JLH 10 Watt class A amplifier

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#9,741
Back in the 1970's, I tinkered with what is essentially the JLH circuit because I had a very limited material resources. But I was never happy with the results, so I moved on to better circuit designs. I had never heard of JLH until recently. So, I find it amusing that the JLH circuit has so many fans today. Given enough resources and hype, most anything will fly?
 
#9,742
The Prof said:
Good morning friends... It was during the Covid lockdown days that I noticed how the John Linsley-Hood ('JLH') Class-A amplifier had come past its Golden Jubilee, and that needed a 'celebration'. Naturally I wanted once again to build an amp that I had built for most of of my friends back in the 1970s. (Ah, then we were all young, and went chasing after high power amp designs...that's another story!) Now I wanted to go 'deep into' JLH's classic design and decide to build the version I thought best reflected my preferences. But sadly scores of useful links were dead, and the DIYAUDIO thread had grown humongous (just a couple of hundred posts to reach 10,000!). So I set about reading that and culling info from the collected wisdom of the group. Also, I dug up my old PC backups.

This took time, and what originally was planned as a Golden Jubilee Tribute to the Master is ready for posting only by now. Better late than never, I guess...

Through my blog posts I wish to share my collection of published material by JLH, with the stress on the "Simple Class A amplifier" first published in the Wireless World, April 1969, issue. My feeling is that for the ardent JLH fan and DIYer, my blog will serve as a "single window" to archival information, other relevant material, and gleanings from the various forums. Hope most of you will find this compendium helpful in your DIY pursuits.

https://jlhclassicdesigns.blogspot.com/2024/11/the-jlh-gateway-golden-jubilee-tribute.html
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Wow, you spent quite some effort on this, well done !
 
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#9,743
@ Bigun
Thank you! How is your 'marathon' design playing now? Yes, I do recall the eight pairs of rivetted power devices and the rest! Do share your current experience if you are still using it, as I suspect you should be. I am sure to mention your build too in one of the upcoming posts, along with other 'landmark' builds/mods ... as soon as I have some free time!
Warm regards
-- UKP
 
#9,744
@ steveu
Good to see an old timer here. But I must admit I was intrigued and tickled, and a bit ruffled (in that order!) by your observations.

IIRC, I was a poor college student back in the 1970s with rather extreme 'resource crunch'. Many of us built the JLH Classic 1969 because of that! My first build had a cheap and crazy heatsink-- a LARGE sheet of aluminium scrounged from the nearby bus body building shop...but it worked!

We somehow wandered away from the JLH, as youngsters are wont to, exploring other alleyways. In my case, I wanted to try out many other simple class-A designs, and that, honestly, was an eye-opening experience, which happened over a few decades of DIY efforts. I am happy that some of those builds which were gathering dust were the ones that caught the ears of the younger set in my personal group, who ended up borrowing it...and not returning it, until I built/helped build one for them! Many of these youngsters in their late twenties and early thirties already owned fairly expensive systems, most of them were keen attendees of audio shows, some of them were amateur musicians, and knew what they were doing.

Good that you are amused that the JLH has a bunch of followers, but kindly note that NOT ALL of them are as old as the design from the previous century. IMHO that concluding sentence of yours, sir, is uncharitable. Your insinuation that it is the 'hype' perpetuated by a bunch of 'devotees' that is keeping the JLH 'flying' is pure bovine excreta. Have you tried listening to a JLH amp anytime recently? If you are not condescending, do find some time and rummage in your 'junkbox' and cobble up a Classic JLH ... and do some listening (of course, if your speaker is not one of those 'high end' behemoths with a bunch of fancy drivers and a complex crossover that easily flummoxes a trained EE!) before you bring your judge's gavel crashing down.

Sir, this, I understand, is a hobbyists' forum. Nobody is selling anything here, least of all snake oil and allied stuff! Nobody here is keeping anything 'flying' with lots of hot air!

Thank God (and JLH!) that the JLH Classic amplifier can hold its own even today!!

Warm regards
-- UKP
 
#9,747
A request to members:
Could somebody point me to some biographical details of Geoff Moss, the 'curator' of things JLH ? How is he-- I had heard he was having some health issues for some years.
Any information in this connection would be of help in making my JLH Gateway compilation more detailed and factual.
Thanks in advance to all.
 
#9,749
#9,750
Personal preferences are exactly that, and don't we all respect those?

If only jxdking was a bit less busy and had gone through the JLH 1996 article in Elecronics World, he would have come across the paragraph where JLH mentions that an easy way to avoid the output coupling capacitor is to change the input circuit topology into a long-tailed pair. In the very next para he clarifies why he did NOT choose to do that.

I quote:
"... I am reluctant to do this because this would alter the overall gain/phase charactristics of the amplifier. It would also require additional high-frequency stabilisation circuitry, with all its incipient problems of transient intrmodualtion or slew-rate limiting."

And while on the topic of the LTP, it would be educative to be reminded that the jury is still out on the question of how the LTP treats the 2nd harmonic, and how that affects the overall harmonic profile of the amplifier, any amplifier. Sadly I am not technically qualified nor trained to prove how that happens.

As regards my posts here, the aims have been clearly set out in my blog as well as in my first post here (# 9,737). Most of the JLH followers would like to try and build the amplifier as per the original design, or, with minimal changes so as not to alter the core circuit topology in any way.
My task here is to help that with a repository of original articles and papers, and not to peddle my personal preferences.

I trust I have been able to do justice to that aim, as well as to pay my own humble tributes to JLH through my blog posts.

Warm regards
-- UKP
 
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#9,751
The Prof said:
And while on the topic of the LTP, it would be educative to be reminded that the jury is still out on the question of how the LTP treats the 2nd harmonic, and how that affects the overall harmonic profile of the amplifier, any amplifier. Sadly I am not technically qualified nor trained to prove how that happens.
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The amp is a class A amp. With ltp and current mirror, it would be <0.01% THD. That means all harmonics are gone. Also always put a miller cap (68pf or 100pf) even the original doesn’t need one.
However, if you are into the shape of the harmonics, you’d better keep the original circuitry.
 
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#9,752
I appreciate that. Yes, I have built and listened to various versions. I had a lot of difficulty deciding between the original version and the one with the CCS. Later with the help of younger members of my personal group here, I came to a "mixed decision" -- I would keep the single supply and the bootstrap capacitor (which in certain ways helps with stability when you test with a variety of output devices), and go for some current injection in the first stage. What I am planning to test next is using a Darlington device for Tr1, and see how that sounds!
All these are on the personal level.
My attempt with the blog is to offer a single access window to the JLH fan and DIYer. Only after all the original material is carried, and after a couple of posts about various well-known mods etc, THEN perhaps I will dare to post details of my build and how that sounds!!

Warm regards,
-- UKP
PS: Maybe the Admins will advise me if I should open a new thread for posting these links ...?
 
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#9,753
An appeal to the esteemed members of the Forum:
As you probably know, I am compiling a "single window source" of the published articles (mostly audio related) of John Linsley Hood. A big lacuna in my present collection is his valued articles published in the UK magazine High Fidelity News and Record Review (HFN-RR) in the 1970s and 1980s.

I shall be most grateful if anybody who has the issues, or has access to the archives of HFN-RR, would kindly share digital copies of those articles for inclusion in my posts. If you would kindly PM or email me, I shall share the issue details. My email ID is : ukpanickar (at) gmail (dot) com

With advance thanks and warm regards.
-- UKP
 
#9,757
The JLH Memorabilia Part-3:

https://jlh-memorabilia.blogspot.com/2024/12/jlh-memorabilia-past-perfect-3.html

A really looong post ... and I haven't covered even half of the 1980s ...
Do read and enjoy, and perhaps embark on a new build or two. Surely some of the JLH designs haven't been matched in subjective sound quality till this day.
Hope you will comment and share (and contribute any material you might have (or have access to) to make the collection complete.
Thanks and warm regards.
-- UKP
 
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