[Q] DEFINE AUDIO AMPLIFIER. STATE ITS TYPE. DRAW ITS FREQUENCY RESPONSE CURVE.

TOPIC : HI FI AUDIO AMPLIFIER/ AUDIO SYSTEMS INTRODUCTION TO AMPLIFIERS: MONO, STEREO DIFFERENCE BETWEEN STEREO AMPLIFIER AND MONO AMPLIFIER. [Q] DEFINE AUDIO AMPLIFIER. STATE ITS TYPE. DRAW ITS FREQUENCY RESPONSE CURVE. An audio power amplifier is an electronic amplifier that increase the strength of low-power audio signals (signals composed primarily of frequencies between 20-20 000 Hz, the human range of hearing) to a level suitable for driving loudspeakers. It is the final electronic stage in a typical audio playback chain. Types of Audio systems : Mono Amplifier system Stereo Amplifier system

[Q] : COMPARE STEREO AMPLIFIER WITH MONO AMPLIFIER

BLOCK DIAGRAM OF HI FI AMPLIFIER AND ITS WORKING. CONTROLS AVAILABLE ON IT AND THEIR FUNCTION, [Q] WHAT IS HI-FI SYSTEM? LIST CHARACTERISTICS OF HI-FI SYSTEM. The word fidelity means faithfulness. In audio system it is used to indicate faithful reproduction of sound. Hi-Fi system stands for high fidelity stereophonic reproducing system. Such a Hi- Fi Sound can be obtained from the recorded stereo tape or live systems from the microphones. High fidelity reproduction is essentially sound reproduction such that the most critical person can listen intensity to it without any distortion. Characteristics of HI-FI amplifier: 1. Signal to noise ratio should be better than 50dB. 2. Frequency response should be flat within +-1dB. 3. Non-linear distortion should not be more than 1%. 4. The system should possess dynamic range of atleast 8dB. 5. Stereophonic effect should be provided. 6. Environmental conditions should be such as to eliminate the external noise in listening room. [Q] DRAW AND DESCRIBE THE BLOCK DIAGRAM OF HI-FI AMPLIFIER. Figure shows the block diagram of a high-fidelity stereo reproducing system. High- fidelity sound can be obtained from the recorded stereo tape or in live system from the microphones. (Stereo signal can also be obtained from the record player.) The stereo signal is fed to two independent amplification channels through a tapemic switch. The amplifier system consist of a low noise high gain preamplifier, equalizer, well design amplifiers giving flat frequency response and little distortion by using negative feedback circuit and then the matching transformer.(a balancing circuit is incorporated to balance out any imbalance in the characteristics of otherwise identical circuits.) The secondary of the matching transformer of each channel is connected to the respective loudspeaker column. For Hi-fi, the loudspeaker columns consisting of woofer, squawker and tweeter are used. All the blocks are design so as to get fat frequency response (from 40 to 15000Hz), little distortion (less than 1%), high signal-to-noise ratio (more than 50db) and high dynamic range (100 db) to achieve the final output of high fidelity.

Balance control: Two amplifiers of a stereo system, although independent of each other are built as matched pair to give equal output for the same input. In spite of the two amplifiers being identical, there may be variations in the characteristics of transistors and ICs and positioning of loudspeakers and furnishings with respect to the listener. The circuit used to compensate for such variations are called Balance control. Loudness control: Sometime music is at low level of volume (as in case of the background music). At low levels there is considerable loss in bass in reproduction. It is therefore, necessary that there should be substantial boosting of the bass at low levels. Boosting at treble may be only nominal because loss at high notes is quite small. The control which provides desired big boosting at bass and a little boosting at treble is called loudness control. Bass and Treble control: It is provided to tailor bass treble as per personal taste of the listener. Master Gain control: a master gain control is used for adjusting overall volume without disturbing the balance. This is achieved by using dual concentric shafts,

the inner shaft adjust the balance control and the outer shaft, the overall gain or volume of the amplifier. A typical master gain control circuit is shown in figure is adjusted for balancing the two channels and then R2 and R3 are adjusted for increasing or decreasing the volume of the channels. R2 and R3 are ganged. Blend control: The stereo effect is diluted by this control when it is too much leftright effect. Diluting is done by disbalancing the two channels. It is shown in figure. Blend control potentiometer is set at zero resistance for balance output. For disturbing the balance, this is advanced further to reduce gain of the left channel. Although blending can be done by balance control also, but once set, the balanced control is not disturbed. Quasi stereo switch: When any one channel signal is made to go into both the channels, one can use both channels and their speakers for a monophonic source of signal. This is done by a switch called quasi-stereo switch. [Q] EXPLAIN THE FUNCTION OF FOLLOWING IN HI-FI AMPLIFIER: I)BALANCE CONTROL II)LOUDNESS CONTROL III)BASS AND TREBLE CONTROL IV)QUASI STABLE SWITCH. Controls available on Hi-Fi system are :- Balance control :- Inspite of the two amplifiers being identical, there may be variations in the output of each channel due to variation in the characteristic of transistors and ICs and positioning of loudspeaker and furnishing with respect to the listener. The circuit used to compensate for such variation is called Balance Control When the balance control is moved down, the output of left channel will increase while that of the right one will decrease, and vice versa when moved up. Loudness control :- Sometimes music is at low level of volume (as in case of the background music). At low levels there is considerable loss in bass in reproduction. It is therefore, necessary that there should be substantial boosting of bass at low levels. Boosting at treble may be nominal because loss at high note is quite small. The control which provides desired big boosting at bass and a little boosting at treble is called Loudness control. It boost audio by +12 db at 50Hz and +3 db at 10Khz. Such a volume control also act as loudness control and is known as Contour Control.

Bass and treble control :- It is provided to tailor bass and treble as per personal taste of the listener. Quasi stereo switch :- This causes any one channel signal to go into all channels. This way, one can use all channels and all speakers for a monophonic source of signal [Q] LIST VARIOUS CONTROL OF HI-FI AMPLIFIER AND EXPLAIN ANY ONE Ans:- Various control of Hi-Fi amplifier:- 01M Balance Control: Master Gain Control: Blend Control: Quasi Stereo Switch: Bass & Treble Control: Loudness Control: Balance Control: Two amplifiers of a stereo system, although independent of each other, are built as matched pair to give equal output for the same input. In spite of the two amplifiers being identical, there may be variations in the output of each channel due to variations in the characteristics of transistors & ICs and positioning of loudspeaker & furnishing with respect to the listener. The circuit used is called BALANCE CONTROL. A simple circuit is shown in fig. The balance control is a potentiometer. When it is set in the center, the current through LED1 & LED2 should be identical, if the signals in the left & right channels are equal. In that case both LED will be equally bright. In case of any inequality, the two brightness level will also become unequal. When balance control is moved down, the output of the left channel will increase while that of right one will decrease, and vice-versa when moved up.

Master Gain Control: A master gain control is used for adjusting overall volume without disturbing the balance. This is achieved by using dual concentric shafts, the inner shaft adjusts the balance control & the outer shaft, the overall gain or volume of the amplifier. A typical master gain control circuit is shown above. R1 is adjusted for balancing two channels & then R2 & R3 are adjusted for increasing or decreasing the volume of the channels. R2 & R3 are ganged Blend Control: The stereo effect is diluted by this control when there is too much left-right effect. Diluting is done by misbalancing the two channels. It is shown in fig. above; blend control potentiometer is set at zero resistance for balanced output. For disturbing the balance, this is advanced further to reduce gain of the left channel. Although blending can be done by balance control also, but once set, the balance control is not disturbed. Quasi Stereo Switch: When any one channel signal is made to go into both the channels, one can use both channels & their speakers for monophonic source of signal. This is done by a switch called quasi-stereo switch. Bass & Treble Control: It is provided to tailor bass & treble as per personal taste of listener. Loudness Control: Sometimes music is at low level of volume. At low levels there is considerable loss in bass in reproduction. It is, therefore necessary that there should be substantial boosting of bass at low levels. Boosting at treble may be only nominal because loss at high notes is quite small. The control which provides desired boosting at bass & at treble is called LOUDNESS CONTROL.

It boost audio by +12dB at 50Hz & +3dB at 10 KHz. The loudness control should be used only when sound level is low

GRAPHIC EQUALIZER CONCEPT- CIRCUIT DIAGRAM AND OPERATION. (5-POINT CIRCUIT DIAGRAM) WHAT IS THE GRAPHIC EQUALIZER? WRITE ITS NECESSITY Graphic Equalizer: A graphic equalizer is a high-fidelity audio control that allows the user to see graphically and control individually a number of different frequency bands in a stereophonic system. A typical graphic equalizer consists of several audio filter/amplifiers, each centered at a specific frequency in the audio range. Most graphic equalizers have two identical sets of filter/amplifiers, one for each channel in a stereophonic system. The gain (volume) controls in most graphic equalizers are slide potentiometers that are adjusted by moving a control button up or down. Gain is increased by sliding the button upwards. The slide potentiometers for each channel are placed side-by-side, with the lowest-frequency unit at the left and the highest-frequency unit at the right. In this way, the positions of the buttons appear to follow a graphical curve that represents the gain as a function of frequency for each channel.

[Q] DRAW AND DESCRIBE OPERATION OF GRAPHIC EQUALIZER [1] In five point configuration, the graphic equalizer breaks up an audio input signal into five different bands covering the range of human hearing. Once this is completed, the signal in each band can be adjusted to provide best sound. [1]A : The center frequencies f1, f2, f3, f4 and f5 of the frequency bands of the graphic equalizer are usually fixed at pre-set values. Once these bands are added back together, they are passed through an amplifier which increases the amplitude of the signal to the point where there is enough power that can be heard through an ordinary speaker. [2] The circuit diagram of graphic equalizer is shown in figure. The graphic equalizer consists of an amplifier for every segment of octave band. Such amplifiers are connected in parallel to cover the complete frequency range. [3] The individual gains of these amplifiers are adjusted such that the required frequency response is obtained. Using five amplifiers for five octaves of frequency may be very expensive. Hence amplifier shown in figure is normally used.

The figure shows, there is only one amplifier with multiple feedback paths. There are gain controls and LC tuned circuits in every feedback path. Thus the gain of every octave band is separately adjusted by the corresponding feedback path gain. [4] The center frequency of the octave band is selected by inductors L1, L2, L3, L4 and L5. The gains of individual octave bands are adjusted by potentiometer controls in the feedback path. [5] The combination of individual control setting for various octaves will provide the required frequency response. The peaks at individual octave bands overlap each other. Hence, phasing distortion needs to be avoided. This distortion occurs due to interaction between the overlapping peaks. The slider controls of each octave band can boost or cut the signals from +15 db to 15 db. [Q] STATE AND EXPLAIN THE CONCEPT OF GRAPHIC EQUALIZERS. Graphic equalizer is used to eliminate unwanted peaks in the frequency response of audio systems. In five point configuration, the graphic equalizer breaks up an audio input signal into five different bands covering the range of human hearing. Once this is completed, the signal in each band can be adjusted to provide best sound. The center frequencies f1, f2, f3, f4 and f5 of the frequency bands of the graphic equalizer are usually fixed at pre-set values. Once these bands are added back together, they are passed through an amplifier which increases the amplitude of the signal to the point where there is enough power that can be heard through an ordinary speaker. The graphic equalizer consists of an amplifier for every segment of octave band. Such amplifiers are connected in parallel to cover the complete frequency range. The individual gains of these amplifiers are adjusted such that the required frequency response is obtained. Using five amplifiers for five octaves of frequency may be very expensive.

DOLBY NR RECORDING SYSTEM [Q] EXPLAIN DOLBY-NR RECORDING SYSTEM IN BRIEF. [1] The Name Dolby is synonymous with Noise-Reduction Systems.(Here Nr represents for Noise-reduction). [2] Dr. Ray Dolby introduced a novel system so the name given. [3] In practice, Dolby system is used for pre-emphasis of high notes and deemphasis of low notes by equalization. Dolby system provides 10-15 db improvement in SNR. [4] In his system, weak signals below 40 db, above noise are boosted before recording. All signals which are 40 db or higher, pass through Dolby system direct without change. [5] Signal in the absence of boosting is shown in Fig.(a) After boosting,recording noise remains unchanged but signal is boosted as shown in fig(b).during playback, signal and nose both are reduced as shown in fig(c).

DOLBY A SYSTEM: fig : Reduction of Noise by 10 db in Dolby System Boosting is done in 4 bands 1) Below 80 Hz. 2) 80 Hz to 2999. 3) 3 khz and above. 4).9 khz and above. Bands 3 and 4 overlap above 9 khz, so that the high frequency noise, in the form of hissing sound is considerably reduced. Each frequency band is processed separately by using low pass, band pass and high pass filters and limiters. Dolby B System: [1] Amore simplified system than Dolby A system has been developed for home use and is known as Dolby B System. [2] In Dolby-B system, the encoded signal covers a signal frequency band from 500 Hz upwards. [3]Signal below a specified threshold value in level are pass through a variable filter which increases the level by about 1 db at 500 Hz. Curve shown by dotted line as shown in fig.

The processed signal from the variable filter and the direct input signal are added. The adder gives the Dolby output. For playback, the signal is inverted and fed to the variable filter. The output of the filter gives to the adder to give decoded output. TYPES OF SPEAKER WOOFER, MID-RANGE, TWEETER Compare between woofer, tweeter and squawker on the basis of following parameters. i) Frequency response ii) Cross over network iii) Cost iv) Application

CROSS OVER NETWORK CIRCUIT AND ITS FUNCTION [Q] STATE THE NECESSITY OF CROSS OVER NETWORK. Necessity of Cross over Network [1] A single cone type speaker is not able to provide uniform response and adequate output power over the entire AF range. [2]A loudspeaker mechanism with a heavy and large diameter called woofer can reproduce low frequencies. [3]A loudspeaker with a light and small diameter cone known as tweeter which performs much better at the high frequency audio frequency range. [4] For proper functioning of a dual speaker system, it is necessary that the frequency range to be covered by the combination of speakers should be split into two ranges at a frequency called cross-over frequency. [5] Hence woofers & tweeters are used with cross-over networks, for getting a uniform frequency response over the entire frequency range [Q] DRAW THE CIRCUIT DIAGRAM OF CROSS OVER NETWORK AND EXPLAIN ITS OPERATION IN BRIEF. When a multiway loudspeaker system is used to get flat frequency response for the entire range of audio frequencies, it is essential to have a crossover network to divide the incoming signal into separate frequency ranges for each speaker. In the absence of crossover networks, the speakers will suffer overheating and the output will be distorted when full power at frequencies outside their range is fed to them. The overall efficiency will be much reduced in the absence of crossover networks. Crossover networks make use of the fact that the capacitive reactance decreases with increase in frequency [ X 2 1 FC increase in frequency (X1= 2 f L). A basic crossover network is illustrated in fig. ]and the inductive reactance increases with

The circuit consists of a low-pass LC filter across the woofer and a frequencies (16 Hz to 1000 Hz) to go to the woofer. The series reactance of L and shunt reactance of C for high audio frequencies prevents these frequencies from going to the woofer. The high-pass filter consisting of C in series and L in shunt allows the high audio frequencies to pass to the tweeter and blocks the low frequencies. The response curve of a typical crossover network (of Fig ) is shown in Fig.. It gives an attenuation of 12 db per octave

CIRCUIT DIAGRAM AND FREQUENCY RESPONSE OF THREE-WAY CROSS OVER NETWORKGIVE THE VALUES OF L AND C [Q] DRAW THE THREE WAY CROSS OVER N/W WITH ITS FREQUENCY RESPONSE GRAPH

Where, R1 is the impedance of a loudspeaker in ohms and fc is the crossover frequency in Hz, L is The inductance and C, the capacitance of LC circuits. A commercial three-way divider network is shown in Fig. In this circuit the capacitor C1 of 1μF in series with the tweeter prevents low and mid-frequencies from reaching the tweeter. Similarly, the inductance Lw of 5 mh in series with the woofer prevents high frequencies from reaching the woofer. Inductances Ls1 and Ls2 of 0.5 mh and 5 mh, respectively in the squawker circuit allow only mid-frequencies and prevent too low and too high frequencies from reaching the squawker. A typical divider curve for a three-way network of Fig. is shown in Fig. A single element in filtering gives attenuation of 6 db per octave and double element in filtering gives attenuation of 6 db per octave and double elements give 12 db octave.

PUBLIC ADDRESS SYSTEM: NEED AND USE, BLOCK DIAGRAM, REQUIREMENTS OF A PUBLIC ADDRESS SYSTEM [Q] EXPLAIN DETAIL PUBLIC ADDRESS SYSTEM WITH SCHEMATIC BLOCK DIAGRAM. A public address system (PA system) is an electronic sound amplification and distribution system with a microphone, amplifier and loudspeaker. The intensity of sound decrease with the distance to receive good intensity of sound for comfortable listening. The system fulfils this function is called public address system. Or simply PA system. BLOCK DIAGRAM :- It is an electronics system in which converts into electrical signal by microphone. The electrical audio signals are amplified and fed into another transducer the loudspeaker which converts the audio signals into the sound wave. The block diagram consist :- Microphone Mixer Voltage amplifier Processing circuit Driver amplifier Power amplifier Loud speaker Microphone:-

It is a electrical transducer it pick up sound wave and converts into the electrical variations called audio signal. Mixer :- The output of the microphone is fed into the mixer stage. the mixer stage is to effectively isolate different channels from each other before feeding into the main amplifier. Voltage amplifier :- It further amplifies the output of the mixer. rocessing Circuit :- These circuit have master gain control and the tone controls Driver amplifier :- It is gives voltage amplification to the signal to such an extent that when fed to the next stage the internal resistance of that stage is reduced. Power amplifier :- it is gives desired power amplification to the signal. it is push-pull type of circuit in general. the power amplifier connected to the loudspeaker. Loudspeaker:- It converts electrical audio signal into pressure variation resulting in sound.

Requirements Sounds from the loudspeaker should not reach the microphone, as a acoustic feedback will result in a loud howling sound. Distribution of sound intensity :- Loudness of the sound is contained in low notes and the intelligibility in high notes. As high notes suffer grater attenuation with distance than the low note intelligibility suffers at further distance. Reverberation :- In a reverberation medium the intelligibility is poor. The Pa system should throw additional power in those areas where the direct gets submerged in the echoes. Dynamic Range Limitation:- The amplifier of a good PA system is equipped with a level limiter which keeps the output level constant when the input level exceeds a certain predetermined value. Matching :- Matching of the loudspeaker impedance with the output impedance of the amplifier is necessary for maximum transfer of energy from amplifier to loudspeaker. Grounding :- Chassis and shields of equipments and coaxial cables should be properly earthed through water pipes Audio systems