The JLH Gateway to Class-A :: 5
Building the JLH Classic -- Moss the Torch Bearer
For JLH enthusiasts and for those who knew him and his work, the year 2004 was indeed a sad one -- they lost the guiding spirit that JLH was to all of them. The Electronics World (formerly Wireless World) magazine declared the June, 2004, issue as a JLH Memorial Issue, and in his honour reprinted the entire article of the JLH Classic amplifier from the April, 1969, Wireless World, a very rare tribute paid to any author.
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| Cover of Electronics World (UK) June, 2004 |
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| Contents Page - EW June 2004 |
JLH had never chosen to be in the limelight, and his admirers were happy to see a rare current photograph of the Master in the Obituary / Tributes column published in the EW June, 2004, issue. But today, looking back, it is heartening to note that the legacy and glory of JLH lives on, thanks to the efforts of a few dedicated souls like Geoff Moss, and the continuing DIY labours of thosands upon thousands of JLH fans the world over.
@ Link to the Electronics World Memorial Issue -- accolades to JLH
https://drive.google.com/file/d/1-tgN0X2r02Z-2eyGskTcesKCNZHB9ypR/view?usp=drive_link
If the hobbyist and DIYer should be indebted to anybody in the post-2004 era, the one name worth recommending is that of Geoff Moss. Once the Master passed, he took it upon himself to establish a site dedicated entirely to JLH and his designs, and went to great lengths to collect, compile and host virtually everything that the Master published. He also was instrumental in promoting improved builds of the JLH Class-A amplifier, tirelessly answering every single question in hobby forums despite his failing health.
Geoff's year-long (and more!) collaboration with Tim Andrew ( a professional concert pianist and audio hobbyist) in taking the Master's design forward has resulted in some definitive hands-on guidelines for the ardent DIYer. Tim's comments as a trained professional musician regarding the subjective quality of his build of the JLH amplifier truly serves as an indicator of the JLH amplifier being very much near to the ideal 'length of wire with gain'. Tim was meticulous in his approach to the modifictions to his JLH build, and evaluated the results over protracted listening sessions, sometimes deciding to stay with the mods, but at times reverting to the earlier state of affairs. The musician's ear, we must admit, is anyday a more trusted hallmark for us than the 'golden ears' of the hacks punting their stuff in 'popular' audio journals. The long and the short of it is that, thanks to the labours of these dedicated souls, the JLH amplifier has once again proven that the classic design is indeed timeless and is equally at home in the 21st Century too playing sweet music.
The Winning Formula
The combination of the technical expertise of Geoff Moss and the musical abilities (and patience!) of Tim Andrew created a synergy of the beauty of art and the precision of engineering -- qualities that JLH himself had cherished much. Let us examine in some detail the contributions of Geoff Moss, which certainly shall serve as a definitive guide to the builder.
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| Offset adjust CCS-1 |
JLH had in his "Postscript" (see the Classic-1996 post for link) had suggested the use of a zener-stabilized current source to set the quiescent current of the Classic-1969. Later in 1996 he did endorse the use of a current source, derived from a three-terminal regulator, to inject the the right amount of current into Tr4, so that the output offset could be nullified easily.
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| Iq adjust CCS-2 |
Both these techniques were enthusiastically adopted by the DIYers. However, it was generally accepted that three-terminal regulators were not exactly famous for their low noise performance, and designers and knowledgeable hobbyists wanted to explore alternatives. Also, 21st century audio design practice favoured a liberal sprinkling of current sources and current mirrors in amplifier circuits. But then curret sources (CCS) offered many advantages, it has to be admitted, and in the case of the JLH, contributed much to the quiescent current/offset stability in spite of the high temperature of operation.
The CCS Magic
Credit for exploring the CCS alley for improvement must surely go to Geoff Moss, whose articles turned out to be the guidebook the serious DIYers took after. As mentioned earlier, the passing of JLH saw Mr Moss devoting his energies to setting up a comprehensive site to host JLH material, and he was most happy to hand-hold the DIYer and clarify his many doubts. The collaborative build by Moss and Tim Andrews turned out in practice to be the accepted "modern standard" JLH build.
In a couple of years the dual CCS mod by Geoff Moss became the de facto standard, and was by common consensus labelled the JLH-2005 for ease of identification. Mr Moss, with the help of others, was able to finalize the design of PCBs that suited either the 'standard' version, or the higher powered version intended for driving difficult loads like the Quad electrostatic speakers.
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| JLH 2005 |
One look at the JLH-2005 schematic will tell you it is nothing but the Classic circuit, with just two modern 2-transistor CCSs replacing the resistor networks for setting the DC conditions. This is exactly as per the recommendations of JLH. The input CCS-1 has a temperature coefficient that is opposite to that of Q4, and so negates the change of output offset occuring as a result of the heating up of Q4. The CCS will also shield the input transistor from supply rail ripple. The bias setting CCS-2 nullifies the increasing distortion at low frequencies that might result if the bootstrap capacitor was used. Also, the maximum output voltage swing of the amplifier is higher with the use of CCS-2, the output current rising from about 1.35 times the quiescent to nearly 1.5 times, thus contributing to better efficiency, and obvious thermal benefits.
Moss and others have perfected a higher power amplifier with two pairs of output devices for driving difficult loads like the Quad electrostatic speakers. This may be tried as an 'upgrade' once you have your 'standard' JLH working nicely. But if your speakers do not demand higher power, it is better to stick to the simple version, as increased complexity/multiple output devices are bound to have some little audible effect. Also, if you ask me, it is better to avoid 'tricks' like doing away with the feedback capacitor. As mentioned earlier, just be sure to use good quality capacitors, and when in doubt, change just one channel and do comparative auditions and decide.
Playing with Power
Any amplifier is as good as its power supply, and more so in the case of a Class-A amplifier. Copious availability of current and voltage stability, coupled with minimum ripple is a given here, and your PSU should approach Jean Hiraga's huge set of batteries when it comes to feeding the JLH! Pay particular attention to using good quality reservoir and bypass capacitors.
Both the capacitance multiplier variety and the fully regulated types are presented. Note that it is possible to start with a cap-multi version, and later convert that to a fully regulated one at your ease. It goes without saying that to derive maximum satisfaction from the JLH, one needs to spend a sizeable chunk of time and lots of TLC, and exercise your 'golden ear' to the max -- all the while without making any major change at all to the Master's original design!
The general consensus developed over a number of years is that monoblock construction brings out the best in the JLH amplifier. Two totally independent amplifier channels have audible benefits, especially when it comes to sharp and focussed sterophonic imaging. It also is beneficial in minimising any ground-related hum and noise. Naturally it is recommended that you follow good grounding practices, and the Star Ground should be the 'real' ground reference point for the amplifier.
A reminder about building dual rail power supplies -- again, consensus goes that it is better to build both the + and - rail regulators or capacitance multiplier circuits identically. See that the the entire PCB is "floating" with respect to earth/ground, and ensure that you connect to Star Ground only the wires from the output side of the PSU PCB -- negative leg in the case of the positive rail module, and positive leg in the case of the negative rail module. This will also guarantee that earth loops are avoided.
Design Notes and Tips
The JLH builder has a golden key that opens the Aladdin's cave of the wisdom of many -- the Design Notes and Tips gathered by Mr Moss. ( In a future post another more comprehensive "Notes" , culled from my archives, will be presented by your truly!) An interesting take on the Classic that is offered here is the option to convert it to a dual supply, output capacitor-less topology. (It's for those of you who suffer from "capaphobiasis".)
Link to the source archives presented here:
Amplifier Options:
# The JLH 2005 comprehensive 'update':
https://drive.google.com/file/d/1UxxMu2pSE_-nQW9ZPZEU8qkekJDeQr1z/view?usp=sharing
# JLH 2005 Setting up Instructions
https://drive.google.com/file/d/1Q_PNv3ZO8d_oyXRGkM_ZylSzkOInBXwH/view?usp=sharing
# JLH 2005 - high power version for Quad speakers
https://drive.google.com/file/d/1nck0f8Q6lJj2fe-LxGIpgJ_Pc257TfzZ/view?usp=sharing
# Typical DC voltage readings for both the 1969 and 1996 versions
https://drive.google.com/file/d/1hg6tztKjp0N2ERu2TmyuYVdA2hBEz8In/view?usp=sharing
# Modern transistor substitutes with notes and suggestions
https://drive.google.com/file/d/1WMvFnRNALyrvmQ-k_JBN10pa7fhIM2zo/view?usp=sharing
# The all important earthing/grounding guidelines
https://drive.google.com/file/d/1HVHSRV3C46v7Yir3H02eyDP2Ch3UGIQx/view?usp=sharing
# A compendium of design tips and notes
https://drive.google.com/file/d/1L8cTpLQ1hRU-1G6-HJ9AkJEHT6D7bxUX/view?usp=sharing
# A Dutch builder's tribute to JLH
https://drive.google.com/file/d/1IEDVtpzMgpYgmmXcMTHeutWvpLLAlevV/view?usp=sharing
Power Supply Options:
# The updated PSU for JLH 1996
https://drive.google.com/file/d/1_MA50V2Cp4LyP9i8ygbkBnFbblxLP_oZ/view?usp=sharing
# The Capacitance Multiplier power supply
https://drive.google.com/file/d/1iLrRoJZGmZc6KE8Q0Femoo54_vNAYX4N/view?usp=sharing
# A Simple Voltage Regulator Circuit
https://drive.google.com/file/d/1rGbuHi5kBRFe3HPM846epy1TQlcf3q0m/view?usp=sharing
# Current boosted LM317/LM337 regulator
https://drive.google.com/file/d/1-53im68qmXKzHMV6J9FBMFVnN4bNeEMy/view?usp=sharing
That I believe is ammunition enough for the avid builder to confidently enter the battlefield and be virtually certain of sweet victory.
Until the next post, do read, share, comment ... and think of starting your build of one the most wonderful amplifiers ever. No need to believe my words; your ears will vouch for that!
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