3.5 Experiment Arrange PA system with multi speakers and microphones (with cord and cordless) Aim: To study performance of Public Address System. Apparatus: PA System kit Dynamics Mic Woofer CRO Theory: It is an electro-acoustic system, in which sound is first converted into electrical signals by a microphone. The electrical audio signals are amplified, processed and fed to another transducer, loudspeaker, which converts the audio signals into sound waves. A block diagram of a basic P.A. System is shown in fig. given below. The function of each block of P.A. System is described below. 1. Microphone: It picks up sound waves and converts them into electrical variations, called audio signals. Generally, amplifiers have provision of 2 or more microphones and in addition, an auxiliary input for tape/record player. 2. Mixer: The output of microphone is fed to a mixer stage. The function of the mixer stage is to effectively isolate different channels from each other before feeding to the main amplifier. It can be either a built-in unit or a separate pluggable unit. There are three types of mixers. The simplest one does not use pre- amplifiers and amplifiers, but uses only gain controls (also called-faders) and isolating series resistors. A little more sophisticated one has separate pre- amplifiers, for separate channels and then after the gain control potentiometers and isolated resistors, there is a common amplifier and an emitter follower. low impedance of emitter follower matches the input impedance of the voltage amplifier of the P.A. system. The function of the separate pre-amplifiers and common amplifier in the mixer stage is to amplify the weak signals. 3. Voltage Amplifier: It further amplifies the output of the mixer. 4. Processing Circuits: These circuits have „Master gain control‟ and tonecontrols (Bass/treble controls). 5. Driver Amplifier: It gives voltage amplification to the signal to such an extent that when fed to the next stage (power amplifier stage), the internal resistance of that stage is reduced. Thus, it drives the power amplifier to give more power. 6. Power Amplifier: It gives desired power amplification to the signal. It uses push pull type of circuit in general, so that the even harmonics are eliminated from the output and transformer core is not saturated. The output of the power amplifier is connected to the loudspeaker through a matching transformer to match the low impedance of the loudspeaker for maximum transfer of power. In some circuits, the design is such that a separate matching transformer is not needed. 7. Loudspeaker: It converts electrical audio signals into pressure variations resulting in sound. A public address system abbreviated P.A. system is an electronic amplification system used as a communication system in public areas. It is an amplification setup with amplifier and loudspeaker used to address a large number of audiences. Consider a P.A. system being used to live musical show. It will be driven from a mixer at which the engineer will put together the incoming microphones and musical instrument signals which will be amplified by the system & provided to the loudspeaker. A simple P.A. system consists of one or more loudspeaker. These three components have a potential for feedback. Feedback occurs when sound from the speaker find it way back into the microphone and then re-amplified this generally manifest itself as a sharp – sudden shrill due to oscillations. This effect can be reduced by proper use of negative feedback. Procedure: Connect the Microphone to the chaises connector of audio input sections Connect patch cord A to A, B to B, C to C, D to D, G to G and G1 to G1 Woofer Connect the speaker to the output point Switch on the supply of the trainer kit. Observe output waveforms on CRO. Observations*: *screenshots of output waveforms to be attached Conclusion:
