There are myriad ways to set up a hi-fi system. A true audiophile (“sound lover”) assembles a complex system over a period of time. Here are some basic considerations that can serve as guidelines when choosing system components.
The simplest type of home stereo system is contained in a single box, with an AM/FM radio receiver and a compact disk (CD) player. The speakers are generally external, but the connecting cables are short. The assets of a so-called compact hi-fi system are small size and low cost.
More sophisticated hi-fi systems have separate boxes containing components such as the following:
- An AM tuner
- An FM tuner
- An amplifier or pair of amplifiers
- A CD player
- A computer and its peripherals
The computer is optional, but it facilitates downloading music files or streaming audio from the Internet, creating (“burning”) CDs, and composing and editing electronic music. A satellite radio receiver, a tape player, a turntable, or other nonstandard peripheral may also be included. The individual hardware units in this type of system, known as a component hi-fi system, are interconnected with shielded cables. A component system costs more than a compact system, but the sound quality is better, you get more audio power, you can do more tasks, and you can tailor the system to your preferences. Some hi-fi manufacturers build all their equipment cabinets to a standard width so they can be mounted one above the other in a rack. A so-called rack-mounted hi-fi system saves floor space and makes the system look professional. The rack can be mounted on wheels so the whole system, except for the external speakers, can be rolled from place to place.
A basic stereo hi-fi system.
Above figure is a block diagram of a typical home stereo hi-fi system. The amplifier chassis is grounded to minimize hum and noise, and to minimize susceptibility to interference from external sources. The AM antenna is usually a loopstick built into the cabinet or mounted on the rear panel. The FM antenna can be an indoor type, such as television rabbit ears, or a directional outdoor antenna equipped with lightning protection hardware.
A tuner is a radio receiver capable of receiving signals in the standard AM broadcast band (535 to 1605 kHz) and/or the standard FM broadcast band (88 to 108 MHz). Tuners don’t have built-in amplifiers. A tuner can provide enough power to drive a headset, but an amplifier is necessary to provide sufficient power to a pair of speakers.
Modern hi-fi tuners employ frequency synthesizers and have digital readouts. Most tuners have several memory channels. These are programmable, and allow you to select your favorite stations with a push of a single button, no matter where the stations happen to be in the frequency band. Most tuners also have seek and/or scan modes that allow the radio to automatically search the band for any station strong enough to be received clearly.
In hi-fi, an amplifier delivers medium or high audio power to a set of speakers. There is at least one input, but more often there are three or more: one for a CD player, another for a tuner, and still others for auxiliary devices such as a tape player, turntable, or computer. Input requirements are a few milliwatts; the output can range up to hundreds of watts.
Amplifier prices increase with increasing power output. A simplified hi-fi amplifier forms the basis for a public-address system. Massive amplifiers are used by popular music bands. Some such systems employ vacuum tubes, because tubes offer electrical ruggedness and excellent linearity.
Speakers and Headsets
No amplifier can deliver sound that is better than the speakers will allow. Speakers are rated according to the audio power they can handle. It’s a good idea to purchase speakers that can tolerate at least twice the audio output power that the amplifier can deliver. This will ensure that speaker distortion will not occur during loud, low-frequency sound bursts. It will also prevent physical damage to the speakers that might otherwise result from accidentally over driving them.
Good speakers contain two or three individual units within a single cabinet. The woofer reproduces bass. The midrange speaker handles medium and, sometimes, treble (high) audio frequencies. A tweeter is designed especially for enhanced treble reproduction.
Headsets are rated according to how well they reproduce sound. This is a subjective consideration. Equally expensive headsets can, and often do, exhibit huge differences in the quality of the sound that they put out. Not only that, but people disagree about what constitutes good sound.
In hi-fi stereo sound equipment, the balance control allows adjustment of the relative volumes of the left and right channels.
In a basic hi-fi system, the balance control consists of a single rotatable knob connected to a pair of potentiometers. When the knob is rotated counterclockwise, the left-channel volume increases and the right-channel volume decreases. When the knob is rotated clockwise, the right-channel volume increases and the left-channel volume decreases. In more sophisticated sound systems, the balance is adjusted by means of two independent volume controls, one for the left channel and the other for the right channel.
Proper balance is important in stereo hi-fi. A balance control can compensate for such factors as variations in speaker placement, relative loudness in the channels, and the acoustical characteristics of the room in which the equipment is installed.
Methods of tone control. At A, a single potentiometer/capacitor combination (X) provides treble attenuation only. At B, one potentiometer/ capacitor combination (X) attenuates the treble, and the other (Y) attenuates the bass.
The amplitude versus frequency characteristics of a hi-fi sound system are adjusted by means of a tone control or controls. In its simplest form, a tone control consists of a single knob or slide device. The counterclockwise, lower, or left-hand settings of this control result in strong bass and weak treble audio output. The clockwise, upper, or right hand settings result in weak bass and strong treble. When the control is set to midposition, the audio response of the amplifier is more or less flat; that is, the bass, midrange, and treble are in roughly the same proportions as in the recorded or received signal. Above figure A shows how a single-knob tone control can be incorporated into an audio amplifier. The amplifier is designed so that the treble response is exaggerated. The potentiometer attenuates the treble to a variable extent.
A more versatile tone control has two capacitors and two potentiometers, as shown in above figure B. One combination is in series, and the other is in parallel. The series connected resistance-capacitance (RC) circuit is connected in parallel with the audio output, and it attenuates the treble to a variable extent. The parallel RC circuit is in series with the audio path, and it attenuates the bass to a variable extent. The two potentiometers can be adjusted separately, although there is some interaction.
If you simply connect two or more audio sources to the same input of an amplifier, you can’t expect good results. Different signal sources (such as a computer, a tuner, and a CD player) are likely to have different impedances. When connected together, the impedances appear in parallel. This can cause impedance mismatches for most or all of the sources, as well as at the amplifier input. The result will be degradation of system efficiency and poor overall performance.
Another problem arises from the fact that the signal amplitudes from various sources almost always differ. A microphone produces minuscule audio-frequency currents, whereas some tuners produce enough power to drive a pair of small loudspeakers. Connecting both of these together will cause the microphone signal to be obliterated by the signal from the tuner. In addition, the tuner output audio might damage the microphone.
An audio mixer eliminates all of the problems involved with the connection of multiple devices to a single channel. First, it isolates the inputs from each other, so there is no impedance mismatch or competition among the sources. Second, the gain at each input can be varied independently. This allows adjustment of amplitudes so the signals blend in the desired ratio.
A graphic equalizer consists of an audio splitter, several bandstop filters, several gain controls, and an audio mixer.
A graphic equalizer is a device for adjusting the relative loudness of audio signals at various frequencies. It allows tailoring of the amplitude versus frequency output of hi-fi sound equipment. Equalizers are used in recording studios and by serious hi-fi stereo enthusiasts. There are several independent gain controls, each one affecting a different part of the audible spectrum. The controls are slide potentiometers with calibrated scales. The slides move up and down, or, in some cases, left to right. When the potentiometers are set so that the slides are all at the same level, the audio output or response is flat, meaning that no particular range is amplified or attenuated with respect to the whole AF spectrum. By moving any one of the controls, the user can adjust the gain within a certain frequency range without affecting the gain outside that range. The positions of the controls on the front panel provide an intuitive graph of the output or response curve. Above figure is a block diagram of a hypothetical graphic equalizer with seven gain controls. The input is fed to an audio splitter that breaks the signal into seven paths of equal impedance, and prevents interaction among the circuits. The seven signals are fed to audio attenuators, also called bandstop filters, each filter having its own gain control. (The center frequencies of the attenuators in this example are at 30, 100, 300, and 900 Hz, and 2.5, 7, and 18 kHz. These are not standard frequencies, and are given here only for illustrative purposes.) The slide potentiometers affect the extent to which each filter affects the gain within its frequency range. Finally, the signals pass through an audio mixer, and the composite is sent to the output.
There are several challenges in the design and proper use of graphic equalizers. The filter gain controls must not interact. Judicious choice of filter frequencies and responses is important. The filters must not introduce distortion. The active devices must not generate significant audio noise. Graphic equalizers are not built to handle high power, so they must be placed at low-level points in an audio amplifier chain. In a multichannel circuit such as a stereo sound system, a separate graphic equalizer can be used for each channel.