JLH Memorabilia -- The Past Perfect! :: 6

Annals of a Life in Tech - Projects from the '90s

        The 1990s saw a clutch of theoretical articles from the pen of JLH, truly a reflection of his penchant for sharing his acquired wisdom regarding design approaches and philosophies in an era that was yet to achieve some amount of maturity in that. Compared to the 1980s, the number of projects published were fewer -- quite likely an indication of his advancing years. Not that his ever-sharp faculties were waning, but rather the designing, building and refining of projects would take so much of time and physical effort.

JLH always insisted on the 'hands-on approach', and it was his habit to first build and refine his projects before sharing them with his beloved readers. But there is no room for disappointment as we have a range of interesting projects here. Let us examine them.

Transistor-driven Valve Amplifier 

        Here is a novel approach by the Master, perhaps a first in the history of audio.(Electronics & Wireless World, Aug 1991) These days we see many such valve-solid state combos, especially where designers are trying to impart a "valve sound" to a solid-state design. But quite likely this approach that utilized the inherent strength of each topology was probably a novel design in that era.

        Having grown up with valves (Remember, his 'first love' was the Williamson valve amplifier that he had built with his own hands, and his Classic 1969 amplifier was the result of his attempts to approach the famed Wiliamson amplifier's realistic sound with a solid-state design !), he knew its strengths and weaknesses well. Here he has set out to marry the excellent characteristics of the class-A solid-state circuitry to the solid performance of the vacuum tube.

        It is a 'short and sweet' project, but I am more than inclined to feel that editorial pressures on space had trimmed the JLH article into just two pages of the most essential stuff. Readers familiar with the style of JLH knows how he takes the reader along on a 'conducted tour' of the circuitry, and here, sadly, we are missing that expert touch from the Master, if I am any judge. But that in no way detracts from our enjoyment of the novel design.

https://drive.google.com/file/d/1hUoF07wbo5tPYx50-2P1lNekCF_bUJ3u/view?usp=sharing

IGBT Audio Amplifier

        The May 1992 issue of the EWW magazine saw another novel design of the period -- an amplifier employing the latest IGBT devices. There is another novelty here for us to notice and be proud of. These days many self-styled 'designers' take application notes of semiconductor companies, make some very minor changes here and there (mostly following the recommendations in the application note itself!), and publish it as their own. Here what we see is Toshiba, the manufacturer of the IGBT, is presenting the circuits designed by JLH for them in the application note !!

        Too bad / sad it is only a single page of the application note. Had it been a full-fledged article from the pen of JLH, we would have been privy to a more in-depth exploration of the IGBT as an audio device. Anyway, we are glad that this serves as an indicator of his stature as a designer of note.

https://drive.google.com/file/d/1FFzWaP0heZJssHGtCzQfvAHY5ONOQmyn/view?usp=sharing

Perfecting Audio with Super-components

        Audiophilia all along had this preoccupation with "gold standard components" -- exotic and exorbitantly expensive super components, cables, and even solder (!) that many audiophiles believed improved the sound of their hi-fi gear. Though such claims belonged more to 'snake oil' territory, still some subjective evaluations of such phenomena had the support of measurable minute differences.

        As we know JLH was primarily a music enthusiast who undertook the design and building of audio equipment that met his strict engineering standards, while at the same time satisfying his critical ears. In fact he happens to be one of the very few designers and engineers who held that a careful balance should be struck between objective engineering design and subjective evaluations of 'quality' or fidelity. The careful reader will note that it was with the rising popularity of the 'hi-fi press' in the modern times that the opinions of the subjectivity brigade started getting high traction among audio aficionados.

        JLH here (ETI, June, 1991) treads the minefield with caution, but with expertise and ease, and patiently clarifies both sides of the argument regarding various such related issues. Any self-respecting audiophile should think that he will not fall an easy prey to the 'audio-fi hype mill' once he has been guided by JLH. A must-read even today.

https://drive.google.com/file/d/1SHq8paJOD7FEBPAbbdkkzGPWnqbsodOx/view?usp=sharing

JFETs and Low Noise Systems

        Moving to July 1992, we see the ETI magazine carrying a detailed article on the current favourite of the designers, the JFET and how it could contribute to low noise stages in audio amplification. In his typical style that the reader is quite familiar with by now, JLH goes into a detailed examination of the characteristics of JFETs and their application in audio circuits.

        As expected, the Master follows this up with a practical design too! JLH rightly predicts that the future of audio amplifiers lies with FETs of one kind or another -- a statement that is borne out by the developments and choices of the following decades.

https://drive.google.com/file/d/1N2KNdJNN6aU8hB-iYkGuuVA0gb17RQ9f/view?usp=sharing

A Modern Integrated Amplifier

        After a long gap, the EWW, June, 1993, issue presented an integrated amplifier that in a way launched the 'modern' trend of the BJT+MOSFET combo. Thankfully this is a 'full-length', detailed project and the reader has enough and more material to introduce him to the new topology. The design is the quintessential marriage of BJT and MOSFET technology, which as we know has garnered much acceptance today. Again, with the carefully designed power supply, JLH reminds the builder that attention to the power supply is what often determines the performance/quality of the amplifier.

        The design, as could be expected, easily raised the bar of performance in comparison with commercial offerings of the period. The minimalist approach of the Master ensured that the subjective sound quality was second to none, and also made it a comparatively easy  and a rewarding project for the experienced DIYer. Come to think of it, when we compare the circuitry and performance of this amplifier with many current offerings of the day, it is heartening to note that the JLH design is more than capable of holding its own despite the passage of more than a quarter century!

https://drive.google.com/file/d/1BqFXKPMEkijXkQJi9oo1ZJdG9daa4Gvs/view?usp=sharing

Low Distortion Hi-fi Attenuator

        As a designer, JLH walked far ahead of most others. The Hi-fi Attenuator design is one such, as it was a concept at the 'cutting edge' of audio design approaches. Designers only much later were accepting the fact that something as simple as attenuating the audio signal (volume control, no less!) would compromise its fidelity unless the methods adopted were truly fool proof.

        The EWW, April 1995, article broke novel ground with a detailed discussion of a low distortion, high fidelity audio attenuator using DC voltage control of audio level. JLH presents an original approach employing a high frequency switching system. Suffice to say, his circuitry took distortion to below measurable limits. Do check it out to gain some insights into the technique.

https://drive.google.com/file/d/1fk_Y7VTQtjZwZxd-9OjoA5EpBKa3_PYO/view?usp=sharing

A Case for the Expert Witness

        The tug-of-war between the BJT and the MOSFET is finally before the Bench and JLH, the 'expert witness' testifies. (EWW, Aug 1995) The audio press at the time was rife with claims and counter-claims regarding the superiority of one or the other, with respected designers spiritedly  taking sides.

        The trigger for JLH's response (through this article) was the claim made by D. Self in favour of the BJTs being more linear in output stages. The Self article is linked to here:
( https://drive.google.com/file/d/1_hLbRNWuwDqkCEHswPXAnEmkj7xU5ilM/view?usp=sharing ), which makes for interesting reading, but it is probably best that the reader tackles that before going to the JLH article.

        JLH presents his foray into detailed study of IGBTs in audio(commissioned by Toshiba, the manufacturer) as ideal devices combining the strengths of both BJTs and MOS devices. It was during those researches that JLH concluded that the MOSFETs were superior to either the BJTs or the IGBTs in audio output stages. A very interesting study for the reader to follow.

https://drive.google.com/file/d/1L5SO3VXuqMWknqudq661QeBdnGnbtRZy/view?usp=sharing

Class-A Power ... again!

        More than a quarter century after the publication of the iconic 'Simple Class-A Amplifier' in 1969, JLH is going for a 'reprise' of his classic design in the September, 1996, issue of the EWW magazine. He has chosen to move with the times, and has elected to use modern components and has upgraded his amplifier to 15W of output power, has gone in for direct speaker coupling, avoiding the one bone of contention that had upset many 'audiophiles', and has designed a better quiescent current stabilizer. Naturally his preference still was for first-rate subjective performance rivalling that of his 'gold standard' Williamson amplifier, and of course, his iconic design, the Simple Class-A amplifier of 1969.

https://drive.google.com/file/d/1-4OTKwH2Lis2SCx23AP7zGXxGo5_KYDT/view?usp=sharing

        [[ This article has been discussed in more detail in the 'Class-A Gateway' posts and the reader is enjoined to refer to that post. Please note that the connection of the AC feedback capacitor C4 is shown erroneously in Fig. 3. (it should go directly to ground, and not the negative rail) The corrected circuit diagram has been appended to the above-mentioned post linked to here:
https://jlhclassicdesigns.blogspot.com/2024/12/the-jlh-gateway-golden-jubilee-tribute-4.html ]]

High Sensitivity Distortion Meter

        As an engineer who set much store by measurements, naturally JLH wanted to have reliable and accurate test and measurement instruments. The state-of-art was evolving at the time, and JLH elected to design and build his own simple and accurate test instruments for measuring and assessing his amplifier designs. EWW, Feb 1998, presented one such high quality / high sensitivity distortion measuring meter.

        As always with the designs of JLH, this too proves to be an excellent theoretical introduction to the topic, concluding with an eminently practical design sketched out and built. A simple and reliable instrument, so typical of the design style of JLH.

https://drive.google.com/file/d/1XUJPCep4K4P5zImSClzw9GBTzoUyR_3I/view?usp=sharing

Gain Stage Investigations

        A designer worth his salt should have an insight into the working of circuits, their strengths and weakness, and how these affect the overall performance of an amplifier. Here ( EWW, July, 1998 ) JLH takes an in-depth look at circuit configurations and their distortion indexes, comparing four common configurations for their contribution to amplifier performance.

        As we all know, the typical solid-state amplifier consists of a VAS followed by an impedance conversion stage to drive the loudspeaker. It is the overall input-output linearity of this system that matters. JLH here examines the factors that govern these aspects, including the critical relationship between feedback and stability of the circuit. Different gain stage topologies are examined, as also the isolation of PSU artifacts.

        Coming to the power output/impedance conversion stages, JLH examines four typical output configurations, studying in detail their distortion performance. From theory, as is so typical of him, JLH moves to practical examples. For the interested reader, the article serves as an excellent "teach-in" regarding the subtle yet critical aspects of design, and how to fine-tune them to obtain a truly 'blameless' amplifier. Very highly recommended.

https://drive.google.com/file/d/1AjxFgjG3T4p-9TLcDr90jBSno3YDl1LQ/view?usp=sharing

Active Filters

        With his survey of active filter circuits in the October, 1991, EWW magazine, JLH fills a lacuna that has existed for long even in text books. As is usual, he begins from first principles and hand-holds the reader through the more involved levels. It has to be admitted that even designers who comfortable juggling the various factors of filter design have a thing or two to learn from JLH. And with JLH, you can be assured that the theory will be followed by an ample dose of practice. Recommended reading for anybody who knows what filters are, but who still feel their foundations of understanding are not that strong.

https://drive.google.com/file/d/1t2LY45hZOqpwclMF9VU-NPpShWExrjjc/view?usp=sharing

Elements of Radio

        The ETI magazine did a great service to newbies by carrying a series on radio theory and practice by one of the best teachers you could find -- JLH! (ETI, March, April, May, 1990) For today's generation who grew up with the Raspberry Pi and suchlike, the nitty-gritty of simple radio reception is somewhat alien territory, and this series expertly shines a light onto the mechanisms of radio theory and practice.

        A signature series by the Master, and quite enjoyable reading for the youngster as well as the old timer!

Part-1: https://drive.google.com/file/d/1DHYyHtJvNLh3zBw8_4xA9FHuT_SGD_V9/view?usp=sharing

Part-2: https://drive.google.com/file/d/1LkktaIqBeUlViJRp5FZcz4I3IpMo2dkX/view?usp=sharing

Part-3: https://drive.google.com/file/d/1xJ_qRlG2lwwE3MoKIAaHpAwb6-3EKWbQ/view?usp=sharing

FM Radio -- Playing A Better Tune

        From simple radio theory JLH moves onto more involved stuff in the series on FM radio design (EW, March, April, May, 1991), examining the trends and designs from the beginnings of FM radio till the modern era. Every aspect of FM radio design receives his attention and the tech world should thank him for his unmatched explication of the intricacies of FM circuit topologies. An easy read, but a highly rewarding one, if you ask me.

Part-1: https://drive.google.com/file/d/1G3MgqDJYjW2eJxkCCvhzalifxdUOAqQs/view?usp=sharing

Part-2: https://drive.google.com/file/d/1tuNwtt8EVA5tnHf6Qtq7mRwNCUpOuiv5/view?usp=sharing

Part-3: https://drive.google.com/file/d/1BNyHKWpyWPiolV9KpjqSPcTosOLnZBVI/view?usp=sharing

Low Distortion Audio Oscillator

        One of the basic tools of the audio designer is the low distortion audio oscillator that enables him to evaluate the performance of his amplifier designs. As we know by now, JLH had this habit of designing and DIY-ing pieces of test equipment when he thought a simple design would serve his need for accuracy and reliability. Here is one such design (ETI, January, 1996) from his pen, and as expected, he takes the reader with him from first principles and accepted practices, through solutions to vexing issues encountered, to a wonderfully simple and accurate practical design -- and that too with a PCB design, which the reader could DIY easily and add to his home lab without breaking the bank!

        A must-have addition to every serious hobbyist's work bench!

https://drive.google.com/file/d/1omsp7kV5DCY7nsCnGsaEKMoreQVQS1MP/view?usp=sharing

Squarer Circuit - An Add-on for the Audio Oscillator

        ETI magazine for October, 1996, carried an add-on for the low distortion audio oscillator JLH had designed -- a much-needed 'squarer' circuit that would make it easy to obtain good quality square waves for testing of amplifiers. It is not particularly easy to derive 'pure' overshoot-free square waves that can serve as a testing standard to judge amplifier performance.

        Here JLH presents an excellent adjunct that would put out 'perfect' square waves derived from the sine wave oscillator described above. Yes, we are reading the Master, and no surprises, we have an excellent practical circuit and a well-laid out PCB to complete the project -- for the DIYer, the theory class is the icing on the cake, so to speak !

https://drive.google.com/file/d/1ehO66VQdBy3Jii_br6cPiavX4DiBOceQ/view?usp=sharing

Audio Dynamic Range Processor

        Dynamics is what gives music its 'life', and the ability of music systems to preserve this often determines their fidelity quotient. As we know all 'real world systems' have their imperfections and compromises, and an absolutely perfect dynamic range cannot be expected from any system, however 'hi' the 'fi' is. Still, ways to overcome the dynamic limitations of the audio chain has all along  tantalized designers.

        Here (EWW, November, 1995) JLH examines this issue that is as old as audio and proposes a novel "chopped attenuator" that expands and compresses by about 25 dB -- of course, with very low harmonic distortion. An excellent article if you wish to update yourself regarding the interplay of noise and signal in the audio chain, and how best to improve your 'fi'.

        Needless to say, JLH is no mere theoretician-- there is a simple, low-THD dynamic range processor design that offers a high standard of performance waiting for your DIY skills there!

https://drive.google.com/file/d/1PiayhjkwGFjsP6gV8dJEOPC7Xs0ztI8e/view?usp=sharing

Power Supplies

        Quite often power supplies are seen as 'necessary evils' by designers, and are often cobbled up at the last moment. But as audio enthusiasts, we know how often JLH has stressed the importance of the power supply if you wish to raise the bar of fidelity. Here (ETI, March, 1994) JLH takes up a general discussion of power supplies, beginning with battery supplies and progressing to mains powered supplies, and series and shunt regulators and their pluses and minuses. An excellent shunt regulator circuit is also presented as a practical 'buildable' example. Surely the hobbyist will benefit from the insights offered by the Master.

Part-1: https://drive.google.com/file/d/1Kf31bRKPJjB0F4QD2wQP5armsj23cutu/view?usp=sharing

Part-2: https://drive.google.com/file/d/1GwiFEeGevTkTlIyxY1LpbwNZsoAVaHv7/view?usp=sharing

        That more or less sums up a detailed survey of the 1990s, though I must admit many more articles remain in the corpus of the life's work of JLH.

        It is hoped that though it may not directly impact the interests of the audio DIYer and enthusiast, these other articles too deserve to be preserved for posterity. Certainly all efforts will be made to update these posts with such material, which will receive much impetus with the active participation and help of my readers who might have access to archival copies of such material from the pen of JLH.

        On that note, let me wish you happy, enjoyable reading !

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