Hi-fi And Music Amplifiers

Sunday, April 15th, 2018 - Analog Electronics

Hi-fi And Music Amplifiers

We start with one of the most familiar uses of electronics, the amplification of speech and music. The picture below shows a block diagram of a typical set-up used by a group of musicians during a live performance. Each microphone converts sound waves into an electrical voltage, or electrical signal, which represents the sound. The electrical signals are conveyed by cables to an amplifier which boosts, or amplifies, the signals before passing them by cables, or radio, to the loudspeakers. The loudspeakers convert the electrical signals back into sound waves which, ideally, are the same as the original speech or music but at much higher intensities.

Hi-fi And Music Amplifiers Block Diagram

Hi-fi And Music AmplifiersBlock diagram of a purely-analog sound amplification system

In picture above, the form of the sound wave at a microphone is shown in a graph of the sound pressure (p) variation with time (t), called the waveform of the sound. The waveform of the electrical voltage (v) generated by the microphone has an almost identical shape. It is analogous to the sound waveform, so it is called an analog waveform, or analog signal. The microphone waveform has a typical peak voltage of some millivolts.

The typical peak voltages needed to provide enough sound from the loudspeakers in a concert hall are a few volts, or tens of volts. So it is quite obvious that the microphone signal voltages need to be amplified by a factor of about one thousand or more. The amplifier has to have a voltage gain of about one thousand or more.

The Voltage Gain Of Hi-fi And Music Amplifiers

Suppose the signal from the microphone has a peak voltage of 10 mV, and the required output voltage from the amplifier is 20 V. What voltage gain is needed?

Voltage gain = \frac{\text {output voltage}}{\text{input voltage}}=\frac{20V}{10mV}=2000

The amplifier also has to be able to provide a relatively high power output. Loudspeakers have input impedances of one of a few standard values, such as 4 Ω, 8 Ω, or 15 Ω. So a speaker input voltage of a few volts, or tens of volts, causes an input power of several watts. Suppose the input was a sine wave of 20 V rms, and the speaker input impedance appeared purely resistive with a value of 8 Ω. What would be the input power?

Power = \frac{V^{2}}{R}=\frac{400}{8}=50 W

The system of picture above uses analog voltages throughout. The analog voltage from each microphone is increased in voltage by the amplifier, but it is still an analog of the original sound, and it is this boosted analog signal which is fed to the loudspeakers. So, The picture above is a purely analog system.

In a typical hi-fi (high fidelity) system, some of the components use analog signals, and some use digital.

Long-playing (LP) records, spinning at 33\frac{1}{3} rpm, and the 45 rpm EP, were the final development of the earliest recording medium, Edison’s phonograph cylinder, followed by the 78 rpm disc. All these recorded the sound as a side-to-side displacement of a groove in the surface; a visible graph, or analog, of the sound waveform. Records are played back by a pick-up in which a ‘stylus’ (sometimes called a ‘needle’) rides in the groove. Its side-to-side displacement produces an analog output voltage from the pick-up. Inevitably, wear and dust in the groove, and surface scratches, lead to ‘surface noise’, a problem which is largely avoided in CDs.

Audio-tape cassettes were a much later development. As in the earlier reel-to-reel technique, the sound is recorded as a magnetic analog signal on magnetic tape. In recording, the electrical signal from the microphone is fed to a coil called the recording head. This generates a varying magnetic field which induces a pattern of permanent magnetism in the magnetic coating on the tape as it passes over the recording head. On playback, the tape passes over the same head, causing a varying magnetic field which induces a varying voltage in the coil. Again the output is an analog voltage, and again it can suffer from noise. The ‘tape noise’ is caused by the granularity of the magnetic coating.

The CD is digital. The CD player picks up the digital signal from the CD, decodes it, and converts it back to an analog signal representing the original recorded sound. The digital signal on the CD consists of a series of ‘pits’ etched into a layer of the disc just below the surface. These pits represent the noughts and ones of a binary code which, in turn, represents the analog signal from the microphone which picked up the original sound. The tiny pits representing the binary-coded signal can easily be obscured by surface dust, finger-prints and the Hke, which cause errors in the recovered digital signal. However, because of the coding process, a great many errors can be tolerated before the decoded digital signal is corrupted. The output from the CD player is an analog signal with a peak voltage of 1V or so, and with a very-low noise content. Hi-fi and music amplifiers has a very-high signal-to-noise ratio.

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