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Old 10th May 2007, 10:27 PM   #21
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John, I think I can more or less agree with all that you said. It is important to know that one cannot rely on a simulation and that it has limitations. But that is true of most tools. Measurements are not always correct or even remotely correct. Just as one has to realize the limitations of measurements and the potential error sources in measurements, one has to realize the limitations and error sources in a simulation. Any tool can give the wrong result if handled wrong. (That is especially important to know if using tools like axes or knives.) A simulation program is basically not very different from other tools. It is just that it is so complex that it is much harder to see if one is using it correctly or not, and if it gives the intended result. Computers do make it easier to mess things up. Spice is a tool that can be very useful, if handled right. But it is not a necessary tool, as both you and many others prove (with the exception of IC design which I think would not be possible today withouth advanced simulation programs).

I think nobody has tried to claim that Spice results are always reliable. There are some people, though (not thinking of anyone in this thread) who seem to believe they are the only ones who are bright enough to realize that Spice results may differ from a real circuit and thus, everybody else must be stupid enough to think that Spice gives a perfect prediction of a real circuit.

Further, don't believe that everybody who likes Spice must be young. I have also used slide rules and mechanical calculators, as I am sure many others here have. That is fun to have done, but I see no place for such tools today, since pocket calculators and computers are much better replacements. There is however one tool that I can never dispense with, and which is the oldest of them all, pen and paper. Pen and paper is one of the most ingenious, useful and reliable tools mankind has ever invented.
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Old 10th May 2007, 10:49 PM   #22
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I would like to thank those who responded to my last couple of posts for participating in an impromptu experiment in reading comprehension and human psychology. (One of my degrees is in psych--old habits die hard.)
I trust the parallels between my post on Science as Religion and some of the responses are obvious enough.
As for my mainframe/network analogy, it was deliberately oblique, though not inappropriate. Again, the responses were predictable. In passing I'll note that it's almost always the ones on the butt-end of an analogy who complain that it isn't relevant.
John, at least, appears to have gotten my point, but that's not surprising, in that he seems to hold some of the same viewpoints that I do. I'm not saying that he agrees with everything I say--he may, he may not; he's a free man and has his own experiences and opinions.
You can't simulate for imaging. You can't simulate for dynamics. You can't simulate for clarity. These things exist...
*****
Brief aside--I'm always amazed by people who claim that imaging doesn't exist. Close your eyes. Point to the musician. Did your finger end up somewhere between the speakers? That's lateral imaging. Close your eyes again. Throw a wad of paper at a well recorded sax player taking a solo. Did the wad of paper end up behind the speakers? That's depth of image. Dynamics is harder to "prove" but I trust that most people who actually listen have heard amps that you can feel in your gut, whereas others don't produce the same visceral response. Etc.
*****
...yet it's not entirely clear how to manipulate a circuit to produce them in a predictable way. It's trivial to say that more capacitance in the power supply might help with dynamics, but there are examples of amps with huge power supplies that don't impress. Likewise it seems probable that lower feedback is beneficial for imaging, but it's not an open and shut case; note that I said beneficial, not necessary (just to head off a few protests--another one of those reading comprehension things...).
Yes, it's true that some of the subjective stuff is poorly understood, and as such hard to design for. That's part of what I'm saying. But the flipside of the coin is that--like the Aleph-X "not working"--you can't depend on simulations to tell you whether the circuit will actually "work." In the case of the Aleph-X, the simulations indicated that the topology wouldn't even idle, much less make music. Those were gross failures, easy enough to spot. But how about more subtle things? I happen to like imaging. There's no simulation software on Earth that will tell me whether one topology will image better than another. Dynamics? Likewise. And so forth.
You can simulate THD until the cows come home (English idiom meaning "for a long time"), but those numbers have only the loosest of correlations with reality because you're dealing with cloned parts; perfect in every way. Monte Carlo analysis can shake up the numbers a bit, but it's still a badly flawed approximation of reality.
How long will it take for the models to get better? I don't know. But it isn't there yet and isn't likely to get close enough in the next couple of years to accomplish the things that we need to do. If you want to know real answers, you've still got to build the thing. And it's likely to remain that way for a while yet.

Grey
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Old 10th May 2007, 10:49 PM   #23
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My sentiments also, Christer.

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Old 10th May 2007, 10:52 PM   #24
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I'd like to say that it appears that Nelson and I hit the Submit key at the same time. There's no possible way that he could have read my post and agreed that quickly.
He is free to agree with what I said or not, but his post should not be taken as approval without further input from him.

Grey

EDIT: Ah, I see he has edited his post. Good.
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Old 10th May 2007, 11:07 PM   #25
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Greg, aren't you missing the point? Assuming there are properties like imaging that do not follow automatically from the usual figures like low THD and low IM etc. they are properties we don't even know how to measure and probably nodody knows how to design for with predictable results. Of course you could not then expect a simulator to tell you in advance how the imaging of an amplifier will be. If it exists as clearly identifiable property, then obviosuly there is something lacking in our theory (or understanding thereof) and in out measurements. Spice is a simulation using very elaborate models of our current understanding of the electronics and semiconductor physics. No more, no less. If our theory is wrong or incomplete, how could Spice know better? Neither Spice nor any other simulator is designed to cope with X-factors, to use Bobs terminology, and neither is the theory in electronics textbooks. Are those textbooks also completely useless for amplifier design? I think not.

Spice also has very interesting uses apart from just simulating a particular design. It seems that it is far beyond some peoples comprehension to even imagine that, so I am happy to see that at least Rodolfo has realized that too.


EDIT: You must have read an earlier version of Nelson post than I did, but I think I can imagine what Nelson edited.
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Old 10th May 2007, 11:19 PM   #26
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Quote:
Originally posted by GRollins
...yet it's not entirely clear how to manipulate a circuit to produce them in a predictable way.
Interestingly, some of the best insights into these issues come
from negatives. You start with a circuit that has the qualities
you want, and you make a small change that degrades them.

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Old 10th May 2007, 11:58 PM   #27
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Right on, Nelson. My greatest leaps in design are often after I make a mistake.
One time, however, a business rival accosted me at the CES where we were both in 'business mode' including wearing business suits, and declared to me that I was a washout and a has-been at the old age of about 40. Two weeks later, I came up with the phono input circuit that launched the Vendetta Research preamp.
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Old 11th May 2007, 12:13 AM   #28
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Quote:
Originally posted by john curl
Bark, Bark! SPICE is OK, but you should know its limitations and that it can sometimes be misleading.

Absolutely. But a few take it's shortcommings a bit too far.............meow.
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Old 11th May 2007, 12:16 AM   #29
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Actually, we just take SPICE a little less seriously than some others. (dog barking)
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Old 11th May 2007, 12:29 AM   #30
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Quote:
Originally posted by john curl
[snip]You, newbe engineers, are taught in school how to use Spice, and you can't see a world without it. [snip]
John,

I am not a newbee engineer. I am probably as old or older than you. I have been using Spice for at least 15 years, using its powers for proof-of-concept work, and researching issues that are hard or vey cumbersome to do in actual hardware, like measuring the sum of several currentsall at once. I am well aware of Spice's limitation.

It's not just the newbees that use Spice. Some of us absorb new tools of the trade as they come available and use them to advantage. Some others stopped moving 30 years ago. That's OK, but then they shouldn't tell us how we should do it.

Jan Didden
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