JLH dual rail version HELP required.

[farmerjack61]

Hello from Tassie.

I purchased 4 of Jim's Audio pcb's to make a later version, dual-rail JLH amp; the schematic an pictures are attached below.

*It is my intention to use a 200VA 12+12 transformer to run all four.
*I would like to use some 2n3055 output transistors that I have on hand.
*I would like to use some 2n3019npn & 4033pnp l.f. driver transistors that I have on hand also. A970 are great for me too.

My plan is to run tight +-14VDC regulated rails, if I can squeeze it out without too much ripple. I'm going to use the big 7.5A regulator LT1083 inbetween two 10mF caps on each rail. I will incorporate a trimmer in to the feedback loop to adjust from say 12-16V and go from there.

Vpk for a 12W amp module is 12V

Ipk for the same amp is 2A.

There are four output transistors in the schematic below, two for current control and two to throw the volts up and down - they are rated at 250V and 16 Amps! which seems a little over the top for my humble machine! and at $3-5 a pop they are far too expensive because the boards not only use these hefty devices, they are doubled-up also. Meaning that, I need 16 output devices for my 4 amps if I use the paralelled devices shown on the schematic.

The two modules that will be driving my tweeters may be quite happy to run on just 2 devices per board. Any ideas welcome. This brings me to the next question.

Thermal considerations: The output transistors specified in Jim's original schematic have a junction to case thermal resistance of 0.7degC/W. And the ones I want to use have a resistance J>C of ~ 1.25degC / W.
Question1; Which transistor of the two will run cooler given a matching heat-sink and operating conditions?
My heatsinks; I have two largish heatsinks of about 0.3degC/w and intend to mount one bass and one treble amp module onto them using a machined 40mm x 6mm angle bracket for max surface contact.
Question4; Exactly how hot will my heatsink be using the data I have supplied?
Question2; Are the 2n3019npn & 4033pnp a fairly good substitute for the C3421 & A1358 ( labelled C3241 on the schematic!)?? I want to go with the tin can drivers because they look vintage (silly, I know...), but if they wont cut the mustard then they can stay in the box and then I will use bd139 or 2sd699 or MJE340 perhaps - any ideas are welcome, what's your favourite driver for this application?

I know that I am asking a lot of questions here, so if each of you could just have a stab at any one question, or make a comment, then I will eventually get there. I am in no hurry and I really do need a bit of help.

I am a wood machinist and joiner by trade and I have made a few stab-in-the-dark assuptions here, based on my limited knowledge, so please rip me into gear in the places where I got it wrong!

Thanks again; farmerjack

[farmerjack61]

I made a blue when I was posting the pictures above and cant fix it in the editor. Here is the correct one;

[Ian Finch]

Nice plans and fair questions but are you sure that following the regulators with 10mF caps is a good idea? Hint: peak charging current and regulator response.
If you are going to have regulation, it may as well work properly, particularly for the negative rail where the front end is prone.

[farmerjack61]

Originally Posted by Ian Finch

Nice plans and fair questions but are you sure that following the regulators with 10mF caps is a good idea? Hint: peak charging current and regulator response.
If you are going to have regulation, it may as well work properly, particularly for the negative rail where the front end is prone.

Hello Ian, I hope you are experiencing some better weather up in the land of the great banana. Last night, here in Devonport (apple country)! I went to bed early, only to be woken with a start. A huge bang.

I arose in a deshevelled state to find my front door wide open and a heap of dust floating around. I think my house may have been struck by lightning!

I am thinking about heading up north soon to study electronics engineering at USQ. I could try to do it by correspondance but I have a mate here doing the same course, and he is struggling without the peer support.

I have been thinking about what you said about the caps and wonder if it would be better to put my regulator after the two big caps and before an 820 - 2200uF cap which the regulator can replenish sooner.

Cheers, and bye for now, Phil

[Ian Finch]

Wow, never heard of that sort of lightning strike
Yup, weather is balmy for the start of spring and skies clear blue here. I'd be out for a surf if the water temp, wasn't still icy. USQ is a good campus but it's different to Tassie in every way. I threw in a scientific career for woodturning but I still wish you every success in your studies - it's a great journey.

The biggest caps. on the amp side, which should be placed close to the output transistors, don't have much benefit over a few hundred uF and that applies to class AB at higher, 50-100W rating. Here you have only around 10W, so the cap could be smaller still. I know it's fashionable to shoehorn in the most humongous caps that can fit but there is a line between useful and wishful thinking.

Spend up and put a couple of nice, low ESR, max. 330 uF beauties like Panasonic FCs or the equivalent in the usual faves from Nichicon, Elna, Rubycon etc. - right on the relevant transistor lead if possible. Actually, when you look at three terminal reg application notes, the maximum cap. normally seen at the reg. output is only in the order of 10uF, to ensure the reg. can beat any likely error and give optimum ripple reduction. Otherwise, regulation becomes decreasingly effective as a noise filter.

[Jonathan Bright]

Hi Farmerjack.....greetings from Brisbane.
Just a thought. Have you tried running your questions etc through the other big JLH thread?
It is called "JLH 10 watt Class A amplifier" or something very similiar.
It is quite large (around 2,500 posts) and all the JLH "Heavy hitters", 1969 and 1996 versions hang out there....just a thought. Also have you seen Geoff Moss's site? Just Google "class A amp site" and it should pop up. It is 99% devoted to this amp'.
Cheers, Jonathan

[farmerjack61]

Originally Posted by Jonathan Bright

Hi Farmerjack.....greetings from Brisbane.
Just a thought. Have you tried running your questions etc through the other big JLH thread?
Cheers, Jonathan

Hi there John. I will go there now. Actually I will go there tonight after I clean up all the horizontal trees on my mates block. Had a bit of a breeze last night.

Can you tell me the name of that book about the mastadons.

Cheers Phil

[Jonathan Bright]

Just so there is no claim against me under any false advertising law the book is not about mastadons!
It is a quote (very slight misquote I realised later, actually) from one of the works of the English author P.G. Wodehouse. Died about 1972. He wrote something like 94 books I think. I have about 57/58 last count. To narrow the search down a bit it was definitely one in the Jeeves/Bertie Wooster series so that reduces the sample size to around 10 I suspect. Those who study Wodehouse maintain that he had a phobia about aunts as they turn up frequently and almost always in a "bad light" with the sole exception of his robust "good and deserving" aunt Dahlia Travers. She's one of the great creations of English literature........Wodehouse himself was "farmed out" as a child to aunts in England by his parents who were stationed in India. The amateur psychologists have a field day. His most distressing Aunt is the fierce Agatha Gregson wife of the delightfully named Spencer Gregson.
So that is the best I can do at the moment. I re-read them every few years so will try and remember to post when I find it again!!!!!

[Ian Finch]

O/T...Hi Jonathan, let me spoil your fun with this from "Jeeves in the Springtime."..Aunt is calling to Aunt like mastodons bellowing across primeval swamps." Google is sometimes amazingly useful but now you'll have to edit your signature! I rather liked your version.

[Jonathan Bright]

Hi Ian thanks, appreciate that. It was going to take a LONG time to track it down "manually". I am suitably penitent and have amended the quote. I normally have a good memory but when I re-read it some time ago I realised it was wrong. I suspect I confused "mastodons" with "mammoths" and subconsciously subsituted "frozen wastes" for swamp.

I would hate to misquote Wodehouse as, like many others, he does more for my mental heath than just about anything else and millions upon millions like myself owe him a large a debt of gratitude.

But as they say; "It is the cracked people of the world who let the light in!"

Cheers, Jonathan