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By Martin Clifford
CHAOS IS A FRIGHTENING word for it conveys a sense of anarchy, terrifying since it saturates us with a sense of impotence, of helplessness. It is fortunate, then. that we are unable to see the madhouse of radio signals surrounding us, zipping through and about us, in all directions, with varying amplitudes and a constantly changing multiplicity of shapes. To compound the confusion, these waves have a fantastic number of frequencies, carrying sound signals with them. And from this polyglot assortment we are supposed to be able to select a single signal, render it completely free of any influence by any other, amplify it, and then transduce or change it into the more familiar form of sound energy. Obviously an impossible job, one that cannot be done, but which is done all the time with precision, with ease, and with superb attention to detail.
The device that reaches out to pick the one desired signal from this seething electronic cauldron is the tuner, similar to, but not quite as completely equipped as a receiver. It is a most important high fidelity component, now refined and improved by electronic manufacturers to a point where today's in-home use tuner would have been a proud laboratory possession just a decade or so ago. The tuner, precision device that it may be, is just one of the group of components that, taken together, form a complete hi-fi system.
We start with the antenna (Fig. 1), a sort of electronic probe, preferably stuck up as high into the sky as we can manage, somewhat like inserting a naked finger into a hot broth, for subsequent licking and sampling purposes. Hurtling through the antenna with about the speed of light, all of the radio waves (and other types of waves as well) that pass through it, induce voltages across its length. Some of these signals are so weak that they perish on their way down the transmission line, the pair of conductors connecting the antenna and the input of the tuner or receiver. Others, more robust, fight for possession. Arriving at the input, some of the signals reverse their direction of travel, go back up the transmission line and are radiated by the antenna out into space. But enough signals remain at the entry portal, the input of the tuner or receiver, to set up an electronic donny-brook (Fig. 2).
The Tuner Input
Not all the signals at the input are of equal strength. Buried in this electronic morass could be one signal, weak, attenuated, yet wanted. Whether your tuner or receiver will be able to rescue this signal from oblivion, hoist it from the clamoring and quite likely stronger other signals, will depend on the sensitivity of the tuner. If the tuner (or receiver) isn't sufficiently sensitive to the plight of this signal, if the tuner is unaware of the presence of the signal, then send not to ask for whom the bells toll. The tuner input becomes the terminal point for that signal. Requiescat in pace.
If the tuner is sufficiently sensitive to that weak signal, it must now rescue it, separate it from all the other, more brutally strong signals. The extent to which the tuner is able to do this is known as its selectivity.
All is not well yet, for the signal, recognized among all the other signals by the sensitivity of the tuner, and chosen from among all the others by the tuner's selectivity, must now be fed and pampered. Before it reaches the speaker it will have been amplified some ten million times, somewhat akin to starting a business career with one dollar and retiring as a multi-millionaire. The tuner, though, accomplishes this in a fraction of a second.
At birth, the signal weighs in at a few millionths of one volt. Divide a volt into a million parts, pick two or three of these parts, and you have some idea of possible signal strength. Not enough there to shock a mosquito. And yet this may be the signal that is supposed to drive two to four speakers with enough volume to shake an auditorium. We have a long way to go.
Generally, tuners have a rated IHF sensitivity of around 3 microvolts, although some are 2 microvolts or a bit less. This has led to the misconception that tuners having such excellent sensitivity can respond to 2 or 3 microvolt signals.
They can, but only to be all but drowned out by the noise level. No rose without its thorn; no signal without its noise. For mono reception the signal should be several times larger that the sensitivity figure, and for stereo a safe number would be at least ten times as large. For most urban areas, signal strengths are generally measured in terms of hundreds of microvolts. However, ultra-sensitivity is essential in fringe areas or for those who tune in distant FM stations as a sport.
The R.F. Amplifier
The first step is to feed the signal electronically intravenously. This is done by an amplifier known as a radio frequency of r.f. amplifier. This section of the tuner (Fig. 3) does two jobs. It helps select the one desired signal, thereby contributing to the overall selectivity of the tuner, and it amplifies the signal. However, it is just a single tuned stage and so it cannot possibly pick out just one signal from among all the others, but at least it narrows the field. At the output of the r.f. amplifier, then, we have a variety of signals (but not as many as at the input) refreshed and invigorated, ready for their next adventure.
Not all tuners or receivers use a tuned front-end. Less expensive units have r.f. amplifiers that amplify only, but do not select.
Capture ratio, a term frequently appearing in manufacturers' spec sheets, is the ability of the tuner or receiver to respond only to a desired signal while rejecting all others.
Capture ratios are expressed in dB and the lower the better.
For quality tuners or receivers, the capture ratio will be about 2 dB, or even less.
Better grade tuners and receivers use a field-effect transistor (abbreviated as FET) as the first amplifying device to which the signals are introduced. FETs achieve current control by an electrode voltage, and so in this respect they are more like tubes than other transistors.
There are two basic types of FETs, the junction type and the insulated-gate field-effect transistor, a metal oxide semiconductor, hence referred to as a MOS-FET. In the FET, the control electrode (comparable to a tube grid) is called the gate, while the output electrode is 'the drain. The input is the source. When a voltage is connected between the source and drain, current flows through the FET from the source to the drain. A bias voltage is put between the source and gate, controlling current flow. When the bias is varied, current flow will change in step. As in a tube, variations are produced by the signal. The gate does not draw current, and so the input impedance is high.
The advantage of the FET is that it doesn't yield as readily as ordinary transistors to strong signals, hence does not be come overloaded, and is less susceptible to cross modulation.
This preliminary treatment of signals (for quite a number are still grouped together) could be repeated by additional tuned amplifiers. This repetitive signal weeding out and amplifying process was used in the early days of radio, circa 1925, but it was never really satisfactory, the end result being a cacophony of two or more signals out of the speaker.
Greater selectivity and better amplification were achieved with the introduction of the superheterodyne circuit, now used in all hi-fi tuners and receivers. The first step in the superheterodyne is the use of a mixer-oscillator. The purpose of the mixer oscillator is to reduce the frequency of all signals, ranging from one end of the tuning dial to the other, to a single frequency. This is done by using an oscillator, known as a local oscillator (just an a.c. signal generator) to heterodyne or beat with the incoming signal. The result of this beating or mixing action is the production of a signal at a different frequency.
The frequency of the local oscillator is always larger than that of the incoming signal by a fixed amount. Assume the incoming signal is 100 mHz. If the local oscillator is 110.7 mHz, then the difference between these two signals is 110.7 100 = 10.7 mHz. If the receiver is now tuned to another station, possibly at 105 mHz, the frequency of the local oscillator is automatically increased to 115.7 mHz. The difference signal becomes 115.7 105 = 10.7 mHz. And so, no matter what signal is tuned in across the entire FM band, the result is a fixed frequency signal referred to as the intermediate frequency and abbreviated as if.
The local oscillator is simply an a.c. generator whose frequency is changed every time you adjust the tuning dial of your receiver or tuner. The signal out of the r.f. amplifier and that from the local oscillator are both fed into the mixer circuit, appropriately named since it beats or mixes the two signals. The resultant signal, the intermediate frequency, is now fed into the i.f. amplifier section.
The I.F. Amplifier
The i.f. amplifier (Fig. 6) consists of a number of fixed tuned amplifier circuits, ordinarily adjusted to 10.7 mHz.
Using a large number of stages, three or more, the signal undergoes increasing selection and amplification.
Budget priced FM sets use double-tuned i.f. transformers, but because of the high intermediate frequency, just a small shift in the adjustable components, generally a variable iron core, can cause detuning. This is more of a nuisance than a serious problem since realignment is then necessary to keep the i.f. transformers working at optimum performance.
Better grade tuners and receivers use fixed-tuned circuits with ceramic or quartz filters or possibly coil-capacitor filters of special design.
The i.f. sections of better grade tuners and receivers use integrated circuits, popularly known as IC's. An integrated circuit is a complete circuit, a sort of block arrangement that does the job of semiconductors, resistors, capacitors, coils, etc. It is a circuit in miniature. By eliminating wiring it avoids the troubles caused by poor connections, plus the unhappy tendency of wires to act as antennas.
The i.f. signal is really a two part affair. A portion of it consists of a wave that has behaved as a transportation medium, much as a truck is used to carry freight. The freight in the receiver is the audio signal. With selectivity accomplished, with amplification partially achieved, the two signals, the carrier and its audio load, are ready to be separated. This is done by a demodulator (Fig. 7) or detector circuit. Here the transport signal is discarded and the audio signal, still somewhat weak, emerges, ready for further treatment.
The Audio Amplifier
Following the demodulator, the audio signal travels through one or more amplifiers whose function is to increase the voltage strength of the signal, hence are referred to as voltage amplifiers. In the case of a tuner, the demodulator might be the final stage or it could have a single voltage audio amplifier. The output of the tuner would then be fed into still another component, a group of voltage amplifiers housed in a single cabinet and known collectively as a preamplifier or pre-amp. In the receiver there is no separate pre-amp. Instead, the voltage amplifier is contained within the receiver.
Voltage is not power. Power requires both voltage and current. You have voltage present at every outlet in your home.
There is no charge for this until you connect some appliance to the outlet, using both voltage and current-that is, power.
Now you begin to pay. Similarly, a speaker requires power in the same sense that an electrical appliance demands it. And so, following the voltage amplifier section is a power amplifier circuit, a circuit that will still amplify the signal, but which is capable of delivering not just an audio voltage, but audio current as well. This is the payoff, for the audio voltages and audio currents supplied to the voice coil of the speaker, produce that long awaited result, sound.
In a receiver, the power amplifier is part of the unit. All that is needed here is to connect the receiver to one or more speakers (Fig. 8). In a separate component system, the tuner is wired to a pre-amp, but the pre-amp, in turn, is joined to still another component, the power amplifier, and that, of course, is attached to the speakers.
There are some variations, of course. You can buy a receiver, in which case you need not concern yourself about pre-amps or power-amps. It is also more economical to buy a receiver and it involves less interconnecting wiring cable problems. But there are also some disadvantages. Most power amps in receivers do not supply high power. 75 watts is rather high for a receiver. Using a tuner and a separate pre-amp, you can select a power amp capable of delivering several hundred watts. Further, the pre-amp, or possibly a pre-amp/power-amp combination, makes the hi-fi system more flexible, since it usually has switched and unswitched outlets (hi-fi systems never seem to have enough available outlets) plus a variety of controls: bass, treble, balance, volume, stereo vs. mono, selector for choosing different inputs (such as phono, tuner, mic, cassette deck, tape deck, etc.), tape monitor, loudness, filter.
The pre-amp (Fig. 9) may be separate or combined with the power amp. The combined pre-amp power-amp can be an integrated unit, with separate outputs from the pre-amp and power-amp, permitting you to substitute a different pre amp or a different power-amp, for the one in the combined unit (Fig. 10). For maximum flexibility in a hi-fi system, separate components are preferable. While this setup costs more, it allows updating individual components when needed.
Power output can be specified in a number of different, often confusing, ways. Continuous power output, also known as root-mean-square or rms power, doesn't really mean what it implies . . . that the power output is kept going ad infinitum. It is just the maximum power the amplifier can deliver for 30 seconds minimum. The amount of distortion during this maximum power delivery time should be indicated, otherwise the power output specification is meaningless. Distortion means both harmonic and intermodulation types.
(A more complete discussion of power ratings can be found in the article "Amplifiers-A Look at Requirements and Specifications," AUDIO, April, 1971.)
So far the emphasis has been on FM or frequency modulation reception. The choice between AM (amplitude modulation) and FM is made by the tuner or receiver. Switching to AM from FM means switching in a whole new set of parts in the front-end, a different i.f., and a new detector. However, the preamp and the power-amp can be utilized for both AM and FM. In effect, going from FM to AM, or switching back to FM, means switching in a different tuner.
Some sets make radio parts do double duty; others have completely separate parts to be used when the switch is made.
Growth of The Hi-Fi System
The most elementary. the most basic hi-fi system consists of a receiver, usually stereo, and a pair of speakers. That is all it takes. A more elaborate system would be one using separate components, a tuner, followed by a pre-amp and then a power amp. These two systems are identical. What is different is the approach.
In the case of the component system, additional units are easily included (Fig. 11). These could comprise a record player, or automatic record changer, a reel-to-reel tape deck, a cassette tape deck, and one or more microphones. For quadraphonic sound (Fig. 12) a decoder-amplifier could be put in as an "add-on" unit, or the decoder could he an integral part of the receiver or tuner. For quad sound, the total speaker requirement now becomes four. No one, though, except the highly affluent, start with a system in toto. Each extra component can be bought separately, when finances permit, but allowing enjoyment of a hi-fi system without all the extras.
( Audio magazine, Dec. 1972)
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