Commercial Audio Amplifier
Index:
Introduction
about Audio Amplifiers
Frequency
Response of an Amplifier
New
Distortion Measurement Method for Audio Amplifiers
Configuration
of the Measuring Circuit
Spectral
Response under Conventional THD Test
Some
Website-Links Related to the Subject
Electronic amplifiers are used mainly to increase the voltage, current, or power of a signal. A linear amplifier provides signal amplification with little or no distortion, so that the output is proportional to the input. A nonlinear amplifier may produce a considerable change in the waveform of the signal. Linear amplifiers are used for audio and video signals, whereas nonlinear amplifiers find use in oscillators, power electronics, modulators, mixers, logic circuits, and other applications where an amplitude cutoff is desired. Although vacuum tubes played a major role in amplifiers in the past, today either discrete transistor circuits or integrated circuits are mostly used.
Introduction about Audio
Amplifiers:
Audio amplifiers, such as are found in radios, television sets, citizens band (CB) radios, and cassette recorders, are generally operated at frequencies below 20 kilohertz (1 kHz = 1000 cycles/sec). They amplify the electrical signal, which then is converted to sound in a loudspeaker. Operational amplifiers (op-amps), built with integrated circuits and consisting of DC-coupled, multistage, linear amplifiers are popular for audio amplifiers.
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Frequency Response of an
Amplifier:
An audio
amplifier has been described as an amplifier with a frequency response from 15
Hz to 20 kHz. The
frequency response of an amplifier can be shown graphically with a frequency
response curve. Figure 1 is the ideal frequency response curve for an audio
amplifier.
Figure 1
This curve is practically "flat"
from 15 Hz to 20 kHz.
This means that the gain of the amplifier is equal between 15 Hz and 20 kHz.
Above 20 kHz or below
15 Hz the gain decreases or "drops off" quite rapidly. The frequency response of
an amplifier is
determined by the components in the circuit.
The difference
between an audio amplifier and other amplifiers is the frequency response of the
amplifier. The transistor itself will respond quite well to the audio
frequency range. No special components are needed to extend or modify the frequency
response.
Single-Stage Audio
Amplifiers:
The first
single-stage audio amplifier is shown in figure 2.
Figure 2
This circuit is
a class A, common-emitter, RC-coupled, transistor, audio amplifier. C1
is a coupling capacitor that couples the input signal to the base of Q1. R1 is used to develop the
input signal and provide bias for the base of Q1. R2 is used to bias the emitter and
provide temperature stability for Q1. C2 is used to provide decoupling
(positive feedback)
of the signal that would be developed by R2. R3 is the collector load for Q1 and
develops the output
signal. C3 is a coupling capacitor that couples the output signal to the next
stage. VCC represents the collector-supply voltage. Since the
transistor is a common-emitter configuration, it provides voltage amplification. The input and
output signals are 180º out of phase. The input and output impedance
are both
medium.
The second
single-stage audio amplifier is shown in figure 3.
Figure 3
This circuit is a
class A, common-source, RC-coupled, FET, audio amplifier. C1 is a
coupling capacitor, which couples the input signal to the gate of Q1. R1 is used to develop the input
signal for the gate of Q1. R2 is used to bias the source of Q1. C2 is used to decouple the
signal developed by R2 (and keep it from affecting the source of
Q1). R3 is the drain
load for Q1 and develops the output signal. C3 couples the output signal to the
next stage.
VDD is the supply voltage for the drain of
Q1. Since this is a common-source configuration, the input and output signals are 180º out of
phase.
New Distortion Measurement Method for
Audio Amplifiers:
After having performed a large number of spectral measurements based on single-tone spectral analysis we have decided to try more complex input signals. The reason is that there is not much correlation between single-tone spectral analysis result and listening preferences during listening tests. This is especially truth for low distortion components (THD of 0.001% order) where audible difference still exists.
Three amplifiers have been tested by now, the class A 20W amp, the error correction amp and Sinclair Z-30 amp. The amplitude on the output of the amplifiers was 14Vp-p (5Vrms), 8-Ohm load. The error correction amp used AD797 circuit in the voltage amplifying stage.
Configuration of the Measuring
Circuit:
Spectral Response under Conventional
THD Test:
The test of these amplifiers (1kHz sine) can be seen clearly with the help of images. To see those images please visit the following link:
http://web.telecom.cz/macura/fmanalysis/freqdist.html
Some Website-Links Related to the
Subject:
For any more information about the subject of amplifiers, and audio amplifiers specifically, you can visit the following suggested links:
http://www.tpub.com/content/neets/14180/index.htm
http://www.electronics-lab.com/
http://www.pha.inecnet.cz/macura/audiopage.html
http://web.telecom.cz/macura/fmanalysis/freqdist.html
http://members.tripod.com/~gabevee/