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18th Apr 2012, 8:23 pm | #41 |
Dekatron
Join Date: Jan 2004
Location: Newcastle upon Tyne, Tyne & Wear, UK.
Posts: 8,194
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Re: JLH Class 'A' Amplifier
Hi Mike, a good current limited power supply is very useful for fault finding, so you can check each channel seperately with the current limit set nicely low.
If there is an overcurrent problem the voltage reduces and no harm is caused. Plenty of home brew designs available. Ed |
19th Apr 2012, 6:40 am | #42 |
Hexode
Join Date: Feb 2007
Location: Luton, Bedfordshire, UK.
Posts: 456
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Re: JLH Class 'A' Amplifier
Cheers ED I will check that out. I spent half an hour last night taking voltages and now have a good idea where the problem lies.
Regards Mike |
2nd May 2012, 4:39 pm | #43 |
Hexode
Join Date: Feb 2007
Location: Luton, Bedfordshire, UK.
Posts: 456
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Re: JLH Class 'A' Amplifier
A bit of an update on my JLH 69 build.....finally done ...apart from maybe a mesh top....found the problem.....I stupidly shorted the power supply & it seems I must have burned up a track on the PS PCB...luckily I had a spare board. How does it sound? always difficult to describe but very good indeed considering I haven't really set it up properly yet. Not quite as smooth as my valve SET amp but not too far off at all & probably more grunt. Seems to drive my old floorstanding 15ohm wharfedales easily....I'm getting 32v out of the power supply which probably helps...no doubt a bit more fettling to do but well worth the effort.... definately glad I went with supersize heatsinks as it certainly gets hot!!!!
Mike |
8th May 2012, 10:40 pm | #44 |
Octode
Join Date: Aug 2006
Location: Stoke-on-Trent, Staffordshire, UK.
Posts: 1,082
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Re: JLH Class 'A' Amplifier
hows it going Mike some say the amps will sound better when run in
regards Trev |
9th May 2012, 9:23 am | #45 | |
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Re: JLH Class 'A' Amplifier
Quote:
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9th May 2012, 6:14 pm | #46 |
Octode
Join Date: Aug 2006
Location: Stoke-on-Trent, Staffordshire, UK.
Posts: 1,082
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Re: JLH Class 'A' Amplifier
I could agree with that comment !
regards Trev |
9th May 2012, 6:52 pm | #47 |
Hexode
Join Date: Feb 2007
Location: Luton, Bedfordshire, UK.
Posts: 456
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Re: JLH Class 'A' Amplifier
Well it's not getting run in and/or I'm not getting used to it... all was going well ....managed to adjust the voltage to half of B+ on each channel then when adjusting the Qcurrent....and the clip probe on one channel slipped off, brushed something on the board and it looks like its fried the rectifiers again I'm getting full voltage at the transformer end of the power supply and nothing past the rectifiers.....I've ordered 10 spares and and some new clip probes. As soon as they turn up I will get the amp back up and running....initial results were very promising and I find the ridiculous levels of heat fascinating. I will report back once done...I might need a steer on a couple of resistor value tweaks to get the current dialled in for the values recommended for a 15ohm load by JLH in his article. Trev I've been spending my time instead working on finishing my attempt at a loftin white 2A3 job....but that's another story!
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9th May 2012, 11:10 pm | #48 |
Octode
Join Date: Aug 2006
Location: Stoke-on-Trent, Staffordshire, UK.
Posts: 1,082
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Re: JLH Class 'A' Amplifier
Yes it seems strange the low grade of heat from a transistor class A transistor amplifier while 30 watts from a valve unit seems easier to get rid of.
I suppose its due to the fact that there is only conduction and hardly any radiation ? Well it has to be said you are a trier i feel sure that the experience is character building good luck with the lofton trev |
24th Sep 2012, 5:00 am | #49 | |
Retired Dormant Member
Join Date: Sep 2008
Location: Cooktown, Australia
Posts: 8
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Re: JLH Class 'A' Amplifier
Quote:
I met Hugh at South Queensferry circa 1976 and second your opinion of a very nice & competent young man. |
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24th Sep 2012, 8:04 am | #50 |
Moderator
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Re: JLH Class 'A' Amplifier
Indeed the same, Ricardo.
Nice to know he's got a good reputation down under. He lives a mile or so away from me, though we don't happen to meet very often. I think the last time was when I spotted his old Humber broken down (ignition system sulking) and I gave him a tow home. The debate between JLH and HPW was interesting. John Linsley-Hood was a talented amateur who put immense amounts of time and effort into what he did, and he did a lot. I never met him, but he came over as a decent guy in his writing. Where he hit his limitation was in the analysis of noise contributions in series and shunt feedback amplifiers. Hugh had been much more deeply engaged professionally in just these things and did a classical analysis. The classic analysis is itself a simplification, blaming all noise on a pair of fictitious noise sources, one a current source, the other a voltage source. A more elaborate study would use simulations of a greater number of noise sources distributed in the circuit. This sort of thing relies on more computational horsepower being available, the classic analyses just need algebra. In 1976, Hugh was working on the first SLMS (selective level measuring set) which had some wonderful design in its receiver, low noise and high dynamic range. If you met him in '76, then I'd have been just over the partition in the next pig-pen. It was a great time to be there If you want to see some of Hugh's best analysis, try "Sources of intermodulation in diode ring mixers" HP Walker, proc IERE. Back to Linsley Hood's class-A design, it was very good for the time, but it carried the ethos of minimising the number of active devices to the point where it missed a few interesting tricks. What Mike's run into is the problem with transistor audio power amplifiers that one hiccup in almost any place, through the wonders of DC coupling, turns everything everywhere into smoke. THanks, David
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27th Sep 2012, 4:29 am | #51 |
Retired Dormant Member
Join Date: Sep 2008
Location: Cooktown, Australia
Posts: 8
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Re: JLH Class 'A' Amplifier
I was still in school when I met Hugh. I think he was either just married or just about to. His wife/girlfriend whose name I'm ashamed to say I can't remember, made me a very nice tea.
A few years later, having designed the quietest moving coil preamp in the known universe and also the Mk4 Soundfield, I was inspired by Hugh to cross swords with Great Guru Baxandall. Fortunately I did it in private cos GGB cut me to ribbons. I used to worship JLH cos I'd built some of his designs. It was only when I'd met him in the flesh that I found out he was just a twiddler like the rest of us and had little of the insight that GGB & HPW showed. However, of his designs, he hit upon a very sweet spot in the little Class A. But I think its not to do with Class A but that it was very simple; only 3 stages, and that the distortion was set by the single ended input stage. Hence the predominately 2nd harm. There are a lot of really awful circuits where the BS is truly liquid, generated by famous names and requiring Unobtainium hand carved by virgins to work. As the designer of the Mk4 Soundfield, I sneer at mikes designed to make your girlfriend sound like Linda Ronstadt. I recognise & like certain old designs like AKG C12 but will never design a microphone to sound like them. I think its possible to capture the 'sound' of the JLH Class A with a Class B design of equal simplicity. This would not need the regulated PSU etc that people seem to think necessary for equal performance & good sound. But as a beach bum in Oz, I'm very unlikely to try this out. PS There are amplifier designs that don't self destruct if one device fails but JLH designs aren't usually of this kind. Last edited by ricardo_lee; 27th Sep 2012 at 4:34 am. |
27th Sep 2012, 9:28 am | #52 |
Moderator
Join Date: Mar 2012
Location: Fife, Scotland, UK.
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Re: JLH Class 'A' Amplifier
Hi Ricardo.
Hugh married Janet, the HP librarian at the time, and thereby set a precedent. Janet left to raise their family and her replacement eventually married another engineer from the lab, Bill Miller, who later left to join Linn, a firm populated with a number of his old chums. Bill headed Linn's R&D dept for a couple of decades. HP, audio, and librarians? Soundfield 4? Calrec? Hebden Bridge? An old colleague Peter Green went there, I seem to remember in the late 70's. The JLH class A works quite well, and is certainly possible to do quite a bit better, but it has a low number of components and a lot of people assume that means simplicity and purity. It gets built by some people who would be scared off by anything using more parts. This is good because it serves as an introduction, it shows people that they *can* build satisfactory transistor amplifiers. The next step is a big one, though, and it's largely invisible to those needing to take it: the concept of a toleranced design, one which will meet its requirements while allowing it to be made of components allowed to have spreads on their parameters. I rather enjoyed playing around with hifi while it was a science, but I went and hid when it changed into a religion. Cheers David
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Can't afford the volcanic island yet, but the plans for my monorail and the goons' uniforms are done |
27th Sep 2012, 1:30 pm | #53 | |
Dekatron
Join Date: Feb 2007
Location: Lynton, N. Devon, UK.
Posts: 7,081
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Re: JLH Class 'A' Amplifier
Quote:
Doug Self has much more insight - again it's clear from reading his writings. Don't overlook, however, that JLH's twiddlings produced some good results! |
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27th Sep 2012, 2:03 pm | #54 |
Dekatron
Join Date: Jul 2007
Location: Evesham, Worcestershire, UK.
Posts: 4,244
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Re: JLH Class 'A' Amplifier
Yes, and Douglas Self does it for a living. Many of his designs get top marks in the hi-fi press, despite his firm anti-subjectivism stance, and he's even starting to be properly respected among the DIY community at last. His books are excellent - highly recommended.
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28th Sep 2012, 1:10 am | #55 | |
Retired Dormant Member
Join Date: Sep 2008
Location: Cooktown, Australia
Posts: 8
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Re: JLH Class 'A' Amplifier
Quote:
I was lucky enough to work with Ted Ashley who designed the last of the great LEAK valve amps and also the first transistor ones. It was illuminating to hear HIS views of what they did right and wrong. Similarly with Peter Walker of Quad. They would have at SE valves etc. The hard part is identifying what is responsible for the 'good' sound. IMHO, for this little amp, it is nothing at all to do with Class A. I don't think LTP i/ps or measures to reduce distortion on this design are beneficial. But stuff to improve reliability & heat - definitely. Provided these measures don't increase complexity too much. And certainly nothing to do with capacitors & wires hand carved from solid Unobtainium by virgins. |
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30th Sep 2012, 10:04 am | #56 | ||
Nonode
Join Date: Jan 2009
Location: Papamoa Beach, Bay of Plenty, New Zealand
Posts: 2,944
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Re: JLH Class 'A' Amplifier
Quote:
Baxandall certainly comes across as having the higher level of insight. I have just re-read the chapter he contributed to Amos (*); he packs a high information density into what is an overview chapter yet it is very easy to read and follow. Strangely in retrospect, I do recall that he was once treated a little dismissively as an “old man of the industry” by one of the younger fellows (maybe Stan Curtis?) in the letters to the editor pages of WW during the amplifier/subjectivism “debate”. Regarding single-ended triodes, maybe there should be some kindness shown to these lonely creatures? In connection with another thread I was looking at the curves for the E55L output valve, and when triode-connected it is still impressively linear over quite a part of the slope. So perhaps with appropriate valve selection and choice of operating conditions, a very small SET could be made approach a “straight wire with gain”, or Self’s “blameless amplifier” state. But then it might not sound like an SET, which might be a problem... (*) Audio Power Amplifier Design Handbook, by Douglas Self Valve & Transistor Audio Amplifers, by John Linsley Hood (**) Chapter 14, “High-Fidelity Amplifiers”, in “Radio, TV & Audio Technical Reference Book”, edited by S.W. Amos Cheers, |
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26th Apr 2013, 9:48 am | #57 |
Tetrode
Join Date: Apr 2013
Location: Reading, Berkshire, UK.
Posts: 56
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Re: JLH Class 'A' Amplifier
I bult the JLH original design when it first came out using MJ481s as the output trannys. When it worked the sound was superb but the output transistors blew up with astounding regularity. Not sure whether it was them or me (as in construction technique) but If I can take some time off from other things I may have a go again! Not that the missus would be happy to have one of my bodges in the living room.
Cheers |