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Old 17th May 2007, 07:35 PM   #111
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Quote:
Originally posted by peufeu


Actually it isn't "mine" ; the idea to apply this to audio comes from Gérard Perrot of Lavardin.

And it does not show up in simulations since the SPICE Gummel-Poon models don't handle non-constant temperature. You can add a function voltage source in the emitter and a RC circuit to model thermal inertia, but hFE and other stuff will not move, so it's really partial.

Still, self-heating of the output transistors is a clean and simple explanation of why amplifiers sound better with higher biasing while theory (and testing with sinewaves of constant amplitude) would predict optimal biasing at a much lower current that what your ears like.

output transistors heat -> Vbe multiplier compensates -> output transistors cool -> you now run in underbiased class B.

See Cordell's paper, for instance, he plots bias versus time after exercising the output stage a bit.

Oh yeah : http://ngspice.sourceforge.net/

Yes, the bias swings in the output stage with program changes have always been a concern to me. I never thought of it this way, but I guess you're saying that this is a good example of memory distortion.

Cheers,
Bob
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Old 17th May 2007, 07:47 PM   #112
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Quote:
Originally posted by PB2



Hi Bob,

The short answer would be yes, but I don't really feel like one, more of a systems/design engineer. My emphasis in school was analog design, RF and microwave design, then got pushed into digital on the job, which lead to computer architecture and custom processors.

I did a lot of board level design, then started chip design in the 90s, mostly VHDL and to gates through synthesis. If people don't believe in simulation what will they think of synthesis?

How about you?

Pete B.

I did linear ICs at Bell Labs, then research on high-speed CMOS SONET and Video ICs at Bellcore in the 80's and early '90's, then did research on linear optoelectronic IC's in Indium Phosphide after that. Now I'm a user of IC's in 10 Gbps optical fiber systems. I managed a group doing synthesis of HDTV chips for awhile, and the success rate using synthesis and Verilog simulation and test-benches was remarkably high, but at times it was a little scary having to put so much trust into the synthesizer, and I never did think that they produced circuits that were as compact and as fast as the technology could provide ... but there was just no other way when deling with such big chips. At Bellcore we were running 0.6-micron CMOS chips at 622 Mbps with custom circuit design and layout, but they were less than 50K transistors.

Cheers,
Bob
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Old 17th May 2007, 07:57 PM   #113
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Quote:
Originally posted by GRollins
[snip].
To put this in perspective, ask yourself this question the next time you take a seat in an airplane: Would you rather have a pilot who has 1000 hours actual flying time and 4000 hours in a flight simulator or a pilot who has 5000 hours experience actually flying?
To me the choice is clear.

Grey

Grey,

I'm with you. I also wouldn't trust a pilot who had never done sim time to learn to react to situations he hadn't met yet.

Jan Didden
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Old 17th May 2007, 07:57 PM   #114
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Quote:
Originally posted by GRollins
Bob,
Isaac Asimov (I believe) once wrote a story about a bunch of armchair scientists who theorized and argued and wrangled and contrived towering reputations based on an incestuous system wherein they analyzed each others' learned papers.
The whole wobbling edifice came crashing down when one maverick decided to go out in the field and do original research.
In essence, the story is a subtle re-telling of The Emperor's New Clothes.
From my point of view, The Emperor's New Clothes stops just as the plot begins to thicken. What happened to the child who pointed out that the Emperor actually had no clothes? Do you really believe the story had a happy ending? My personal experience from when I was young, coupled with later book learning, agree that adults do not look kindly on children (or adults, for that matter) who deconstruct their world views. Children rebound nicely from finding out the truth about Santa Claus. Adults get nasty.
I well understand the attraction of simulations. I used to use them, myself. Then, as now, people were claiming, "Oh, sure, simulations used to be flawed...but now we've got [insert flavor of the month] and we're golden." Then, as now, there were glaring gaps and inaccuracies. Yet every month or two the cry would go out, "If you haven't tried Version XX, all your criticisms are invalid, because now we've got [you guessed it...the new flavor of the month]."
Well, no, the criticisms aren't invalid and there are quite a few of them. I hope that someday the list gets whittled down to nothing, but I'm not going to wait. I've got far too many hastily scribbled notes I need to try out.
Your point about people who simulate, people who build, and people who do both is valid within limits. One thing that disturbs me is simulator/builders who count their experience simulating as though it were experience building. "Oooh, look at me! I spent one hour building and seven hours simulating this week, so I have eight hours of experience." The not-so-subtle implication is that he is better than the fellow who spent a mere five hours building.
To put this in perspective, ask yourself this question the next time you take a seat in an airplane: Would you rather have a pilot who has 1000 hours actual flying time and 4000 hours in a flight simulator or a pilot who has 5000 hours experience actually flying?
To me the choice is clear.

Grey

Actually, to me the very clear answer is neither. I'll take the pilot who has 1000 hours on the simulator and 4000 hours in the air.

Cheers,
Bob
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Old 17th May 2007, 08:01 PM   #115
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Split second posting ....
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Old 17th May 2007, 08:03 PM   #116
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Default 2SC3264-2SA1295 SPICE Models

Does anybody here have SPICE models for the Sanken LAPT 2SC3264 and 2SA1295 power output devices? If so, I would deeply appreciate it if you would mail them to me at bob@cordellaudio.com.

Thanks,
Bob
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Old 18th May 2007, 01:41 AM   #117
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Do you mean to tell me that there are no commonly available spice models for these devices?
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Old 18th May 2007, 03:48 AM   #118
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Quote:
Originally posted by john curl
Do you mean to tell me that there are no commonly available spice models for these devices?
I looked for them and couldn't find them either. What's worse, vendors will sometimes supply models that are totally wrong. Examples of this are the OnSemi MJL3281A/1302A. I've documented the problems with these and fit new models to the datasheet parameters on one of my web pages.
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Old 18th May 2007, 03:52 AM   #119
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Well, I hope this gets me off the hook! I didn't want to 'Spice' the design anyway. It already works.
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Old 18th May 2007, 04:10 AM   #120
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LOL! You weren't thinking of becoming one of "The SPICE Guys" were you?
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