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Old 8th November 2002, 03:04 PM   #1
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Default I hold at your neck the gom jabbar.

Fred,

Is your attached picture implying that this is a painful discussion? Or possibly that only in the "Empire" does the "Spice" clarify issues through heightened sensitivity and prescience whilst in our world of electronics it seems to muddle and confound even the simplest of explainations?

Later,
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Old 8th November 2002, 03:11 PM   #2
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There is an expression in England which goes along the lines of:

"Only a bad workman blames his tools".



Jim.
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Old 8th November 2002, 03:17 PM   #3
jam is offline jam  United States
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What do you say to the above, Harry! .....Er. I mean Fred.

Jam
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Old 8th November 2002, 03:54 PM   #4
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One big problem with Spice is, as Fred points out, is that the
transistor models are not very good. I have looked at a number
of manufacturers models of BJTs and none of them correlate
well with the data sheets. I have DIYed models for some BJTs
and I cannot find any way to make them correlate well to the
data sheets, getting about the same problems as the
manufacturers models have.

I am sure one can improve on the simulations by learning more
about how Spice works, tweaking the simulation parameters.
however, no simulation is better than its models, and this is
a weak link in the chain. The models can still work fine in many
cases, if you make sure that the transistors operate under the
conditions where the models are OK.

As for distorsion measurements, I have run a number of
large-signal analyses on some amps and FFt'd to get a
spectrum. I do not assume that this gives distorsion figures
that coincide with the real circuit. However, I think it can give
a good idea of the relative performance of two circuits. What
happens to the distorsion if I change a particular resistor value?
Which has the lower distorsion of two similar circuits? I think
even the relative change of even vs. odd order distorsion may
tell some truth.

All of us do not have spectrum analyzers or the time and money
to build hundreds of different amps. I think Spice can give some
help to choose what to build, but it cannot tell you how an amp
will sound of course (at least not without a lot of experience at
correlating real amps with Spice results).
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Old 8th November 2002, 04:35 PM   #5
mlloyd1 is offline mlloyd1  United States
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I think the various simulators are NOT useful at all for determining actual distortion numbers, but can be VERY useful (when properly applied) for seeing effects of changes in a relative fashion.

The trick is "properly applied" implies some learning/understanding that takes a bit of time and practice to acquire.

mlloyd1
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Old 8th November 2002, 04:41 PM   #6
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Default Thanks to Christer......

and yes......Phred. And I see my ol' buddy up north just jumped in.

Go read Bob Pease's book. Phred and I are not the only engineers out there that know better than believe the gibberish that Spice can spit out.

Yes, you have to know how to ask it the right question. You also have to be smart enough to know when it gives you the wrong answer. Something us old-timers learned how to do in the days before everyone started using, and believing, it without questioning it.

Now.....I want to see you argue with Bob Pease. That would be highly entertaining. I think we would all be amused by that.

Jocko
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Old 8th November 2002, 04:49 PM   #7
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Hi,

Quote:
Phred and I are not the only engineers out there that know better than believe the gibberish that Spice can spit out.
And I wouldn't even rely on the output of a simple spreadsheet as some MD's do oh sooo religiously.

If it looks a bit odd than check it out the old fashioned way and soon enough you realize something doesn't compute.

Now I'll have that glass of Claret,
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Old 8th November 2002, 07:12 PM   #8
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Default He who controls the Spice Controls the Universe

Folding space to travel without moving........ and now running Spice to design without measuring.

Again I say..... check your simulations against some real measured numbers.

http://www.borbelyaudio.com/borb502.pdf
http://www.borbelyaudio.com/ae699bor.pdf
http://www.passdiy.com/pdf/balzenpre.pdf

I have used Spice since 1976 by the way. This was with punch cards and the school's (UofA, Fayetteville Arkansas) IBM 360! The first serious assignment was to design a single transistor amplifier with Spice. I proved that Hfe variations made a single transistor circuit a poor choice for the stated. This would have required measurements on a large group of transistors with sufficient Hfe spread. Spice is a wonderful tool.

A pair 6 inch Vise-Grip pliers is great tool, but I wouldn't try to hammer a nail with them.

The spice must flow,
Fred

PS Low distortion oscillators are pretty easy to build with an op amp and a light bulb. Aa sound card and a shareware FFT program makes distortion pretty easy to measure. You don't have to buy a bunch of expensive equipment to make good distortion measurements.
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Old 8th November 2002, 09:00 PM   #9
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I've used a 6" pair of vise grips for a hammer. Sometimes ya gotta use wots ya got.

But only if I had to...........

Jocko
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Old 8th November 2002, 09:15 PM   #10
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Ok Fred, but what has these to do with the I/V converter the topic is about

Funny to see some old wise men hanging around here that seem to be arrived in DIY-audio heaven. Knowing all there is to know and in the end only be able to give their opinion by one-liners without proper reasoning and making no sense concerning the topic. Everywhere they appear in a topic, the topic gets lost in space.

Go ahead fellows, amuse yourself by spitting around your frustrations. I will go back to the music itself. There are funnier places to be.

Maybe later…

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