The JLH Gateway to Class-A :: 2

Building the JLH Classic --  The JLH Archives

        Looking back, it can be seen that 1969 was an epochal year for JLH fans -- the Master published the iconic "Simple Class-A Amplifier" in April and later, in July of the same year, the "Modular Pre-amplifier Design" in the Wireless World magazine, the grand old dame of UK "wireless" publishing. As they often say, "the rest is history"! History continued to be made in the long fifty years plus after the publication of that article by ardent enthusiasts and DIYers.

        True to the article title, the design presented an extremely simple (and elegant!) class-A power amplifier with just four transistors and a handful of passive components, which could often be scrounged from the 'junk box' of the average DIYer. As is often the case with class-A amplifiers, the expensive parts were the rather substantial (when compared to class-B amplifers) heatsinks and the heavy power supply. Construction was simplicity itself as point-to-point construction on a piece of 'universal circuit board' served the purpose -- in fact, many constructors have opined that the p-2-p was often better than a PCB, with the ease of component swaps/'adjustments' and wiring directly to 'nodes' to keep hum at bay (the input node, psu node and output node are particularly important; we will refer to them later).

        Right at the outset JLH acknowledges the seminal transformerless amplifier design by Tobey and Dinsdale (Wireless World, Nov, 1961) -- a Class-B, push-pull quasi-complementary design. This is another design that should pique the curiosity and interest of the JLH fan, as later JLH himself proposed some mods that apparently improved its performance much. (We will take a look at this also later.) By late 1950s, solid state amplifiers were taking baby steps while the 'incandescent bottles' ruled the roost comfortably. Perhaps the only stumbling block there was the output transformer, which took real expertise to design and build, if the results were to approach 'hi-fi' standards.

        Having perhaps analyzed the design, and in all probability having built and 'tested/tasted' the Dinsdale amp, it is no surprise that JLH decided to choose a Class-A design that had no crossover artifacts. (Do refer to the 'Class-A versus Class-B' observations in the appended Notes/Links elsewhere.) Of course, as most of us know, the simplest Class-A amplifier ('single-ended') is just a single device with a load resistor, but with very low efficiency (Fig-1).

        That means a lot of power is wasted as heat, with only about 12% going to drive the loudspeaker. The more efficient topology with an inductance replacing the load resistor (Fig-2) unfortunately is plagued by the same issues as that of a transformer -- a really good inductor is bulky, expensive AND needs expertise to design and build, if it is not to affect audio quality adversely. Probably the best topology is to use a transistor as an 'active load' (Fig-3), a technique that has remained popular till the present. To further increase efficiency, JLH adapts the tried and trusted 'concertina phase splitter' topology for the driver stage so that the output devices are driven in push-pull.

The 'Concertina Phase Splitter'

This also ensures that the linearity of the amplifier will be good even in the absence of feedback/low feedback.

( https://en.wikipedia.org/wiki/Phase_splitter 

 https://www.tubecad.com/2009/07/blog0167.htm )

Harmonic Profile- S-E v. P-P

        This topology of the JLH amp has led to many a debate about the amp being single-ended or push-pull. Naturally, the arguments focussed on the more benign (pleasing to the ear!) harmonic profile of the single-ended amplifier, versus that of the typical push-pull amplifier. An excellent analysis of this quandary has been presented by Geoff Moss. His take on the JLH circuit:

        "... A lot depends on how you analyse the Q1/Q2/Q3 output stage, which can be looked at in several different ways, ie Q3 acts as an accurate current phase splitter, Q2 is a voltage follower with Q1 as a variable current source emitter load or Q1 is a common collector stage with Q2 acting as a variable current current source collector load.

        Whichever way you look at it, there is a degree of effective 'push-pull' action which results in a higher maximum output current, for a given quiescent current, than would be the case for a single-ended output stage. With perfect transistors, the maximum output current could theoretically be twice the quiescent current but unfortunately we do not have any perfect transistors available.

        Many simulations using a variety of power transistor models indicated that in the real world the peak output current could be between about 1.2 and 1.5 times the quiescent current, with the majority of models giving a figure between 1.3 and 1.4, so I chose 1.35 as a suitable multiplier to use for design purposes.

        At the same time as I was carrying out these simulations, Rod Elliott was developing his DoZ design (a similar output stage to the JLH) and he determined empirically that the optimum quiescent current was about 0.75 times the required peak output current so our conclusions (mine from simulation and Rod's from measurement) were in agreement ..."
(See also link below about the single-ended/push-pull conundrum.)

        Subsequent to the publication of the iconic amplifier, various clarifications and suggestions have been presented by JLH himself in the 'Letters to the Editor' pages of the Wireles World mag, all of which have been linked to below for reference. A link to the debates on Class-A versus Class-B too is also there.

        The December, 1970, Wireless World carried a full article ( a 'postscript') relating to the Class-A amplifier and the Simple Preamplifier, both published in1969. Of course, links to the full articles on the Pre-amp and the later comprehensive Modular Preamplifier will be presented in future posts. The 'Postscript' article is required reading for the Class-A amp builder, the primary reason for its being presented now.

Links to the JLH Archives:

1. The Classic 'Simple Class A Amplifier' -- Wireless World, April, 1969 

https://drive.google.com/file/d/1aNJdalEF2OKVmq4GCa02ggDmLwYMM1t2/view?usp=sharing

2. Letters to the Ed - Class-A versus Class-B -- WW, May, 1969 

https://drive.google.com/file/d/18qXA03jSbgZJC0dXy2RLk6ryG0urt6a0/view?usp=sharing

3. Letters - Audio Amplifier Performance -- WW, July, 1969 

https://drive.google.com/file/d/1Ie5eHwwbWRxYZag4WUyZWV1yNcVD7m3W/view?usp=sharing

4. Letters - Minimum Standing Current in Class A -- WW, Sept, 1969 

https://drive.google.com/file/d/1ZS9WQLPYJ08tkP8LF9uZBt2YZy0QnvWr/view?usp=sharing

5. Letters -  Linsley Hood Class A Amplifier -- WW , Oct, 1969 

https://drive.google.com/file/d/1nJVx-bY8ggJRlFjtgtY7JdrvCzT5rsBc/view?usp=sharing

6. The JLH Class-A Output Stage – Single-ended or push-pull? -- Geoff Moss (2004) 

https://drive.google.com/file/d/1FsukIOC9mAcgpAAloAqqWK8w3ds9gyMz/view?usp=sharing

7. A Postscript to the Class A Amplifier and the Modular Preamplifier -- WW, Dec, 1970 

https://drive.google.com/file/d/1T-HFYdhOKJLR2SB4uT3UJAUhhjCyxfiN/view?usp=sharing

8. A series of Letters to the Editor and replies by JLH throw light onto various aspects of amplifier design. It is hoped that these provide useful insights to DIYers too. For the sake of 'completeness' they are linked to here.

https://drive.google.com/file/d/1KAVoCKXPwkDQKe520QywUA42tsfWRV42/view?usp=sharing

        The above provides enough material for the newbie or the serious builder who is keen to go through the original published material from half a century ago. The insights gleaned from the wisdom of the Masters will serve to guide the builder, I am sure.

        The upcoming post will present comprehensive notes distilled from the discussion in the forums, as well as present various circuit mods and tweaks and builds by many designers and enthusiasts. Take them for what they are worth. The material is posted in good faith, and for use by the DIYer, and not for profit. All care has been taken to ensure the authenticity of sources/information presented here. Any error is open to correction. Those who are privy to additional information/material are requested to share it, so that the Blog could be updated to include those also.

        Do read, comment, share  -- and build and listen!!

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