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 tape-

mic 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 left-

right 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 de-

emphasis 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 [FC

X2

1]and the inductive reactance increases with

increase in frequency

(X1= 2 f L). A basic crossover network is illustrated in fig.

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