The JLH Gateway to Class-A :: 12
Playing Musical Chairs ...
Playing musical chairs with a medley of class-A amplifiers is a tantalizing, thrilling prospect. Basically the game is a metaphor for deciding the winner by elimination, and most agree that there is no better fun game than musical chairs to zero in on Numero Uno. Here is an opportunity (and a challenge!) for the class-A enthusiast to play musical chairs with a set of class-A amplifiers, including the all-time favourite the JLH Classic! The winner, as they say, takes it all!!
... with the Many Flavours of Class-A
Naturally, the decades since the JLH Classic amplifier appeared on the scene in 1969 have spawned a profusion of class-A amplifier designs. But then you might think, what are these designs by other designers doing here in a site devoted to JLH and his classic amplifier ??
As many a diehard JLH enthusiast would put it (based on their own experience), after trying out everything else, you are likely to go back to the simple, classic JLH design of 1969 !
So, consider this as an opportunity to be introduced to the cream of class-A amplifiers published in the hobby Press since 1969. These range from the single-ended lower powered amplifiers to higher powered push-pull designs, including some behemoths putting out 300 Watts! Many of these amplifiers have had a good following in the DIY community. Honestly I have not built nor auditioned ALL of them, though I have tried out one or two simpler designs and compared them with my "gold standard", the JLH Classic-1969.
A Challenge to the Ears
Yes, this is an opportunity and a challenge to the DIYers and builders out there. Here is a selection of popular class-A amplifier designs from around the world by designers of note, and some of them happen to employ newer circuit techniques and modern devices. By all means build and enjoy the ones that catch your fancy. But do NOT forget to compare them with the JLH Classic, and be sure to post your experiences. This sure will prove to be of much interest to the ardent DIYer and the JLH enthusiast.
(Here I am not in any way forgetting the classic amplifier designs by Jean Hiraga like the Le Monstre, or the many modern designs by Nelson Pass, all of which enjoy enthusiastic following in the DIY community. But please note that these are very well documented and information is available to the enthusiast readily at various sites and at passdiy.com. My attempt here is to bring to the notice of the DIYer class-A amplifier designs that have not been the subject of discussion widely, or those that have been forgotten with the passage of time, or got buried in the avalanche of information.)
The Sugden Class-A Amplifier for DIYers
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| The Famous Sugden A-21 Amplifier |
Let me kickstart this Class-A cavalcade with a design from a young contemporary of JLH -- James Sugden, who had started his own company in 1967. The rightly famous Sugden A-21 was a pure class-A transistor amplifier that established the reputation of the company in no time, and was in production for a long long time. Mr Sugden was requested by the editor of Hi Fi News and Record Review (the UK audio magazine, at the time very popular with serious DIYers) for an up-to-date class-A amplifier design, which was published in two parts in the April and May issues of 1970.
Part-1 presents the design in some detail, with an analysis of the circuit topology, while Part-2 detailed the building , testing and setting-up of the amplifier. The amplifier boasts of impressive (for the day!) specs, which would stand it in good stead with its peers even today. Check out the link to the article here:
https://drive.google.com/file/d/17ddjvtERI2FEnH4RJ3fpXEV7_tqiX-R2/view?usp=sharing
The Mullard Class-A Amplifier
Simple and sweet! Here is a tried-and-trusted design from one of the stalwarts of early audio design -- Mullard, the UK division of Philips. With an 8 Ohm load, the amplifier outputs 15 W at less than 0.1 % THD. With a 4 Ohm load, the amplifier switches to class-AB to give about 20 W power. A simple circuit that gives good results and which was very popular with hobbyists in the previous century.
https://drive.google.com/file/d/15R8RxsC9US8yzsH9x57VHH8apb0zrcRZ/view?usp=sharing
The Calvert Class-A Amplifier
A little gem of a 10 W amplifier by A. H. Calvert appeared in the Wireless World, June 1976, as a Circuit Idea, with claims of "unconditional stability ... made from uncritical, easily available parts ... and with short-circuit protection"! A great formula indeed that takes it into the enviable position of being a 'younger cousin' of the JLH Classic amplifier (if we borrow a little from Grand Old Man Pass in naming conventions) ! What more do you need from a simple amplifier?
The circuit is very similar to that of the JLH Classic. As always with class-A, the heatsinks need to dissipate about four times the output power of the amp. Considering all its positive aspects, it is surprising and disappointing that it did not get the attention and following it rightly deserved.
Perhaps DIYers today will be enthusiastic in taking up a build and do a comparative audition with the JLH Classic -- the elegant design certainly deserves that IMHO.
Class-A Amplifier With No Feedback
The Wireless World published a not very well noticed design by the young Russian designer Miloslavskij for a wideband class-A audio power amplifier in July, 1980. It was a theoretical design and not a constructor's project -- perhaps the reason why it was not enthusiastically followed neither by the DIYers nor by the designers of the time. The 1980s, on might recall, were times when designers were chasing higher output powers with their solid-state designs, and it is no surprise that this excellent idea got buried in the flood of other 'super power' designs.
The design targets more accurate reproduction of transients (hence the wideband requirement) and the designer for various reasons has chosen an active crossover system. The article provides enough grist for the mill of the knowledgeable hobbyist, who could easily attain results that possibly could raise the performance bar. It is for this reason that I have chosen to include it here. Do follow the link below and study and build the circuit that comes in both SE and P-P versions.
https://drive.google.com/file/d/1097XNC5SFrkQ0Dvfg67e-GsbjlZ41DUy/view?usp=sharing
Here is also a link to Paul Kemble's excellent web page where he refers to Miloslavskij's Class-A amplifier for your reference:
https://www.angelfire.com/sd/paulkemble/sound2d.html
The Lang Class-A MOSFET Amplifier
A popular amplifier design that appeared originally in the ELRAD Magazine in 1985, the Lang amplifier is a low feedback, 'no compromise' design utilizing lateral MOSFET output devices (very popular in the modern era) and a fully symmetrical circuit topology. The author analyzes the circuit well, making it easy for the hobbyist to follow the design. Overall a very interesting design that provides high quality amplification. Strongly recommended.
https://drive.google.com/file/d/1R43fnrAE0nrUGAj76D9YtrNXPGKt_E9v/view?usp=sharing
The 'Blameless' Class-A Amplifier
The 'Blameless' name immediately invokes D. Self to even those who have not read his articles on design carefully. There are designers who do not agree totally with all the views that Self puts forward, but overall it cannot be denied that his is a respected name in audio.
Here is a 'Blameless' class-A design that Self presents as a "worked design approaching the straight wire with gain" in Part-8 of his series on reducing distortion in power amplifiers. (Electronics + Wireless World, March, 1994)
Self agrees that the only real disadvantage of class-A is its lack of efficiency. He proceeds to examine the various circuit topologies that the designer could pick from, but ultimately, for his own well-considered reasons, settles on a high-bias class-B, push-pull design.
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| Self's novel Iq Control Circuit |
The article is worth a detailed and careful read, as a corollary of which a construction project could be attempted. Surely a post-build comparative audition will establish the 'blameless' credentials of Self's design -- if that really is the case.
Here is the link to the original article:
https://drive.google.com/file/d/1dPOgu7hHr45YIj-ouxIyVm1Txq1sOmuO/view?usp=sharing
The Five Amplifiers
The number Five has a significance of its own -- Five often represent Universal perfection, as in the Five Elements and the Five Senses, making it a powerful symbol in various cultures and contexts.
The turn of the century saw a short article outlining five versions of an ambitious 'universal' class-A design published in the Electronics World (formerly Wireless World) magazine of March, 1999 -- as they said it on the cover 'Class-A for All' !
It was a wonderful design by Colin Wonfor, which, as far as I know, unfortunately did not receive the kind of attention it rightly deserved -- despite its simple design and modular approach, most likely the reason being that it was NOT a 'hands-on' hobbyist project.
The five amplifiers, all based on the same uncomplicated topology, ranged in output power from 20 Watts to a huge 300 Watts of class-A power (which burned up a whopping 1,500 Watts per channel)!
The circuit is fairly simple and uses readily available components, and employs very large, convection cooled heatsinks. Heat and heatsinks, and huge power supplies are daunting factors that raise cold sweat on the brows of intrepid constructors merely contemplating a class-A build! But all said and done, I can't see any reason why the average builder should not try his hand at building at least the 20 Watt version -- and do some comparative auditions. To that end, here is the link to the original article, which sadly, many hobbyists do not seem to recall.
https://drive.google.com/file/d/192AJW8hfiQXUNbj8bfMDfSS2-lFVy_ST/view?usp=sharing
20W Class-A amplifier by Leo Simpson & Peter Smith
Everyday Practical Electronics was a hobby magazine that prided itself on highly detailed and practical projects for the DIYer that often approached pro standards in design and presentation. True to form, the 20 Watt class-A amplifier by seasoned designers Leo Simpson and Peter Smith published in the EPE ran for five months from October, 2008 to February, 2009, presenting a comprehensive design for the dedicated DIYer. I am very happy to present it here as the design and presentation are indeed top-notch and give the hobbyist enough guidance to achieve near-pro standards. Here is the link to the full set of articles:
https://drive.google.com/file/d/1RukrlnJWg2T6ljjDuhxToS5A4QE-4Be3/view?usp=sharing
The POWER FOLLOWER topology at one time had its many followers. Or, shall I put it this way -- there was no serious class-A enthusiast who had not tried the Power Follower amplifier at one point in time. The topology offered 'truth in reproduction' and it was mostly the tube aficionados who often switched to the solid-state Power Follower output stage, as its inherent inefficiency meant you had to have loads of preamp driving voltage, a condition easily satisfied by tube preamplifiers. Plus, it preserved the 'tube sound' they were in love with!
Whatever, the Power Follower offers a simple and easy project for the DIYer. Here is a quad (no, not the Quad from Acoustical Manufacturing of UK!) of 'followers' which I am sure will gather a set of followers in its wake in due course!
The Ciuffoli SE Class-A Power Follower
Power follower topologies were popular with DIYers off and on, and perhaps the most popular one on the Web was the design by the Italian designer and hobbyist Andrea Ciuffoli, which he first published in 1999. It had some unique design features, and the designer was once the darling of the "tweaker community", if I may say so.
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| Ciuffoli Powr Follower |
This is touted as a 20-25 W single-ended 'no feedback' amplifier, and was featured as a prize-winning design in the Electronics World, May,2000, issue.
Here are the links to the original article, as well as to Ciuffoli's website presenting the amplifier again -- after 20 years, in 2019.
https://drive.google.com/file/d/1FOxwsMtJOADuVAUbmqq-fgALraD3_LHA/view?usp=sharing
https://www.audiodesignguide.com/PowerFollower/index1.html
Pavel Macura Power Follower
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| Macura Power Follower |
Another popular power follower amplifier was from the much respected Czech designer and DIYer Pavel Macura, which he presented in 2009. The MOSFET follower has benefits of single-ended, common Source class A amplifier with less distortion, lower output impedance, higher input resistance and, of course, no (global) feedback! Not a bad combo, and an easy circuit to build (and tweak too)! No wonder the Macura follower, just like the Ciuffoli follower, had generated many followers !!
Here are the links to Macura's site:
https://pmacura.cz/follower_e.html
https://pmacura.cz/audiopage.html
The Mo-Fo by Mike Rothacher
There are versions of power followers that many are fond of, and in 2017 Mike Rothacher came up with a simple MOSFET design. It sure is an ultra simple circuit, with just one active device (IRFP250) and a fairly large inductor, and practically little else -- audio amplifiers couldn't get any simpler! Those who have built similar circuits swear by the subjective quality such simple designs offer. For the hobbyist, this is a new/old wrinkle that s/he could explore afresh. Here is the link to the original article:
https://drive.google.com/file/d/1tzDGnfvuIXguiokmUJYnaVzkOZjibVE5/view?usp=sharing
The SEWA Power Follower
The Norwegian hobbyist/designer 'Mad_K' admits that he was inspired by Nelson Pass to create a truly simple amplifier. It happens that this is no amplifier, but a power follower. The simple circuit outputs 7W, and comes in a couple of flavours --either with a resistive load, or with an inductive load (like the Mo-Fo above).
Many have opined that the 'pure resistive load' produces purer sound. It has to be admitted that it is one amp that sure is 'noob' friendly. Check it out by following the thread on DIYAUDIO.
https://www.diyaudio.com/community/threads/sewa-seven-watt-amplifier.66822/
The TUMOS Amplifier
With the availability of high power MOSFET devices, many designers came up with interesting designs of class-A amplifiers. One such was the TUMOS from Doede Douma, who called it 'the MOSFET power amp module with real Tube sound'. Well, here is an opportunity to put that statement to the ultimate test--go, build and listen to it!
It sure is an interesting design as Mr Douma was planning to use the design for all the modules in an active crossover setup. His formula was simple: a hybrid amplifier -- tubes for voltage gain and MOSFETs for power -- and no global NFB either! A simple phase-coherent 6 dB filter at the input took care of the crossover points admirably. Check it out here:
https://drive.google.com/file/d/10B6-ENAhDswkA6s6RfqSnFXIxjmtfF2B/view?usp=sharing
https://doede.de/dddac/op06.htm
The Ovation SX Amplifier
Originally published in August, 2012, and later updated in March, 2016, the Ovation SX is a low distortion, wide bandwidth CFA topology amplifier by Andrew Russell. The full PDF may be downloaded from:
http://178.62.51.182/wp-content/uploads/2014/03/The-sx-Amplifier-V2.10.pdf
The article goes into topics like 'Why class A' and examines the designs of JLH and Jean Hiraga, and also opens the PIMD debate in some detail. After that the circuit is discussed. Overall a very interesting read for the hobbyist looking to update himself regarding the latest design trends and philosophies. Highly recommended.
Hifisonix hosts the files and here is their link:
https://www.hifisonix.com/projects/sx-amplifier-audio-power-amplifier/
The Alpha Nirvana Amplifier
Towards the end of 2019 Hugh Dean, the well known Australian audio designer, published his Alpha Nirvana class-A amplifier in the DIYAUDIO forum. It was an efficient (43 %) design able to drive 39 W into an 8 Ohm load, and was enthusiastically taken up by the DIY community who held the designer in high esteem for his AKSA series of amplifiers. The output consisted of a standard complementary MOSFET stage, but operating in S-E class A with an active CCS.
The design has had its devotees around the world and certainly merits study. Currently the post runs to 186 pages, and culling the fine points of design and build support would take some amount of patient reading. Do go to the DIYAUDIO link for more information:
https://www.diyaudio.com/community/threads/alpha-nirvana-39w-8ohm-class-a-amp.344540/
Lateral MOSFET Class-A Amplifiers
Let me now introduce a gifted and popular designer from Serbia who goes under the handle Juma in the DIYAUDIO forum. I have built and listened to a few of his designs, which I must say are very good ('musical') in my subjective experience. Here are a few noted designs from Juma.
Class A amp with Lateral MOSFETs
Here is an early design (2011) from Juma which uses lateral MOSFETs and outputs about 25 W. He was trying to get the qualities of the famous F3 and F5 amplifiers by Nelson Pass, and to judge by the results, he did achieve that. It is not a long thread and so it's easy enough to gather all the data you need before you start the build. But first be sure to get hold of a few good Lateral MOSFETs -- of late they have become inordinately expensive and "un-obtainium".
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| LatMOS Class-A Amplifier |
Here is the Forum link:
https://www.diyaudio.com/community/threads/25w-class-a-amp-with-lateral-mosfets.183482/
Cubie -- the F5 Variant
Juma published the Cubie in 2014 as then he was looking for a small 10 W amplifier. The name reflected the 'cube' structure of the built amplifier. This was a very popular project, and very soon he came out with Cubie-2 and Cubie-3 versions, which also we shall examine.
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| Cubie Amplifier |
Here is the Forum link:
Cubie-2
As Juma puts it quite candidly, if Cubie sounded great, Cubie-2 managed to sound 'a bit better'! It is left to the builder to verify that statement by the designer!
The design saw the light of day in early 2015, and suffice to say, it garnered a lot of interest from the Forum members.
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| Cubie-2 |
Do visit the Forum link for your edification:
https://www.diyaudio.com/community/threads/cubie2.271543/
Cubie-3 (Class-AB)
Juma was busy in 2015, and August saw the posting of the Cubie-3 version. But please note that this was NOT a class-A amplifier, but a class-AB one. I have no idea if Juma was carrying on with the Cubie nomenclature based purely on the build geometry, or whether he found the Cubie-3 to be so good as to beat the class-A Cubie-2, and so he was justified in naming it thus. Perhaps the answer lies with the findings of the builder! So with this caveat, let me give you the link to the Forum post:
https://www.diyaudio.com/community/threads/cubie3.278034/
Trans-Nova Schade Amplifier
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| Trans-Nova Amplifier |
The year 2020 saw another great class-A design from Juma, based on the Trans-Nova topology by Jim Strickland.
It is a simple design employing Lateral MOSFETs that delivers sterling results, if I may share my own experience with it. Do note that it needs two separate floating power supplies, powered from two separate windings of the power transformer.
Here is the link to the Forum post:
https://www.diyaudio.com/community/threads/transnova-schade-os-amp.359000/
The 'Ultimate' Class-A Amplifier
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| Bliss 4-layer PCB |
To make such a claim as above needs supreme confidence in one's art! We indeed have come a LONG way, DIYers! If you want to check out the state of DIY and sample a design by modern designers/hobbyists, make a beeline for the 'Bliss' class-A amplifier on DIYAUDIO. It has 'everything' that the old-timers didn't have -- SMDs, Lateral MOSFETs, 4-layer 'industrial grade' PCBs ... the works! And the sound? Way above the stratosphere, if we are to believe the designer and the builders.
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| Bliss Amplifier |
I am happy and proud that DIY has indeed achieved 'pro' levels in design and build. Though it is 'beyond my ken', I greatly appreciate the work. It is an ongoing project and here is the link for those intrepid solder slingers amongst us:
Single-Ended 6W Power Amplifier
John Broskie the tube Guru is known for his adept handling of both tube and solid-state circuitry. I dare say there are very few designers with his understanding of both technologies, and who could mix and match the old and the new devices to come up with novel circuits.
Here he explores a simple single-ended MOSFET amplifier with a CCS. Nothing extraordinary there, you might think, but the circuit bears the unique Broskie signature. The way he hand-holds the reader step-by-step through his design is nothing short of fantastic. A sure candidate for a lazy evening's build. Do explore the circuit here:
https://drive.google.com/file/d/1ERmRdZnbSu14wDxwWPDm-snYPda9GIlx/view?usp=sharing
A Low Distortion Class A Amplifier
Here is another high quality class-A amplifier from the prolific Czech designer Pavel Macura, originally published in 2016 and later modified in 2022 . The PM-A4 was a successor to his previous designs (PM-A1 and A2) where he wanted to explore the possibility of lowering non-linear distortion further and extending the bandwidth.
The PM-A4 is a 2-channel class A amplifier with 1.5A per channel idle current, and class A operation is fulfilled up to 3A output current. The circuit is noteworthy for its novelty -- the entire output stage operates within the feedback loop of the input JFET op-amp. (circuit pic) Do explore the meticulous presentation of the amplifier at:
https://pmacura.cz/pma4_en.htm
and also in the DIYAUDIO Forum:
(<--- check Post # 18 for schematic)
Class-A/AB Amplifier
Very few readers are likely to have seen the hobby magazine Electronics in Action, much less remember its contributions. It was perhaps the shortest-lived hobby magazine. They had even roped in JLH in an effort to garner readership. (see earlier Post-5 in 'JLH Memorabilia' where we have surveyed the series of articles by JLH in the mag)
Here is a MOSFET amplifier design by Andrew Armstrong, which appears at best to be a 'rush job'. It is an average class-AB design, which could be biased higher (of course, with adequate heatsinking!) so that the first few watts could be output in class-A.
Why I have included it here is for historical reasons --many are ignorant of its existence, or forgotten completely about it. But it must be admitted that it is an easy build for the novice hobbyist, who would be happy to have a blending of two 'avatars' in one -- a low power class-A amp for critical listening, and a class-AB workhorse for those loud party blasts.
It is obvious that whatever be the bias level, it will never be a match for the JLH Classic amplifier. Anyway, here is the link:
https://drive.google.com/file/d/1_78Q5icvXUofoZgHp5a6-Jjh1VY7tmKt/view?usp=sharing
The Norman Thagard 100W Class-A Amplifier
In 1995 the AUDIO magazine published a three-part article (Jan, Feb, Mar) authored by the famous NASA astronaut, avid audiophile and designer, Dr Norman Thagard, outlining a 'pure class-A' amplifier putting out 100 Watts! It was a landmark design with many unique features, about which more anon.
Earlier he had co-authored with none other than Nelson Pass another 'pure class-A' amplifier, the A-75, in The Audio Amateur magazine (April, 1992 and January, 1993). Dr Thagard has to his credit many high quality audio designs published over the years.
[#] Here are some more details for the interested reader to explore:
# Dr Norman Thagard Bio: https://en.wikipedia.org/wiki/Norman_Thagard
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| The A-75 Amplifier (DIY) |
# A-75 Class-A amplifier - Parts 1 & 2:
Part-1 : https://www.firstwatt.com/wp-content/uploads/2023/12/art_a75_1.pdf
Part-2: https://www.firstwatt.com/wp-content/uploads/2023/12/art_a75_2.pdf
Building the A-75 : https://mackiefamily.unospace.net/a75/index_a.html
# A MOSFET Update for the A-75/Forte-1a Amplifier - (Audio Xpress, May, Jun, 2003)
# Phono Pre-amplifier for the CD Era - Thagard (Electronics World, April, 2001)
# A Case Study in Audio Amplifier Design - Thagard (Audio Engineering)
# A 100W/Channel AB Differential Amplifier - Thagard (Audio Xpress, Nov, Dec, 2002)
# Single-ended to Differential Made Easy - Thagard (Audio Xpress, Dec, 2004) [#]
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| A-100 Output PCB |
Now let us get back to the A-100 Class-A amplifier referred to above. Do check out the link below to the set of original articles that appeared in the AUDIO magazine. Dr Thagard clarifies that the topology was dictated chiefly by his design goals.
Naturally 100 Watts of class-A output demanded a hefty paralleled output stage. He was particularly fond of the 'cascode' circuit topology as that ensured very low distortion levels, and here used a novel 'Bi-MOS cascode' format for the output devices. This easily enabled him to meet most of his design goals, especially wide bandwidth and very low distortion.
In fact, the one distinguishing mark of this amplifier is that it employs the 'cascode' topology from the input to the output -- the input stage is a dual-differential 'bi-FET' cascode. Just as he had said about the A-75 circuit elsewhere, here also the stress is to present the circuit schematic to the builder for his understanding of the design choices.
To sum up this excellent series, it may be said that every serious hobbyist should consider this a 'must read'. The A-100 is not simple, nor inexpensive. To undertake a build means you need to have some real experience under your belt, and also somewhat deep pockets!
But once built, this amplifier is likely to give you 'an out-of-this-world' experience of fidelity -- the designer is a 'spaceman audiophile', no less!
All of the above (except the two AB amps) are fairly simple Class-A designs that guarantee great subjective audio quality and merits the consideration of the serious hobbyist. As mentioned earlier, the aim of the JLH enthusiast shall remain the basic consideration in such an endeavour-- to compare and see where these designs stand with respect to the JLH Classic amplifier.
This is "Work in Progress", and so you may expect additions to the above list of delectable Class-A amplifiers.
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Very valuable article
ReplyDeleteAmazing insight into world of class A
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