Variations on the Classic Theme
In the history of DIY audio, no other amplifier has achieved such cult status and spawned so many variations as the Classic JLH has -- at least not to my knowledge. The tribe of upstarts aiming to upstage JLH, a "forgotten designer from the glass age", has been too many in the last fifty years plus; but we may safely ignore/forget them, as not even one such 'design' (mostly cobbled up in a circuit simulator, which to most of them was nothing more than an exotic toy!) has had any acceptance or has managed to survive in the chaotic hobby world.
On the other hand, devoted "JLH-ers" like Geoff Moss have made sterling contributions to the JLH legacy and his philosophy of design. We may also consider with respect the countless "tweakers and modders" who have attempted in their own way to adapt the classic JLH design to their times, with the components available at the time, and who have meticulously kept in their minds the uncoloured 'JLH sound' as their ultimate aim. It is these 'working angels of DIY' who have kept alive the JLH spirit and have inspired the younger generation of hobbyists, who have given a new lease of life for the classic design in the 21st Century.
Tweaker's Delight, Modder's Paradise!
My attempt here is to present in one platform the countless mods and adaptations and tweaks that have occurred since the publication of the Classic JLH amplifier in 1969, and which have been virtually 'lost' in the ocean of discussion and correspondence over half a century. Only the more interesting and relevant variations have been selected for inclusion here (in no particular order or chronology), primarily to serve as a source of information and inspiration for the ardent JLH DIYer. Also, this is likely to help avoid the waste of time spent re-inventing the wheel. Wherever needed, short descriptive comments are appended to the circuit drawings.
As the icing on the cake, I have also elected to post pictures of many builds by enthusiasts. Some are typical workbench prototypes, but many are notable for their mechanical ingenuity, and a few easily reach 'pro' levels. As they say, it looks like it has all been done before!
BIG Warning !!
All circuits are presented 'as is'. I have NOT built, nor tested these circuits for correct and proper working. The hobbyist / DIYer is advised extreme caution while implementing these circuits or modifications. Tread softly and carefully even though the DIY field is not exactly a minefield; but who knows, there might be dangerous incendiaries hiding within plain sight!
Always use a 'series-bulb' test setup while switching on new designs for the first time, and watch the quiescent current (and the heatsink temperature) with hawk eyes over a period of time, lest it runs away with your precious devices into electronic Hades or Valhalla!!
Do feel free to explore and be inspired by the creativity of a slew of ardent DIYers.
Boldly go where many a brave soul has gone before !!
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Here is a clever mod by
Bigun -- a capacitance multiplier just for the driver and the input device, which greatly reduces the rail ripple. The bootstrap capacitor enables the driver to swing higher than the output, which is all very good. An excellent and simple mod which could be tried with good effect.
Bigun's mod replacing the driver (Tr3) with a CFP. Something that could be tried and evaluated by the DIYer. Apparently Bigun reverted back to the original circuit. But don't let that deter you from trying the modification, as so many factors come into play. Who knows, you might stumble onto an improvement in performance!
Here is a circuit by the tube / solid-state Guru John Broskie -- very much like the classic JLH.
Some very helpful test DC voltages for the DIYer -- of course, from a simulator; real life voltages may show slight variations, but this is very good as a guide when you get stuck with some issue or other.
A good series from
Mike (fredbloggstwo) who has been experimenting with the various versions of the JLH classic and has evaluated a set of 'boards', which makes experimentation easy and painless. Highly recommended for DIYers. (For details refer to the
Notes in Part-10.)
Here is the 'standard' driver board.
Here is what he terms the S-pair (Sziklai pair) driver board.
The standard single output pair board.
Single Darlington pair output board.
High power dual output pair board.
Surely Mike's technique could be adopted by the DIYer so that it becomes easy to try various combinations of output devices and topologies. It is something that I have personally found to be very useful and practical. Once the design has been fine-tuned, you may think of a dedicated single PCB. (or, a P-2-P implementation!)
Highly recommended!
The implementation by Graham Maynard, whose GEM amplifier is a famous design.
Mr Maynard was highly inspired by JLH while designing his version of the class-A amplifier, and later the acclaimed GEM amplifier.
An Ali Express kit circuit. These kits are great value for money, no doubt, but the quality of the bundled devices and the accuracy of the circuit are to be double-checked.
For the knowledgeable hobbyist (or the careful tinkerer!) the kit offers a short-cut to having everything to hand cheaply. Some kits come with fairly good heatsinks and cabinets.
Another Ali Express kit.
Be warned that the kits are not 'fool proof'; extreme caution is called for while working on it.
Some simple mods by Banat -- try it for what it is worth.
Here is something from the respected designer John Ellis.
Another Ellis mod to the classic 1969 amp.
Mr Ellis has been instrumental in exploring the finer aspects of changes/mods to the JLH amplifier. You may refer to the
Notes in Part-10 to know more about his take on many aspects of design.
Ergo Esken, another hobbyist, suggesting some mods.
More from Ergo Esken.
Note the use of the modern LM317 for providing a rock solid reference for the input stage, as also the JLH-style current source for the driver transistor.
These mods are sure to improve the PSRR of the amplifier and also improve the stability of working.
Contributions from Grunf.
He has opted for JFET current sources. I haven't been able to test how far that will affect the sound quality of the amplifier. Anyway, it is a novel approach and hobbyists are advised to experiment and also audition with some care to discover the 'truth'.
Grunf has also confidently used modern 'high speed', high power output devices (which were reported to be unstable) with excellent results, according to him.
For the builder with a choice of modern hi-spec devices easily available, the explorations of Grunf could prove to be inspirational.
Hennady Kovalsky is a modern JLH enthusiast who has done a lot of work with JLH amps.
Here is his take on the famous Geoff Moss 2003 version.
A power supply design from Hennady.
Earthing / grounding has always been the bugbear of hobbyists and constructors. Only when persistent hum through the speakers that defy all attempts to track it down fails do we realize the importance of good earthing practices.
Earthing / grounding is supposedly a 'black art' as made out by many 'pundits' on the Web, who display extremely complicated, mind-boggling grounding schemes. Be wary.
Here is a good, simple scheme that may be followed with good results.
Here is another 'single point' earthing scheme, which may work, but which IMO is not very ideal. It is always better to keep the sensitive input side 'signal earth' a bit 'above' the 'Star Earth' with say, a 10 -- 15 Ohm resistor.
Whatever you do, always plan your earthing scheme well to avoid later trouble!
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