon connector.