The JLH Gateway to Class-A :: 6
It would be interesting to take a look at the Class-A amplifier scene around the time JLH was thinking of an amplifier to replace his trusty Williamson amp. Of course, a few solid-state Class-A designs were there, but none had achieved cult status, so to speak. Contemporaries of JLH included well known and respected designers like Nelson-Jones, Dinsdale, I.M. Shaw, Arthur Bailey, J. E. Sugden, Stan Curtis and others.
It was in the mid-sixties that the young engineer James Sugden founded his company, coming out with the famous A21 amplifier, famed to be the first production Class-A amplifier. But the company website today claims that it was the Si402 model (not a very popular model, nor produced in large numbers, apparently) that has the honour to claim that title.
 |
| Sugden Amplifiers - Old and New |
Whatever, Mr Sugden published a design for DIY in the Hi-Fi News magazine for April/May 1970. In the article Sugden does acknowledge the designs of both JLH and Nelson-Jones, and comments in some detail about the JLH Classic amplifier of 1969.
 |
| Sugden 10 W amplifier |
It is evident that the young designer has a lot of admiration for JLH's design, and admits his own design has some similarity to that by JLH. Those who are interested may compare the two circuits, after ignoring the first emitter-follower stage of the Sugden design. It certainly is educative to study thoroughly the HFN article.
https://drive.google.com/file/d/1Yq1zlC8mkmHl1SOt67-LFGSALrrjOHWd/view?usp=sharing
Another class-A amplifier that 'hogged the headlines' in the 1970s was the Stan Curtis design, which went into commercial production as the Lecson AP-4, and attained some sort of a "high end" following at the time. Of course, it was a higher powered behemoth, heavy (15 mm thick front panels and half a dozen huge heatsinks!), hot and expensive! Soon after the company folded, the Electronics Today International magazine featured it as an advanced construction project. This put out about 60W plus of pure class-A power, and much higher output in class-B. According to the designer, it dissipated about 400W, with a quiescent current of 5A, and served the purpose of a room heater very well!
It may here be noted that neither the circuit topology nor other factors put it in the same ballpark as the JLH, or other similar amplifiers of the period, or of later times. It is presented here only for the sake of "completeness" and to provide the reader with a fuller technological picture of the era.
https://drive.google.com/file/d/1IEg_m1EZDOt1uXI5VN_L6rurVTR0Ruq4/view?usp=sharing
A year after the publication of the JLH Classic in 1969, Wireless World magazine carried the design for an "ultra-low distortion" class-A amplifier by Nelson-Jones. He too preferred the superior quality of the class-A configuration "... in an attempt to get a little nearer to perfection". Here he quotes JLH "... the basic linearity of the amplifier should be good, even in the absence of feedback", so that feedback should NOT be viewed as a shortcut to overcome the ills of a poor design.
Nelson-Jones opts for a quasi-complementary topology for the output, as better NPN devices were available at the time. The excellent explication of the circuit will be very helpful, particularly for those who are not well-versed in the working of solid state amplifiers. A noteworthy plus point of the design is its ability to withstand a full short-circuit of the output.
It is interesting to note that Nelson-Jones adopts the ploy of not insulating the devices from the heatsinks in order to maximize thermal transfer from the device to the heatsink -- the heatsinks being mounted on insulated standoffs. As was the practice in those days, there was a lot of hand-holding for the constructor by the author. As promised in the title, the amplifier did meet the very low distortion and noise thresholds, no doubt. But when it came to matching the disarming simplicity and elegance of the JLH design, it left quite a bit to be desired.
https://drive.google.com/file/d/1evuQRxaJYbLzCaj7Fsn7ZId7qRXB0se8/view?usp=sharing
Those among you who have a few grey hairs (or, like me, no hairs!) will recall the name of John Curl -- the famous audio engineer who was the 'soundman' of The Grateful Dead rock group and who dreamed up their "wall of sound" stage setup. He was a respected designer with some unconventional, but technically sound, views on sound and fidelity and all that, especially when it came to the "sound" of solid state amplifiers. He was the designer of Parasound amplifiers since 1989, as many of you know.
 |
| Class-A Amplifier by John Curl |
Here is something "straight from the horse's mouth":
" ... All of my Parasound designs are based on a 'formula' that I have developed over the decades. There are other approaches that work as well, and although I do disagree with Doug Self and Bob Cordell on a number of things, their writings can be helpful as well, so long as you keep an open mind about my approach, which they will often criticize.
I make amps, complementary jfet input, complementary balanced throughout, fast, and as Class A as possible. I tend NOT to use an output coil or an input cap. I servo the output for DC offset, instead. That is about all there is to it..." To get a better idea of Curl's take on amplifiers, do read the interview linked to here: (text of interview posted by Parasound) You may check out his early class-A circuit to get a taste of his design approach, but be warned that it may not be an easy formula for the lone DIYer. Here is an interview with John Curl, in which he shares his design outlooks.
https://drive.google.com/file/d/1JN5oZa7fCLUpOQaVdcJzY2iQjW_xFEiA/view?usp=sharing
Then, of course, there is the little known (hidden!) gem of a 10 W amplifier by A. H. Calvert -- a 'younger cousin' of the JLH, perhaps! The circuit was published 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! What more do you need from a simple amplifier?
 |
| The Calvert Class-A Amplifier |
As always with class-A, the heatsinks need to dissipate about 4 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 design certainly deserves that IMHO.
Now it would be interesting to take a cursory look at a couple of non-Class-A contemporary designs. Here are a couple from Arthur Bailey and another class-AB design from our own JLH.
Though Dr Bailey's amplifier designs of the period were NOT class-A, they are examined here as they were indicative of the state-of-the-art of the 1960s, a period into which the JLH Classic with its Class-A topology and "blameless qualities" emerged. The simple 20 W Bailey amplifier (WW, Nov 1966) was a class-B design with a driver transformer (a leftover from the valve days), employing PNP Germanium output devices. The author also offered a sturdier output stage with Silicon NPN devices for when "... severe operating conditions are encountered". The performance of the amplifier is "... well up to the standards expected of the best valve amplifiers", to quote Dr Bailey.
https://drive.google.com/file/d/1MjaUpDmxKuxL68PJRdagCPsueKMQ_-Zz/view?usp=sharing
The later 30 W amplifier design offered by Bailey in the Wireless World, May 1968, edition was more complex, and bears a greater resemblance to the currently popular solid state circuit topologies. The design did away with the performance degrading driver transformer, and employed the latest fully complementary output devices in a symmetrical, dual supply topology. This had a salutary effect in reducing distortion and ensuring good high frequency performance. The circuit also uses what is called a 'Vb-e multiplier' circuit to set and stabilize the bias for the output devices, a 'modern' practice followed till today. It basically is a class-B design, with some initial class-A bias ("standing quiescent current") to reduce crossover distortion. The amplifier has far better 'paper specs' than the best valve amplifiers of the period, but, the author admits, is subjectively only slightly better than the best valve amplifiers.
https://drive.google.com/file/d/1MK5oi7vkvjb3In8txdCjyr35jvdcmIlK/view?usp=sharing
It will be of some academic interest to compare these and other class-AB designs of the era with the class-AB designs of John Linsley Hood, which did "approach the performance of class-A amplifiers". ( 15-20 W Class AB Audio Amplifier -- A design with class-A performance, but reduced thermal dissipation -- by JLH, Wireless World, July 1970) The article is a testimony to the genius of JLH in finding clever solutions to design problems. He bases his design on his proven class-A design, but devises a circuit topology to precisely define the standing current of the class-AB output stage. The reader is invited to study the circuit explication presented by the Master here. The result is that class-A level distortion and other performance standards are met for the essentially class-B stage, with a higher standing current.
https://drive.google.com/file/d/1q9yM5yPROTghu26l6D_dJ8V1ao0-ppRe/view?usp=sharing
Though our primary concern here is with Class-A amplifiers, an overall picture of the hi-fi design approaches of the era and the comparative merits and demerits of the two topologies are well worth following. Also how some of the techniques of class-A design was applied to improve the performance of the class-B designs by JLH is illustrative and indicative of his insights and mastery of circuit design; hence their inclusion here in some detail.
Now that the JLH Classic amplifier has been studied in some detail in the background of the prevailing trends of the era, perhaps it is time to take up the thread of how the timeless design by JLH has set in motion a whole string of designs and modifications and tweaks by scores of designers and knowledgeable hobbyists over the ensuing decades till the present. Needless to say, a healthy interest in the little amplifier has been sustained through the last half century and more, and quite a few designs and mods have emerged, which deserve in-depth examination, so that it might add to the corpus of JLH-style class-A amplifiers, and surely serve as guideposts for the DIYer and the JLH enthusiast.
We shall bend our shoulders to that task in the upcoming posts. In the meantime, read and enjoy, and gather a deep breath, ready to dive deep into the waters of the JLH ocean!
* * * * * * * * * * * *
Comments
Post a Comment