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Old 19th May 2007, 03:26 AM   #11
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Conrad, your resolution is not good enough. You MUST be sure of your common mode rejection with frequency, AND you must VERY CLOSELY match the RC time constants. This is why we use an instrumentation IC op amp with 120dB common mode rejection.
If you want to see nothing at all, why bother to measure in the first place?
For the record, mylar vs polypropylene is easy. polystyrene vs polypropylene is tough.
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Old 19th May 2007, 12:09 PM   #12
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John, I can force conditions where I can see a difference, but IMO, it's a red herring. (The 1A7A is a matched diff amp of excellent specs BTW, especially if you take care to adjust it properly) Both our methods of testing are fundamentally flawed anyway. D=omegaRsCs, or Rs/Xs if you prefer. We know from measurements that capacitors tend towards a semi-constant D over a wide frequency range. Not exact, but it doesn't vary all that much. Thus, Rs *must* vary with frequency. That means that these circuits can only be balanced for one frequency, and will always show a difference for caps of differing D and a non-sinusoidal waveform.

Anything will look "bad" compared to Teflon, polystyrene, or polypropylene, but no 'X-factor" has been revealed, only the difference caused by conventional capacitor parameters. And, sure, with enough sensitivity you can even see the difference between those three. Non-linearities in things like electrolytics will show up, but they also do when you look at the residual in any 4-arm bridge method. Not sure about DA, but I suspect it's not much of a factor with reasonable frequency AC signals- little soakage occurs per cycle. Thus, if I want to get down to insanely low values of D, it's easier to just use a bridge designed for the purpose, remembering that the reference cap in most bridges will be a very low DF mica, with high DA, yet it has no real influence on the results.

Still, an interesting test would be to find two capacitors of exactly the same value, and exactly the same DF, but of different dielectric types. Measure those against each other in your circuit, and there should be little if any difference. IMO again, whatever difference does show up will be below audibility for any decent film caps, and I'm not aware of any properly conducted listening tests that suggest otherwise (exact matched levels, blind, etc blah blah). I'm not saying there is zero measurable difference on lab equipment, just zero difference in practical application.

Going a bit OT, I once breadboarded a circuit for a research application where capacitor defects were an issue. It involved using two identical caps, one in the circuit of interest, and one in a bridge configuration that matched the operating conditions of the circuit. The bridge was nulled, and the residual amplified. The amplified residual was then injected into the circuit of interest, cancelling out the flaws of the capacitor. It was too sensitive to drift in the balance to be practical as anything other than a lab curiosity, but the concept is interesting.
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Old 22nd May 2007, 01:52 AM   #13
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Conrad, find Cyril Bateman's series on capacitors. That will give you an additional (and alternative) set of measurements that, should you be so ambitious, could give you a reliable rule of thumb regarding capacitor sonics.

Read: mylar caps, among others, suck.
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Old 22nd May 2007, 02:30 AM   #14
jcx is offline jcx  United States
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Bob Pease reported some sucess compensating DA in S/H caps:

http://www.national.com/rap/Applicat...570,28,00.html

also:

http://www.intusoft.com/nlhtm/nl65.htm
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Old 22nd May 2007, 04:12 AM   #15
Hartono is offline Hartono  Indonesia
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jcx :

"Bob Pease reported some sucess compensating DA in S/H caps"



yes, unfortunately we can not use that method for signal coupling caps.

Cheers
Hartono.
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Old 22nd May 2007, 06:45 PM   #16
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second that
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Old 22nd May 2007, 07:49 PM   #17
rdf is offline rdf  Canada
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Default Re: Another cap comparison

Hi Conrad. A question about process, was the spectrum of the differential examined? Your note on the 60 Hz component implies 'yes' but it wasn't explicitly stated. If so what was revealed? With the help of solid state CCS loads tube amplification stages with a 2nd harmonic 65 dB down, 3rd well below that and nothing above are possible. If the spectral ‘difference’ between caps, assuming the worst, is a tight band of harmonics at -85 dB off the right hand side of the frequency scale (as I've measured of one poor sounding amp) it’s still arguable the coupling cap defines the sound of the circuit. I’m willing to bet a cluster of -85 dB harmonics from 5th to 15th far outweighs the sonic thumbprint of a -65 dB 2nd. The Tek 1A7A might not be sensitive enough to reveal all meaningful differences.

A suggestion for expansion, a 2 volt excitation signal doesn't come close to mimicking the full range a coupling capacitor sees in normal tube circuit duty. Another forum member describes driving capacitors in series with an eight ohm power resistor directly from the output of a power amp and typically hearing the bodies ‘sing’. This might be one non-linear effect a 2 volt noise test doesn’t capture. DC bias might reveal (or hide) another.

A final suggestion, consider high order low-pass filtering the excitation signal and examining the residual differences in the stop band. This might provide greater insight into what's happening. BTW, it's good to finally see a test in which one end of the cap isn't tied to ground, shunting the signal the test is intended to capture.
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Old 23rd May 2007, 04:57 AM   #18
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Rdf- good stuff. I didn't do SA, and that would almost have to reveal more. I'm building up my test arsenal with another computer, so I should be able to do decent SA via an external sound card soon. The diff amp has an output, so I should be able to start with that. My assumption, not actually tested, is that once anything is down in the noise floor of my (listening) system, it just doesn't matter. OTOH, I'm also working at reducing both the distortion and noise floor of my system, so the bar does get raised. You mentioned our thermionic friends, which have their own requirements, but I'm pretty much concerned with solid state inputs in the 10-20kohm region. I probably won't mess with tubes again unless I fall into some deal on a prebuilt amp. I used to have modified Dyna Mark IIIs, but sold them many years ago for what seems like a song today.
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Old 23rd May 2007, 03:22 PM   #19
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Looks like a job for a wiki author to tackle ... entries to the various links and articles that rate / evaluate / quantifing caps by material v. reactance, reluctance, inductance, capacitance, etc ... No?
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Old 23rd May 2007, 03:26 PM   #20
jcx is offline jcx  United States
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Default Re: Re: Another cap comparison

Quote:
Originally posted by Hartono
jcx :

"Bob Pease reported some sucess compensating DA in S/H caps"



yes, unfortunately we can not use that method for signal coupling caps.

Cheers
Hartono.
What exactly do you think needs Correction in Linear audio coupling applications?

Bob Pease’ DA correction is based on his linear approximation of DA as applied to highly nonlinear S/H circuits where the effect is especially clear with multiplexed input signals

Using Bob's same 6 RC branch linear approximation model of the DA of a mylar cap it is easy to show that the frequency response variation from that of an ideal cap ranges from >1 mdB at 20 Hz to <1 udB at 3 KHz with Bob's 1 uF Mylar cap model and 100 KOhms of input impedance that would be common in a audio coupling applications, with ss amplifiers at lower impedance the deviation from the ideal C response is still 10X less than the DBT threshold of 0.1 dB frequency response variation at the -3dB RC response point, much less in the pass band

unless you have some new data showing ppM frequency response variations are audible, I have to conclude that the linear model of DA does not explain why different film dielectric capacitors could be audibly different in active filters or coupling applications at common impedances



Quote:
Originally posted by rdf
... BTW, it's good to finally see a test in which one end of the cap isn't tied to ground, shunting the signal the test is intended to capture.

I'm also curious about why people who do have decent equipment want to handicap it while trying to make sensitive measurements; "ground" is a human applied label, not a fundamental E/M concept, the label can be “moved” without affecting circuit operation

Moving the "ground" from one side of the signal generator to the other in the RC coupling circuit test usefully removes the voltage source from the measurement, reducing the CMRR requirement and allowing a much more sensitive measurement within the limitations of common test equipment - when we are interested in differences it makes sense to arrange the measurement to measure the difference - not the common part
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