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Old 17th December 2002, 03:38 PM   #1
Joel is offline Joel  United States
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Default Steve Bench cap "tests"

Ok all,

This website of "tests" is talked up by nearly everyone who wants to win a technical argument about whether different capacitors sound different.
My question is: is it valid, from a technical point of view, as a test of things in the audio range? I am not claiming to be an engineer. But I have questions about the following:
1) the voltages and currents seem high for a simulation of an audio amp.
2) he doesn't say what the scale we're looking at is (or did I miss it?).
3) He does not show a schematic of the test circuit, and I found his description of it hard to follow. Can somebody draw it for us?

Quote:
Originally posted by dorkus
yeah, well it's necessary to exaggerate the distortion products, because it's hard to see very small distortions on a scope. e.g. a sine wave can look perfect, but easily have 1% THD on it, which is very audible.
I'm worried about any test that needs to be exaggerated to show results... honestly, I'm not trying to start another fight here. It seems that this website, along with the Jung and Marsh article is used as the seminal evidence of these differences, and if it's valid, it can't hurt to look closer at the methods used.
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Old 17th December 2002, 03:41 PM   #2
SY is offline SY  United States
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Straw man. There's lots of measurable differences between caps- no one with even the slightest technical competence argues against that. The question is the AUDIBILTY of those differences. That's not addressed.
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Old 17th December 2002, 03:46 PM   #3
Joel is offline Joel  United States
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Quote:
Originally posted by SY
The question is the AUDIBILTY of those differences. That's not addressed.
That's what I'm talking about. As I asked, is this a valid test, and setup, for an audio amp?
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Old 17th December 2002, 04:28 PM   #4
Joel is offline Joel  United States
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Sorry, I forgot to post the link:
http://members.aol.com/sbench102/caps.html
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Old 17th December 2002, 04:34 PM   #5
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Since this thread deals with Steve Benchs test, perhaps we
should have a link to this test also:
http://members.aol.com/sbench102//cap.html

(Edit: Seems Joel posted the link while I was writing this.)

I agree with you Steve that this test raises a number of
questions. It is unclear exactly what he measures and how he
measures it. He talks about "sampling" the voltage, for instance.
Does he simply mean measuring the voltage, ie. feeding it to
one channel of the scope, or does he indeed mean sampling?
In the latter case, how, and why is this important? He says he
integrates the current, but not how? If he does it the analogue
way he needs a capacitor to do the integration, raising the
question how this capacitor may affect the test. Controversely,
if he samples and integrates numerically, there are other
potential sources of errors which he does not say anything about.

One also wonders if all capacitors had the same value, and
what value this was? For the test to be meaningful, I guess one
has to use components with the same capacitance, but this
will lead to the use of somewhat extreme values for certain
types of capacitors, which may be another source of error.

As for the exaggeration, I think that is reasonable, since the
point was to show the results on the scope. one must then,
however, estimate how to scale the results to reasonable values,
since we seem not to deal with a linear function.
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Old 17th December 2002, 05:10 PM   #6
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Default STEVE'S TEST

Hello,

Christer,here is a partial answer to your QQ:

"I measured several different types of parts, and captured the results (simply by training a camera at the scope). The value of each of the capacitors was constant, 0.1uF. The signal level was held constant at about 70 volts RMS at 600 Hz across the capacitors. (for about 26mA signal current). This is probably more than you would normally expect, and serves to show the results better. A number of capacitor types were used in this experiment. The first series of curves show paper and oil, polycarbonate film, polyester film, polystyrene film, polypropylene, 100 v olt and 1000 volt ceramic and silver mica."

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Old 17th December 2002, 05:15 PM   #7
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Frank,

You're right, I missed that he actually did specify the capacitance.
Still, there are a lot of things that are unclear to me and that
should be addressed, by Bench or someone else repeating
the test. Note that I do not claim the results to be wrong, just
that "proof" itself is wanting. There clearly is something he
has captured on the scope, I just want to know more precisely
what it is.
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Old 17th December 2002, 05:24 PM   #8
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Default CAP TEST

Hi,

Well,I don't think S.B. claims it is a scientific test.
When it was first published most people agreed that the reuslts correlated with what was expected.

At the bottom of the page there is some explanation.
Btw,I don't think he had numerical (digital) sampling in mind.
In American English "to sample" also means use somtehing as a sample.

"The "current monitor" resistor used for all curves was 100 ohms. The "integrator" was passive, using a 100k and 0.047, 0.1, or 0.47 uF capacitors depending on the frequency and level. The "X" input to the scope was fed with an additional series capacitor and adjusted for exactly "90 degrees" phase shift (typically ran 0.01u into the 10 meg scope probe resistance) to reduce systemic errors. The generator is absolutely isolated, being the secondary of a transformer capable of 300 volts into 1k from 15Hz to 1kHz. The bias voltage was added "in series" with this from an HP regulated and isolated supply. The junction of the current monitor resistor and the capacitor under test serves as the scope and system "reference" point. "

Ciao,
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Old 17th December 2002, 05:33 PM   #9
Joel is offline Joel  United States
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Yes Frank, that is the description I was talking about. Can you or someone else please convert that into a schematic?
And he does not say what values the hysterisis shifts represent (value/cm on the scope's display), or the time factor.
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Old 17th December 2002, 05:47 PM   #10
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Default Re: CAP TEST

Quote:
Originally posted by fdegrove
Well,I don't think S.B. claims it is a scientific test.
When it was first published most people agreed that the reuslts correlated with what was expected.
No he doesn't, at least not in the text linked to, but some people
both here and at audioasylum uses the results in debates as if
it were a scientific truth. BTW, as far as I remember there was
quite some discussion on audioasylum about this test and its
validity, so I don't think everbody agrees to the correlation.

Quote:

"The "current monitor" resistor used for all curves was 100 ohms. The "integrator" was passive, using a 100k and 0.047, 0.1, or 0.47 uF capacitors depending on the frequency and level. The "X" input to the scope was fed with an additional series capacitor and adjusted for exactly "90 degrees" phase shift (typically ran 0.01u into the 10 meg scope probe resistance) to reduce systemic errors. The generator is absolutely isolated, being the secondary of a transformer capable of 300 volts into 1k from 15Hz to 1kHz. The bias voltage was added "in series" with this from an HP regulated and isolated supply. The junction of the current monitor resistor and the capacitor under test serves as the scope and system "reference" point. "
Where did you find this info? It certainly adds a lot of valuable
info and should have been published along with the test.
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