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Old 21st October 2009, 01:17 PM   #11
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Second episode of the long-running tutorial.....

Quote:
Should I? Are the mechanisms different from the ones in other amplifiers?
Perhaps I'm misunderstanding you here - you said the offsets should be similar on both ICs. So you could have been meaning one of two things - that its a design goal to make them similar, or that the implementation we're discussing (I take it from AN-1192) will result in similar offsets. I'm though wondering why offsets are a problem here, given that they'll be of the order of low tens of mV worst case whereas gain errors will result in greater imbalance when using 0.1% resistors and a 100uF cap to determine gain.


Quote:
You should probably ask the designer or the datasheet author. I can only guess that they tried to match Ri to Rb in an attempt to improve CMRR and thus decrease DC offset.
I can't see how 'improving CMRR' makes any difference to DC offset - they're quite unrelated in this particular configuration. I wouldn't dream of asking the appnote authors, its obvious to me they're a bit out of their depth here.


Quote:
What do you base yours upon that he didn't? Infallibility, because his name is well-known?
No, because he's a highly competent, experienced analogue engineer and would be very unlikely to make the really basic errors these writers have made. So, back to my original question - what's your hypothesis based on?


Quote:
I don't know, how it is going to cause a sharing problem, and I didn't write that it does.

It makes little sense to filter the non-inverting input at 3,38 Hz and the inverting input at 0,16 Hz. Those two filters should have similar corner frequencies and add up to a reasonable quality factor. More than half an octave of separation between them should be an exception.
I'm totally lost as to the rationale behind these remarks - to me it looks like some dogma having no bearing on reasonable design. In the world of electronics, 'quality factor' (or 'Q') means something totally different. So what meaning have you ascribed to the term here?


Quote:
It has no advantages that justify its size and cost.
We're speaking here of the 1000uF capacitor - I've already set out the advantages of it - improving the low frequency current sharing between paralleled amplifiers. Its physical size can be fairly small as the working voltage isn't great. Alternatively if you object to the size and cost, there's always the option of scaling up the values of the gain setting resistors. Increasing the 20k to 100k would mean a five fold reduction in this capacitor for example so a 220uF would do fine. Or you could go the way of the other designs in the appnote and use servos - bear in mind though they're considerably more complex and costly than the bigger cap.


Quote:
You will find few amplifiers with a 1 mF capacitor in the inverting leg, if any.
That might be for varied reasons - I've given one above - the gain setting resistors might be chosen higher. Another reason might well be that the amp uses a servo rather than a big capacitor - electrolytic caps aren't by and large considered high-end components and plenty of designers prefer to avoid them all together. In any case there's no need for such a large value when amps aren't being paralleled. I think you'll find few commercial amplifiers out there based on paralleled chip amps.


Quote:
Are you prepared to accept the accumulated experience of the majority of amplifier designer's as proof?
Not as proof, no, but certainly as strong evidence. However your statement above doesn't follow logically from your earlier claim since I've explained a few reasons why commercial amps may well not have 1mF caps. Nevertheless, since you've introduced evidence of amplifier designers into the conversation, have you heard of Doug Self ? He sometimes contributes here and he's written quite a lot on amplifier design. I happen to have a copy of his book 'Audio Power Amplifier Design Handbook' open in front of me. On page 220 of this tome, he presents a fully discrete design ('Trimodal Amplifier') which does indeed use a 1mF capacitor in this very position. Which amplifier designers are you going to cite who disagree with Mr Self ?


Quote:
I also refer to those resistors as load-sharing resistors out of habit. The truth is probably more that those resistors act as buffers between the IC outputs, so that any imbalances can be converted into heat in them, instead of the ICs' output stages. Close matching them is necessary, when you take the ICs to their voltage limits as I explained before.
But you've not explained at all why close matching the sharing resistors is necessary. If you did the math, you'd find out why its not. In this case, I'm not going to do it for you.

Last edited by abraxalito; 21st October 2009 at 01:20 PM.
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Old 21st October 2009, 04:15 PM   #12
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Originally Posted by abraxalito View Post
Not really, since a lecture is generally unidirectional. Here I'm interacting with your remarks so its more of a tutorial.
Seems I was at least right about the nit-picking part.
I am not sure, whether I want to spend a lot of time with your 'tutorial'. I expect that in the end we will rather come to realize that we interpreted each other wrongly, than really learning something new.

An example for what I mean is your following contribution...

Quote:
Originally Posted by abraxalito View Post
A 20% change in the 100uF cap gives of the order of a 0.5% change in the gain at 20Hz, so your relatively expensive 0.1% resistors are rather wasted.
... which led me to the conclusion that you find the use of 0,1 % resistors a waste, while you wanted to convey that you find the use of a 100 F capacitor inadequate.


Quote:
Originally Posted by abraxalito View Post
This is the same (to the number of significant figures in the calculation) as having no capacitor at all, so the value of the relatively expensive 0.1% resistors hasn't been thrown away.
What makes you think a 0,32 % gain error means throwing away the value of the relatively expensive 0,1 % resistors? Compared to the ~ 2 % error with 1 % resistors the improvement is significant. We can only speculate about how and why the engineers at National came to the conclusion that they did not need a gain error as small as 0,2 %.


Quote:
Originally Posted by abraxalito View Post
I accept your more accurate figure above of 0.32% - which only makes my thesis stronger and yours weaker.
I don't even remember setting up a thesis. And calling anything in your posts a thesis is strongly overstating its importance.


Quote:
Originally Posted by abraxalito View Post
I wouldn't dream of asking the appnote authors, its obvious to me they're a bit out of their depth here.
Now that is a bold statement. If you think, you can do better than they, go ahead. You will find many eager readers here. Although you lost my attention completely by that. Save your time on completing your 'tutorial', because I won't read or answer it any further.
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Old 21st October 2009, 10:32 PM   #13
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Quote:
Originally Posted by pacificblue View Post
Seems I was at least right about the nit-picking part.
I am not sure, whether I want to spend a lot of time with your 'tutorial'.
You're not the only person here - if there are others building paralleled chip amps, its possible they might find my contributions helpful. As for the 'nit-picking' claim, you've no evidence that I'm nit-picking - this is attention to important design details.


Quote:
I expect that in the end we will rather come to realize that we interpreted each other wrongly, than really learning something new.
What I've come to realize so far is you've interpreted me wrongly. Another thing I've come to realize is you're not interested in admitting to many of your mistakes. Never mind, there are others here who may learn from them.


Quote:
An example for what I mean is your following contribution... ... which led me to the conclusion that you find the use of 0,1 % resistors a waste, while you wanted to convey that you find the use of a 100 F capacitor inadequate.
Yep, its obvious you made an error of interpretation.


Quote:
What makes you think a 0,32 % gain error means throwing away the value of the relatively expensive 0,1 % resistors? Compared to the ~ 2 % error with 1 % resistors the improvement is significant.
I see you're still confused, even though I've explained a couple of times so far. Well, maybe third time lucky! I've never considered 1% resistors adequate to the task here - the sharing will be way too poor if they're used. So I agree with you, but bringing 1% resistors in here is a red herring. What you don't seem to realise is that the 0.32% error is due to the capacitor primarily, by increasing its value we reduce that figure.


Quote:
We can only speculate about how and why the engineers at National came to the conclusion that they did not need a gain error as small as 0,2 %.
You may continue to speculate, I have no need to because its obvious - they're not competent in this particular appnote. If speculation's your thing, here's one : perhaps they were a couple of students who National gave a summer job to. If you look at that appnote again, you'll see they didn't specify a 100uF capacitor as you're suggesting, their circuit has 68uF which makes the matching worse than 0.32% at 20Hz.


Quote:
Now that is a bold statement. If you think, you can do better than they, go ahead. You will find many eager readers here.
I can do better, as I've been demonstrating these past couple of posts. I'll continue to explain to any interested readers here exactly how that appnote is flawed and how they can fix it.


Quote:
Although you lost my attention completely by that. Save your time on completing your 'tutorial', because I won't read or answer it any further.
Again you seem to be under the bizarre illusion that I'm only doing this for your benefit. That's kinda quaint Your loss that you're not paying attention, not mine.
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Old 22nd October 2009, 08:07 AM   #14
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Quote:
Originally Posted by abraxalito View Post
Do show your calculations. I'm guessing they went something like this :
Quote:
Originally Posted by abraxalito View Post
Do please show your reasoning.
Quote:
Originally Posted by abraxalito View Post
Have you looked at the LM3886 datasheet to understand what causes an offset at the outputs?
Quote:
Originally Posted by abraxalito View Post
Do please show your reasoning, bearing in mind
Quote:
Originally Posted by abraxalito View Post
If he did indeed proof-read this AN, he's let various other howlers through along with these,
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Originally Posted by abraxalito View Post
Do please explain
Quote:
Originally Posted by abraxalito View Post
But you're missing the point here
Quote:
Originally Posted by abraxalito View Post
Do the math.
Quote:
Originally Posted by abraxalito View Post
I've just had a look at the AN-1192 and it seems they didn't get this one proof-read
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Originally Posted by abraxalito View Post
you've interpreted me wrongly.
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Originally Posted by abraxalito View Post
Yep, its obvious you made an error of interpretation.
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Originally Posted by abraxalito View Post
I see you're still confused,
Quote:
Originally Posted by abraxalito View Post
You may continue to speculate, I have no need to because its obvious - they're not competent in this particular appnote.
Quote:
Originally Posted by abraxalito View Post
I can do better, as I've been demonstrating these past couple of posts. I'll continue to explain to any interested readers here exactly how that appnote is flawed and how they can fix it.
Quote:
Originally Posted by abraxalito View Post
Your loss that you're not paying attention, not mine.
I am probably also wrong in assuming that this is a forum? Seems to be abraxalito's ego-shooter instead.
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Old 22nd October 2009, 08:56 AM   #15
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Originally Posted by abraxalito View Post
perhaps they were a couple of students who National gave a summer job to.
When you look at the top of AN-1192, you can read that the authors are John DeCelles and Troy Huebner.

Joh deCelles was already the author of AN-898 in 1993, so it is pretty unlikely that he was still a student on a holiday job, when he wrote AN-1192 in 2004.

From: All About Class D Audio Amplifiers
Quote:
Troy Huebner has been with National's Audio Group since 1996 originally working as a product engineer but switching to applications in 1998. His main area has been the high power class D and Overture amplifiers. Troy obtained a BSEE degree from Brigham Young University, Provo, Utah (1996).
Obviously the co-author was also not a student on a summer job anymore by 2004.
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Old 22nd October 2009, 10:06 AM   #16
AndrewT is offline AndrewT  Scotland
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can you two bring your combined knowledge together and propose what must be done with a parallel chipamp design to make it work reliably?
In particular detail any differences from the application note, an1192.
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Old 22nd October 2009, 11:13 AM   #17
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Quote:
Originally Posted by pacificblue View Post
When you look at the top of AN-1192, you can read that the authors are John DeCelles and Troy Huebner.
Yep, I already did that research. That's what hypotheses are for, to be shot down. Now, how about doing a similar exercise for your hypothesis that it was indeed proof read by Bob Pease ?
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Old 22nd October 2009, 11:30 AM   #18
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Quote:
Originally Posted by AndrewT View Post
can you two bring your combined knowledge together and propose what must be done with a parallel chipamp design to make it work reliably?
In particular detail any differences from the application note, an1192.
Sure, any particular implementation shown in that appnote?

Some of my comments just apply to how the components have been over-specified in various places - the tolerances on the output current sharing resistors and on the various input resistors. Correcting those won't affect the reliability, just improve the performance per dollar spent.

However, the issue of the capacitor tolerances affecting the gain matching does affect the reliability and performance. I'll post up something more detailed on the simplest paralleled schematic in that appnote soon.
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Old 22nd October 2009, 02:22 PM   #19
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Quote:
Originally Posted by AndrewT View Post
what must be done with a parallel chipamp design to make it work reliably?
From AN-1192:
Quote:
The objective is to provide simple high power solutions that are conservatively designed, highly reliable and have low part count.
For a beginner or a one-time-builder, who does not have sufficient knowledge to draw his own conclusions, I keep up my recommendation to stick to the datasheet and rather take a too conservative approach.

Here is another quote from AN-1192:
Quote:
1% gain setting resistors (Ri and Rf) will give good results but it is recommended 0.1% tolerance resistors be used for setting the gain of each op amp for closer matched gain and equal output current and power dissipation.
Once the gains are matched so that both ICs deliver virtually the same output voltage swing, the load sharing resistors come into play. The tighter the heatsink and the higher the rail voltage, the tighter should the tolerances of those resistors be.

In the end each designer has to decide for himself, whether his amplifier can tolerate a current difference of 5 % or if he needs those hard to source 1 % power resistors. If he cannot assess that, he should take the safer approach or be prepared to spend time and money for repairs, re-designs and/or re-building.
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Old 22nd October 2009, 02:35 PM   #20
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A stereo pair of parallel chipamps requires a minimum of 4 output resistors.
Buy 6 (two go into stock) and series connect them using a 317 CCS passing 500mA. Set your DMM to 200.0mVdc and measure the voltage drop (~50.0mV) across each resistor.
Pick a pair that have as near as possible the same voltage drop. Now pick a second pair.
That will give you two pairs of near +-1% tolerance 0r1 resistors.

Alternatively, buy 40off 1r0 1% 600mW metal film. Just parallel up 10 of each to give a 6W +-1% 0r1 resistor. The simple act of paralleling 10off will statistically give <0.5% variation.

That sort of useful information can be used by any competent builder.

If he/she can't do that then they should not be building a parallel chipamp.
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