Sound System Course: Audio Amplifier

fig.1

Audio Amplifier (Boosters):
see fig.1
An audio amplifier is an electronic amplifier that amplifies 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 and is the final stage in a typical audio playback chain.
While the input signal to an audio amplifier may measure only a few hundred microwatts, its output may be tens, hundreds, or thousands of watts.                                                                                                   

And to understand what is an Audio Power is, we need to go through the following definitions:

1- Gain The gain of an amplifier is the ratio of output to input power or amplitude, and is usually measured in decibels.
amplifiers are often specified in terms of the maximum power gain obtainable, while the voltage gain of audio amplifiers and instrumentation amplifiers will be more often specified (since the amplifier's input impedance will often be much higher than the source impedance, and the load impedance higher than the amplifier's output impedance).
The gain of a good quality full-range audio amplifier will be essentially flat between 20 Hz to about 20 kHz (the range of normal human hearing).

2- Bandwidth The bandwidth of an amplifier is the range of frequencies for which the amplifier gives satisfactory performance.

3- Efficiency Efficiency is a measure of how much of the power source is usefully applied to the amplifier's output.

4- Clipping When the signal drive to the amplifier is increased, the output also increases until a point is reached where some part of the amplifier becomes saturated and cannot produce any more output; this is called clipping, and results in distortion.

5- Noise This is a measure of how much noise is introduced in the amplification process.

6- Output dynamic range It is the range, usually given in dB, between the smallest and largest useful output levels. The lowest useful level is limited by output noise, while the largest is limited most often by distortion. The ratio of these two is quoted as the amplifier dynamic range.

if S = maximal allowed signal power and N = noise power, the dynamic range DR is:
DR = (S + N ) /N

7- Slew rate It is the maximum rate of change of the output, usually quoted in volts per second (or microsecond). Many amplifiers are ultimately slew rate limited.

8- Stability It is an issue in all amplifiers with feedback, whether that feedback is added intentionally or results unintentionally. It is especially an issue when applied over multiple amplifying stages.

9- Negative feedback It occurs when the output of a system acts to oppose changes to the input of the system, with the result that the changes are attenuated. If the overall feedback of the system is negative, then the system will tend to be stable.

10- Distortion It is the alteration of the original shape (or other characteristic) of sound wave. Distortion is usually unwanted, and often many methods are employed to minimize it in practice.
Audio Amplifier Applications: 
  • Public address systems.
  • Theatrical and concert sound reinforcement.
  • Domestic sound systems. 
  • The sound card in a personal computer.

What are the types of Audio Amplifiers? 
  • Tube Amplifiers.
  • FET Amplifiers.
  • Transistors Amplifiers.
  • Class A, Class AB, Class B Amplifiers.
  • Class D Amplifiers.
  • Stereo, Monoblock, and Multichannel Amplifiers.
1- Tube Amplifiers see fig.2
Tube amplifiers use vacuum tubes to convert the AC signal to a DC current that can power speakers. Combinations of multiple tubes increase the overall gain of the signal. Higher powered tube amps often use a pentode circuit design, while lower powered tube amps use a single ended triode (SET) design. SET amps are very popular with people who have high efficiency speakers (typically horns). 

fig.2

2- FET Amplifiers see fig.3A FET (Field Effect Transistor) is a solid state design amp. Solid state amps run much cooler than tube amps (which create a lot of heat from the output tubes). A MOSFET (Metal Oxide Semiconductor Field Effect Transistor) is probably the most popular solid state amplifier design. MOSFETs can create very high gain amplifiers.
fig.3
3- Transistors Amplifiers see fig.4Bipolar transistors are another type of amplifier, though not as common as tubes or MOSFETS. Bipolar transistors are current devices, while MOSFETS and tubes are voltage devices. 

fig.4

4- Class A, Class AB & Class B Amplifiers see fig.5The class of an amplifier refers to the circuit design. In a Class A design, the amp is always producing its maximum power output, whether it has a source playing or not. Class A amps are very inefficient, but people like them because they have extremely low distortion. Class B amps idle at no current, then turn on when an input source hits them. Class B amps are much more efficient than Class A, but produce more distortion. Class AB is a combination of the two, where the signal drives on circuit in Class A, then when it turns off, the amp switches to the second circuit.
fig.5(Class AB)

5- Class D Amplifiers see fig.6Another circuit that is being use in amps is Class D. Contrary to popular opinion the D does not refer to "digital." Class D amps are very efficient, even more so than Class B, and due to their topology don't need heavy heatsinks, so the amps are more compact and light.
Stereo, Monoblock, and Multichannel.
fig.6

6- Stereo, Monoblock, and Multichannel Amplifiers see fig.7Amplifiers can be created in several configurations. Stereo amplifiers, which can drive a pair of speakers, are probably the most common. Some people prefer monoblock amplifiers, which typically have higher power ratings. Monoblock amplifiers drive a single speaker, so you need a pair of monoblock amps in a stereo setup. Multichannel amplifiers are used in home theater applications and are typically designed to drive five channels, though some can drive seven.

fig.7 (Monoblock Amp.)

Amplifier Application in Public Address SystemsAll amplifiers are designed to deliver their maximum amount of power into a certain number of speakers. This number is indicated (usually located near the speaker jacks) as impedance and is rated in Ohms. The most common amplifier impedance is 4 Ohms. This means that the Amplifier will put out the most amount of power (be loudest) when it has a total of 4 Ohms worth of speakers connected to it. It will put out less power (be quieter) if the total of the speakers calculates to be more than 4 Ohms.

The goal in any P.A. setup is to try and get the speakers to calculate up to, but not less than, the amplifier’s rated impedance. If the speakers add up to less than the amplifier’s rated impedance, the amplifier tries to put out more power than it was designed to do and it overheats and can be damaged. Generally speaking, the more speakers you add to your P.A., the lower the impedance number becomes.

This is why you cannot just add more and more speakers to make everything louder. Fortunately, most modern day amplifiers have built in protection to shut down the amplifier when it gets too hot. This is called load protection and many amplifiers are equipped with this, in fact, they might even be protected right down to zero impedance (also known as a short). Some amplifiers have even been designed to put out lower power when the impedance gets lower so that your system doesn’t shut down. 

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