JLH Memorabilia -- The Past Perfect! :: 2
Annals of a Life in Tech - the 1970s
Here is a peek through the window of time into the distant past when the likes of John Linsley Hood walked the earth, and did his best to give us nuggets of audio finery, and shared his considerable wisdom with us. Through these authentic archives it is hoped that the rich legacy of JLH shall be kept alive for the benefit of the generations to come.
Open Sesame !!
https://drive.google.com/file/d/1jDp1FazjjTyQHg5TluSN8E9GgchZMs7I/view?usp=sharing
Modular Pre-amplifier
The year 1969 marked by the publication of the classic amplifier saw another interesting circuit for the DIYer -- the first of the Modular preamplifiers that JLH designed. (The later design with its "Clapham Junction" tone control circuit was to garner far greater accolades.)
The article begins with a fine introduction to Intermodulation Distortion (IMD) in amplifiers which may not be ideally linear. To go with his high quality Class-A amplifier, he has designed a few "modular" pre-amplifier stages of high quality. The builder may mix and match the "building blocks" to arrive at a pre-amp that satisfies his needs or preferences. Inputs for magnetic and ceramic pick-up cartridges, as well as higher level inputs are catered for. The tone control stage is the 'standard' Baxandall type, but with a novelty-- the active device is a JFET which provides lower noise and better linearity. Even the low-pass filter at the output side receives much thought so as to minimize distortion levels. It is subtle touches like this and the attention to minute details that mark the design thinking of JLH.
https://drive.google.com/file/d/1p3pQhhMPciYdILy4Qbmu30NzR8Q3LAwQ/view?usp=sharing
A Postscript published in December of the same year updates both the Simple Class-A Amplifier and the Modular Preamp. The design is embellished with a steep cut low pass filter to cut down on record surface noise etc. Other tips to fine tune the performance of the preamp too are discussed in detail.
https://drive.google.com/file/d/10Z75sN-Vv0t4_oIk58lDdt2PcjhS3c9g/view?usp=sharing
Also, here is a Circuit Idea (WW, March, 1970) from JLH
relating to his preamplifier discussed above:
Combined low-pass and high-pass filter
The circuit employed for magnetic pick-up equalization in my pre-amplifier design (WW, July 1969) can be modified to provide simultaneous low distortion low-pass and high-pass filtering.
The capacitor value given (5% tolerance) can be altered proportionately for other turn-over frequencies. Mid-point gain is 50 and the filter slopes 18 dB/octave. -- JLH
This preamp was the outcome of the requests of his friends and readers for a simple preamp for use with a ceramic pickup and tune and other standard inputs. Of course, the request must have come as the "standard offerings" in the technical Press at the time must have a left a lot to be desired when it came to subjective quality. He goes into details of matching the high output ceamic pickup to the input stage, and also comments on their sensitivity to 'rumble', which is likely to mar the low frequency performance. In order to alleviate this, he goes for a meticulously designed filter section.
Naturally, a cheap JFET was used as the input element to maintain a high input impedance suitable for the ceramic cartridge. In order to cut costs he employs a simple zener stabilized power rail. It is heartening to note how he goes into detail regarding the design of the 'simple' preamplifier -- for him even a simple circuit demanded careful thought and planning and wise compromises, if it was to perform well much above its cost and 'class'. No wonder even the simple preamp had a huge following in those days as the JLH design stood head and shoulders above most others that peppered the pages of tech and DIY journals.
It is wonderful to note how thoughtful JLH is regarding the constructor who builds his simple preamplifier. Though originally designed to cater to a high output 'cheap' ceramic cartridge, the foresees the situation when an upgrade might be made to a higher quality magnetic cartridge (with typically less than 5 mV output) . He offers a magnetic pickup matching preamplifier using one of the latest ICs of the era from Motorola. And one may note that the performance of the humble (!) module follows the RIAA Eq curve within 1 dB!
https://drive.google.com/file/d/1xpdt1ZPZ9xeOryDB-Fr8lbphERtmPJzN/view?usp=sharing
Amplifiers and Classes
JLH here throws light on the different amplifier classes and how they impact distortion figures. The class-B amplifier, despite having a low figure of THD at maximum output could easily exhibit unbearable levels of distortion at typical listening levels of maybe a Watt or so. As we all know, some amps may look pretty impressive on paper, but they suffer when it comes to tonal quality.
An involved discussion is presented regarding the transient instabilities brought on by complex speaker loads. Factors affecting performance are dealt with in detail and JLH proposes solutions, which will be demonstrated with a 15-20 W class-AB amplifier. It is educative to look at his analysis of the issues at hand, and how he transcends those to emerge with a faultless design.
https://drive.google.com/file/d/1ohgrI8CZb5Vqq3G2FaLkkMnWi-nDeVnd/view?usp=sharing
Proof of Concept 15 W AB Amplifier
All the theoretical aspects discussed previously found answers in the 15-20 W amplifier design, which though a class-AB one, approached class-A performance, as boldly stated byJLH in the article headline itself! JLH employed a special topology using complementary emitter followers driven from a voltage source in order to minimize distortion at low output levels. The special arrangement ensured that at low levels the stage would operate in class-A, but with the efficiency of class-B, so that heating was reduced.
The detailed theoretical discussion of the stage topology is a good "design class" for the student. The outcome was that the amplifier excelled in performance in comparison with the "standard" class-AB offerings of the time. Also, it may be noted how the engineer in him favoured evaluation by measurement, but he never ignored listening trials where the measured phenomena were related to observed audible effects. He goes into detailed tips for the constructor and also explains how to calculate power levels, and other aspects of the design.
https://drive.google.com/file/d/18BDvKT9vbNuTFqRkJEsWUwh2N8NSbopK/view?usp=sharing
The Liniac is an excellent example of the forward thinking of JLH. Marrying the nascent IC design approaches and solid state design to realize fantastic results for linear amplification with mundane devices available then was true sleight of hand in those early days. Having perfected the design, he goes on to detail a slew of 'application' circuits for audio.
Even if you cannot understand or appreciate the design fine points, it would be more than useful for the hobbyist to follow the article and build a few modules, which I am sure will offer performance far exceeding the 'standard' circuits usually employed in audio. And at the end of the exercise you will be raising your hat and bowing to the genius of JLH! Come to think of it, how many such unique designs have you seen from other designers?
https://drive.google.com/file/d/1a1UFUyQI-xY27jqBDSVJinRyWMLLqStK/view?usp=sharing
Reference Standard Vinyl !
JLH had favoured the UK magazine Hi-fi News & Record Review with a series of articles on audio and virtually all of them had a good following. Unfortunately for us, it is very difficult to get access to the archives of HFN+RR, and that is a sad loss for us. Here is a rather poor quality copy of one of JLH's great designs. The very title itself is indicative of its high standard of quality, approaching pro levels in following the RIAA Eq curve, and its noise and distortion performance. It may be noted that JLH has chosen to implement his Liniac technology here!
The design has stood the test of time as it is capable of holding its own even in our 'modern' times. It is quite likely that the carefully and knowledgeably designed discrete circuitry might outperform some of the current offerings. The design has some sort of a renewed relevance in our times, what with the resurgence of vinyl today! The more intrepid among DIYers should by all means build one and listen, and perhaps start a new trend (like the Classic 1969 Class-A amp did!) -- which might turn out to be the best tribute you could offer to the Master!
https://drive.google.com/file/d/15i_iiFipdMpwNV33ivoAnhEPpMa6dte3/view?usp=sharing
Low Cost 15W Amplifier
Another design that calls out to be included in this lot is from the Texas Instruments engineers Hardcastle and Lane published in the Wireless World, Oct, 1969, if only as an example of current design thinking. This is more like an application note from TI, and is included here as an illustration of the evolution of audio design.
The authors call it a 'low cost' amplifier using their latest range of silicon devices, DC coupled, with a symmetrical output stage, and that cutting-edge topology of the time, a differential amplifier input stage -- all for the measly sum of 5 UK Pound sterling! The circuit holds not many surprises for us today as it bears much resemblance to modern designs. From that point of view, the designers deserve our kudos, as also for their detailed "hand-holding" of the DIYer. A simple design with respectable performance and specs that could be DIYed today with satisfaction, if I am any judge. Do check it out.
https://drive.google.com/file/d/1TwTmkMrnT7cW5tDZM7Yx_0YlqtaCdIdm/view?usp=sharing
The JLH Cassette Revolution
Before we talk about the JLH foray into the popular Compact Cassette scene, I think it is fitting that we take a bird's eye view of the cassette scene. As probably most of us know, the Compact Cassette was a Philips invention of the 1960s. The 'dictaphone'-standard format soon transformed itself into a music medium, with the coming of the pre-recorded Musicassette. Of course, improvements in mechanical design, advanced playback heads, and improved tape formulations all played a part in this. The 1970s saw the cassette ruling the music world, with most brands releasing hi-fi cassette decks for home use, and names like Akai, Nakamichi, Pioneer, Yamaha, Tandberg, Revox etc etc had machines that approached the quality levels once reserved for reel-to-reel machines.
Another tectonic upheaval that turned the world of audio topsy turvy was the introduction of the Sony Walkman. Sony released models one after the other, each smaller than the previous and with unbelievable audio quality, and some of the later models had even the recording function. This cemented the position of the cassette at the very top of audio playback and enjoyment.
A major factor for its popularity we may note was the availability in the 1970s of high quality deck mechanisms, heads etc for the hobbyist and the resulting enthusiasm in the DIY circles. The 1970s (when yours truly was a University student!) were indeed heady days, and a careful build by a DIYer gave a cassette playback performance second to none! The cassette was king of the entertainment world, and the DIYer had everything going for him in a major way. (This was to continue like that until the CD entered the scene and killed the cassette. But today we witness the cassette rising Phoenix-like from its ashes once again. Well, that is another story.)
This was the background into which JLH introduced his low cost / high quality cassette system for the hobbyist, and coming from JLH, this was likely to be a design that was very much out of the ordinary. (Wireless World, May, June, August, 1976.)
In typical JLH style, a simple, yet high quality and high-tech circuitry was designed for the DIYer's enjoyment. It is interesting to note that the design was conceived primarily to meet his own need for quality entertainment while on vacation to remote areas. (This personal aspect is true of most of JLH's designs, as it was in the case of the Classic Class-A Amplifier.) The circuitry is described and discussed in detail and it sure is "total package" for the discerning hobbyist. The Addendum (Letters to the Editor) presents the reply JLH gives to a reader who questioned his selection of certain design criteria, which makes for a very entertaining read.
The design assumes significance in the light of the present-day cassette revival, and I have a feeling that a DIY build will easily beat some the of current designs that are being pushed into the market by Chinese companies wishing to cash in on the cassette revival.
1. JLH Cassette Deck - Wireless World, Part-1 : May, Part-2: June, Part-3: Aug, 1976:
https://drive.google.com/file/d/1_5rMarVL-D2b3dmekfRJMXiyCDbXgKgE/view?usp=sharing
2, Letter to the Editor, WW, Aug, 1976:
https://drive.google.com/file/d/1u7OXeX9K5ViGu9UkBJ4BEUzGciz-Jkgp/view?usp=sharing
Cassette Deck Postscript
The Wireless World magazine published a postscript to the popular Cassette Deck article by JLH in February, 1978. As we could guess, JLH has been using the cassette deck for his personal entertainment and likely he had noticed a few areas which could benefit from a redesign. JLH takes an in-depth look at replay noise and discusses techniques to minimize that. The role of HF bias in determining the noise threshold too receives attention. Step by step, his old design gets various improvements that takes it to another level.
Recommended reading even if you have not built the cassette recorder, as the detailed 'lecture class' is an excellent means to update yourself regarding the fine points of magnetic recording.
https://drive.google.com/file/d/1GZInXsegMNUxMEEMipiHkuPRrM6V-RdW/view?usp=sharing
A lost gem (for us here!) from the 1970s is A Direct-coupled High Quality Stereo Amplifier published in four parts from Nov, 1972, thru Feb, 1973, in the Hi-Fi News & Record Review (HFN+RR) magazine. Unfortunately archives of the articles could not be sourced till now. I am hoping to add them ASAP.
Later, in the May, 1973, issue JLH published 'Afterthoughts and Additions' to the High-Quality DC Coupled Amplifier, which again, has failed our attempts to source a copy.
Next year, in the Apr, 1974, issue of HFN+RR, JLH added 'A Retrospective Look: Practical Construction & Kit Considerations' to the above amplifier design. Try as I might, I could not obtain copies of any of these articles, which would surely have proven to be of deep interest to the JLH fan as it outlined his design approaches, and the practical realization of the design is sure to have topped many commercial designs of the period.
However, a kind reader has shared a detailed (with circuits and excerpts from the original text from HFN+RR) kit build manual released by Powertran Electronics. The document is shared here so that the reader could get some insights into the above articles and also study the circuits designed by JLH. Powertran has dubbed the amplifier 'Linsley Hood 75 Watt Amplifier - De Luxe', a 'slimline' build as was quite popular at the time. I am posting it here for what it is worth.
Do go through and update yourself about the design approaches of JLH, and how he had decided to go for a direct-coupled version, following a lecture by E.J. Jordan, the famous loudspeaker designer, regarding the merits of such a system of amplification. JLH then moves onto a detailed discussion of the topology and how he arrived at the final design. An easy and enlightening read not only for the JLH enthusiast. Here is the link:
https://drive.google.com/file/d/1hmzOMeOe94zi9EhGUfzsl4jfAHBPl9A0/view?usp=sharing
Note: A series of Letters to the Editor and replies by JLH (Wireless World, 1970 issues) throw light onto various aspects of amplifier design. It is hoped that these provide useful insights for DIYers too. For the sake of 'completeness' they are linked to here:
https://drive.google.com/file/d/1KAVoCKXPwkDQKe520QywUA42tsfWRV42/view?usp=sharing
High Quality Headphone Amplifier - HFN&RR, Jan 1979
As we know, the Simple Class-A Amplifier by JLH had caused quite a stir in audio and DIY circles, and in a very short time it had established itself as the 'go to' amplifier where quality was the first criterion. A natural outcome was its use by headphone aficionados. Surely this called for better power supplies with lowered rails and a much reduced quiescent current. Many had reported excellent results with it also.
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| High Quality Headphone Amplifier |
It is quite likely that the demand for a good headphone amplifier with the lowest possible distortion levels also reached JLH's ears. JLH offered a New Year gift to his fans and published the High Quality Headphone Amplifier in the Hi Fi News & Record Review magazine in Jan, 1979, a good ten years after the Classic amplifier saw the light of day. The design was indeed a modified version of the classic 1969 amplifier tailored for superior performance with the higher impedance and lower power demands of the headphone.
Here was a reference standard small amplifier against which other power amplifiers could be easily judged. The measured (miniscule!) distortion was predominantly second harmonic, giving it a 'valve-like sound'. It easily accommodated headphones from 8 Ohms to infinity.
Too bad that I could NOT source the original article, try as I did. Let us hope that one of our readers will soon amend that by sharing a copy of the original JLH article from HFN+RR.
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| Headphone Amp with Tone Controls |
There was a second version with simple Bass and Treble tone controls added before the input (that could be switched out) which added to the little amplifier's usefulness.
The Headphone Amp v2
Another headphone amplifier that came from the mind of JLH was bundled with the Audio Design Amplifier "demo build" featured in the Electronics Today International (ETI) magazine, June thru September, 1984. I am linking to it here for the convenience of the reader -- please turn to P-29 of the June, 1984 issue for a design that JLH himself termed had "an excellent sound", and little distortion.
Here is the link:
https://drive.google.com/file/d/15svJTW35AfrY0PF4LYvjHUIl5dqbYn06/view?usp=sharing
Linear Voltage Controlled Oscillators
VCOs are today an accepted circuit block that designers use without giving much thought to their linearity or stability. Not so back in the 1960s and 70s. JLH had explored this new territory and had designed his own highly linear VCO, particularly for use with the PLL FM demodulator. Here is a fine opportunity for the circuit enthusiast to learn the innards of a VCO. This design was published in WW, November, 1973.
https://drive.google.com/file/d/1kQZ9jhVd3__LY2HhGjwfBGZSdSYZURg3/view?usp=sharing
WW, September, 1979, had in its pages an improved VCO design by JLH that improved the S/N ratio. As can be expected, all his designs conclude with a practical application circuit, and that is the case here too. Both the articles are recommended reading for the hobbyist.
https://drive.google.com/file/d/1x6Z6MzvGPTF8-3m6_0wt6owsMdoSdYkA/view?usp=sharing
Twin Rail Stabilized PSU
Power supplies, as we all know, are the mainstays of any circuit's performance and stability. JLH had stressed this aspect vis-a-vis high quality audio amplifiers and test instruments. Here is a twin-rail stabilized power supply designed by the Master himself. (WW, January, 1975)
Once again, in typical JLH style, the reader is taken through the pluses and minuses of various PSU topologies. The design has proved to be an accurate and precision stabilized PSU that could serve many needs in the hobbyist's workshop.
https://drive.google.com/file/d/1GzP2XgaRSEs_obcAWmkcbOjOhNzOUb-d/view?usp=sharing
And here is a novel and refined One-shot Timer from JLH:
(The material was kindly made available by our reader and DIYAUDIO member Mjona; my special thanks to him.)
Until next time, do read, build and enjoy, share and comment!
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