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| HEATHKIT'S 'Continuing Education Course, EE-3104: Electronic Circuits,' is a fitting conclusion to the three courses I reviewed in Issue 1, 1977, pp. 26. The course is offered by the Heath Co., Benton Harbor MI 49022 for $54.95; with a kit trainer $109.95. All four courses and the trainer are available as ETS-3114 for $224.95. These four courses will not train an electronics engineer technician, despite Heath's claims, but they will provide an extremely thorough grounding in the various aspects of electronic theory, and their practical experiments will allow the student to understand how the various circuits operate. On opening the box one finds a large padded three-ring notebook, a package of pages and dividers, several packages of parts for the experiments, and six flexible records. An understanding of direct current, alternating current, and semiconductor devices is prerequisite for this course, whether you acquire the knowledge from Heath's other courses (EE 3101, 3102, 3103) or elsewhere. INSTRUMENTS NEEDED Even though all the necessary parts are supplied for the various experiments, their use requires access to some sort of breadboard, variable plus and minus DC supplies, 30 volts AC with center tap, a source of sine and square waves, and several potentiometers. These are all included in the Heath Experimenter Trainer (ET 3100) discussed in my earlier review. The experiments also require the use of a high impedance voltmeter and a good oscilloscope (5MHz, triggered sweep, and calibrated vertical amplifier and timebase). In a number of cases scope readings are taken at more than one point in the same circuit; I found Heath's 4550 dual-trace scope helpful since it enabled an immediate check of phase relationships, comparative voltages, etc. However, a good single-trace scope is all that is needed. Course units include Basic Amplifiers, Typical Amplifiers, Operational Amplifiers, Power Supplies, Oscillators, Pulse Circuits, and Modulation. Most of these are of interest to anyone who wants to understand TAA''s circuits. Personally, I was not interested in pulse circuits or modulation and not very interested in oscillators, but they are all important for a thorough understanding of the field. Each unit's format is much the same as in the earlier courses. A 33.333 rpm, 7' record introduces the unit with basic information and a survey. (These contain some material which is important background not discussed in the main body of the text. The ones I heard in earlier courses simply duplicated material in the unit.) After an introduction and list of unit objectives and activities, the body of the text, with adequate illustrations, is broken at appropriate points by programmed review sections and by experiments which give the student hands-on verification of important points of the unit. A unit summary, a unit exam with multiple choice questions, and the exam answers follow. These are important for someone who studies entirely alone, since they give not only the correct answer but the reason for that answer or the correct way to compute the proper value. The reinforcement of information in the record, text, programmed review, experiments, unit summary, and unit exam with answers is important for the success of these courses. This format also provides an interestingly varied program to prevent boredom in those who tire when reading great chunks of text. WHAT'S AN AMPLIFIER? Unit 1 Basic Amplifiers, examines the general background of this most basic of electronic units. After a general introduction to the purpose, varieties, and uses of amplifiers, the three basic types of amplifiers are described with their major characteristics. The different characteristics of common-collector, common base, and common-emitter circuits are the heart of this unit as they are of primary importance for electronic circuits. Separate sections treat the important questions of biasing and coupling. One of Rule-of-Thumb Design Procedure for an Amplifier with a Voltage Divider and Emitter Feedback the few genuine design procedures in the course, 'Rule of Thumb Design Procedures for an Amplifier with a Voltage Divider and Emitter Feedback', takes the student through the design and biasing of a single stage common-emitter amplifier. It is marred by Heath's persistent avoidance of standard formula for computing resistance, capacitance, and frequency, - 1 /~ 2 pi RL. To my mind, in this age of calculators it would be much clearer if the basic formula were always used instead of the various derivative formulae which cloud the primary relationship. Experiment 1 demonstrates the relative thermal stability of the basic biasing circuits while it gives practice in using the Experimenter Trainer. Experiment 2 measures the operation and characteristics of a common-emitter amplifier circuit: AC gain with and without emitter bypassing, and input and output resistances. These are fairly simple experiments, but they also teach several other things. The rest of the course gives only circuit diagrams, not the additional parts layout sketches which were in the first three courses. Presumably we all are grown-up now and can put away such childish things. (Why did I have trouble with RF oscillation at several undesired points?) Moreover, using an oscilloscope to measure values is useful practice at this early stage. UNIT 2 Single transistor circuits are of limited value, and Unit 2 quite properly builds on Unit 1 by giving thorough exposure to typical amplifiers for specific applications. Amplifiers can be divided into categories by the frequency range they handle and the power they can supply. Unit 2 surveys this scene with a fair amount of detail. Direct current, audio, video, radio frequency, and intermediate frequency amplifiers are examined in turn. Various complicating aspects of these circuits are gradually introduced. Thus, the first discussion of a single stage DC amplifier also considers the effects of internal resistance in the source and of the external load resistance. The inability of a single stage to give enough gain leads naturally into multiple-stage circuits. The problems of the various kinds of DC coupling are discussed with appropriate illustrations. Direct coupling, resistance coupling, Zener coupling, complementary coupling, and Darlington and differential coupling are briefly discussed. I wish Heath had given circuit values in the illustrations of these circuits or had provided experiments which gave practice with their use. Experiment 3, DC amplifiers, demonstrates the way the input resistance of a common-emitter stage is affected by the Beta of the transistor, the value of the emitter resistor, and the use of a Darlington circuit. Measuring an RC time constant with an oscilloscope provides useful practice with the Experimenter and the scope. Since there are no more layout pictures, which also showed parts changes, do read circuit values carefully. The results are wrong if, like me, you don't change a .001 uF capacitor to 100pF when the circuit calls for the new value. The discussion of audio amplifiers treats these circuits in only moderate detail. The entire course must cover a lot of ground, but I feel not enough attention was given to these circuits which are basic to almost any home. Alternating voltage amplifiers, their major types of coupling, power amplifiers (single-ended and push-pull, complementary and quasi-complementary), heat sinks, volume control circuits, and tone control circuits receive basic coverage. Experiment 4 demonstrates crossover distortion and its reduction by the use of a stabister and by raising or lowering the power supply voltages. The remainder of the unit gives a similarly general coverage of video amplifiers and RF IF amplifiers. Proofreaders should have corrected p. 2 84, tenth line from the bottom, to read T2 for T1. UNIT 3 Unit 3, Operational Amplifiers, gives a similar wide-ranging but shallow look at the ubiquitous op amp, especially integrated circuit op amps. Basic to an understanding of op amp operation is a grasp of the various differential amplifier configurations. From single input, single output to differential input, differential output, they are given a general discussion supplemented with the use and effect of voltage and current sources, practical differential amps, common mode inputs, and common mode rejection. Experiment 5 provides the chance to play around with and learn the operation of a differential amp made with discrete components. Balanced plus and minus supply voltages simplify circuit operation, but I'd have liked to see the effect of a single-ended voltage on operation. The unit then turns to IC op amps, since they are the most practical and widely used form of what is actually a highly complex circuit. A block diagram and typical schematic of the inside of an op amp help clarify how the IC functions; the major electrical characteristics are briefly discussed, as are the basic op amp families. Experiment 6 measures input and output resistances of an op amp and investigates using a 741 as a comparator. This type of open-loop operation is rare, and the rest of the unit explores the more customary closed-loop circuits, discussing inverting and non-inverting con figurations with feedback operation, the virtual ground, input current, resistor ratios and input impedance follows an examination of bandwidth limitations. The varied gain obtainable in inverting and non-inverting configurations. Experiment 7 measures DC and AC gain and the operation of inverting and non-inverting circuits. Other applications considered include summing amps, active filters, and difference amps. Experiment 8 illustrates low-pass, high pass, and band-pass filters. The simple VCVS filter circuits are not as interesting as those Jung has worked with in TAA, but they require only a single op amp and do give some experience generating your Own response curves. UNIT 4 The next unit gives a thorough treatment of Power Supplies, discussing and comparing the characteristics and operation of half-wave, full-wave, and bridge rectifiers. The effects of capacitors as filters are shown as they influence RMS voltage and ripple. Resistor capacitor and Test Reports inductor capacitor filters are compared in terms of their effectiveness and comparative cost. Voltage multipliers, doublers, triplers, etc., conclude the unit's first section, and Experiment 9 gives practical experience with these aspects of unregulated power supplies. This experiment calls for a 30 volt AC source with centertap which the Design Experimenter provides. The next section considers voltage regulation. Discussion of load regulation with Zener diodes, series regulator, the use of feedback in regulators with discrete components or op amps, and the regulation of current to the load lead to Experiment 10 which investigates these aspects of shunt and series regulation. Next come other aspects of power sup plies: various types of fusing, crowbars, more elaborate shunt regulators, and IC regulators. Finally the unit examines several complex power supply circuits; discussing an oscilloscope power supply, a TV supply, and the supply in the Experimenter in a fashion which helps one understand how the various voltages are obtained and regulated. The last three units, Oscillators, Pulse Circuits, and Modulation, give a similar detailed treatment of their respective Practical Tank Circuit; 12, LC Oscillators; 13, Crystal Oscillators; 14, Wien Bridge Oscillators; i Waveshaping; 16, Rectangular Wave Generators; 17, Ramp Generators; and 18, Amplitude Modulation. I wish I could discuss them in detail, but feel they may be of less interest to TAA readers and that the detail given here for the first four units will suggest the coverage of the rest. CONCLUSIONS As I indicated when I began this review, Heath's claim that these courses will train an electronic engineer or technician is overstated. They lack a pattern of setting design problems and giving examples of correct solutions after the student has tried to solve them. In the same way, insufficient effort is made to deal with the kind of low distortion taken for granted in the current state of audio. However, even if they don't fulfill the hopes they inspire, these courses have a great deal to offer the non-professional, including those of us who weren't trained in this field and are trying to get a basic grasp of electronics. With our own parts collections, the parts from the various courses, the Experimenter Trainer, test equipment, and articles from magazines like The Audio Amateur or other construction publications, we can really mess around with projects and, by changing component values, really learn to roll our own. Heath's courses themselves don't train engineers, but the four courses' basic coverage does give a thorough and systematic grounding in the fundamental principles. I remember from the earliest days TAA readers have asked about such thorough training in electronics. Heath's courses give us a moderately priced way of learning. The opportunity to select for ourselves or borrow the necessary test equipment is an important element in keeping down the cost of this study program. It deserves consideration from anyone trying to understand the principles of our hobby. -Ronald H. Miller HOW TO EVALUATE SPEAKERS by FREDERICK M. GLOECKLER, Jr. Contributing Editor BACK IN 1975 when we published Roger Russell's two part article (Issue 1,2) on speaker evaluation we thought his opinions would elicit a lot of reader comments. We tentatively scheduled a reader's forum for the 3.7 75 issue. Well, the response was a bit underwhelming and we didn't do the forum. Fred Gloeckler's comments were to be part of that interchange-and we think his comments are too laden with wisdom to allow them to languish longer in our files.-The Editor. THE AUDIO AMATEUR often must evaluate speakers in the following situations: 1. Buying speakers for music reproduction in the home; 2. installing speakers for music reproduction in the home; 3. building speaker systems; 4. installing speaker systems for sound reinforcement / production in public areas. Each situation requires somewhat different test methods. The primary inputs to a purchase decision are listening tests and other people's data (manufacturers, dealers, reviewers, and friends). Mr. Russell (Issues 1, 2, 1975) has noted the pitfalls of listening tests and comparisons with live music. I've found I can determine sonic differences better if I listen to recordings before attending a concert, rather than after. You should experience live music in a variety of acoustical settings to get an idea of the wide variety of natural sound. Perhaps the best source material for evaluating speaker accuracy is high quality master tapes of musicians you hear often, such as your church choir. A recording of friends' spoken voices, not your own voice, can be a severe test. Extraneous natural sounds can help in your evaluation. For example: one of my tapes, recorded on a hot day, captures the noise of a fan. One highly-touted speaker converted the noise into a hiss. RECORDINGS Unless you listen to master tapes exclusively, include phonograph records in your evaluation. The various audiophile publications list records with superior sound. I won't list my favorites, but rather note each has its own defects. Listen to several records with different sonic perspectives to get an idea of the system's "naturalness." Don't limit yourself to those recordings which sound ''best'' on your present system. Such a listening evaluation takes a fair amount of time and should be divided into several sessions to accommodate changes in mood, etc. Naturally, your dealer won't be able to handle extensive listening sessions during peak traffic hours. Un fortunately, many, if not most, dealers' listening facilities are less than optimum. Perhaps the best solution is to join an audio club and audition the various members' systems. Listen for a well-defined stereo image which does not wander, with a natural sense of depth. While Mr. Russell advocates wide dispersion, others feel dispersion should be controlled to minimize interaction with the room. Dispersion characteristics should be smooth throughout the listening area and should not vary rapidly with frequency. DOGMA For a given budget, the best sound will be obtained by investing in better speakers and room treatment rather than electronic equalization. If a speaker needs equalization (other than that provided by the designer), it often is deficient. Room equalization often is valid only for a relatively localized listening area and will not eliminate the ringing associated with room resonances. Small, relatively closed areas such as hallways and stairwells often cause severe coloration. Single, sharp handclaps are a good test signal for ringing. Try damping material on various surfaces until the ringing disappears. Use only as much damping as is necessary to control the ringing. The test methods described by Mr. Russell are very useful when trying to determine the best speaker locations. The CBS STR 140 RIAA Pink Noise Acoustical Test Record (1) is another good source of 1/3-octave bands of test noise. Measure the response of any test record before you use it: my Soundcraftsman record was far from flat. If you don't own a good, calibrated, omnidirectional microphone, any microphone with a smooth bass response, located at your listening position, can help you identify major room resonances. When trying different positions, don't forget that elevating speakers a foot or two can often clean up the low end. TEST METHODS Observers have noted that the traditional frequency response and distortion measurements used to design speakers do not fully characterize the sound. New test methods, such as impulse testing coupled with transform techniques (2), provide additional information. Unfortunately the equipment required is expensive, so most amateurs will have to use the traditional tests along with extensive listening. The situation changes for sound reinforcement. Normally, speaker location is constrained for various reasons, room changes become very expensive, and power handling capability, speaker efficiency, and amplifier power are important economic considerations. Mr. Russell's measurement techniques are a good starting point for balancing the system using electronic equalization. The system microphones are used instead of special measurement microphones. Once the system is balanced, additional fine tuning to minimize feedback modes is often required to raise the system gain before feedback to an acceptable level. Notes: 1. Available from Old Colony Sound Lab, Box 243, Peterborough NH 03458 for $15. 2. See Audio's speaker reviews or the Collected Papers of the 50th Audio Engineering Society Convention, 1975. -------- Also see: Audio Aids, by readers Squires, Hill, Caldwell, Thompson, Winn, Hardwick, Moritko, and White
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