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Old 3rd February 2013, 04:41 AM   #1691
davada is offline davada  Canada
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Quote:
Originally Posted by richiem View Post
Yes, I too doubt the Lynx results. As to C0G/NP0 -- I have been told numerous times by folks I trusted that all of the ceramic cap materials are to a greater or lesser extent piezoelectric. I think this is not a Good Thing. But again, in the absence of facts....
Well the best way to find out is to stick them in one our oscillators and see what happens.
Any volunteers?
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Old 3rd February 2013, 05:07 AM   #1692
DDB is offline DDB  United States
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If someone has a 1K Victor oscillator it is pretty easy to remove the red film caps and reinstall cogs there for a test.
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Old 3rd February 2013, 06:50 AM   #1693
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Quote:
There has been discussion that capacitors do create distortion but I'm not clear about the mechanism that causes this.
In fact there are several mechanisms. Consider this equivalent circuit:

Click the image to open in full size.

At audio frequencies, only the two R and the basic C element are of importance. Any of these may show a voltage coefficient and thus contribute distortion.

The exact physical explanation why the basic capacitance element exhibits a voltage coefficient is not clear to me either. One possible contribution could be that the electric field contained within the plates of the capacitor may slightly bend/displace the plates, and thus cause capacitance modulation with signal voltage.

The leakage resistance is formed by parasitic elements of not well known chemical composition. These not necessarily follow Ohm's law (which in essence applies to metal conductors only) very well, and may show significant voltage coefficient.

The ESR is at least partially formed by end contact resistance. Bruce Hofer points out in his "Building Analog in the 2010s" slides that, particularly for film capacitors, these may be unreliable and cause some distortion contribution.

Which mechanism dominates depends very much on the circuit impedance, i.e. the corresponding R which forms the effective time constant. At medium impedances (1-10 kOhm), the basic voltage coefficient of the C element seems to be the most significant in my experience. Nonetheless, this discussion shows that you can't fully characterize capacitor distortion at a single frequency. Capacitor distortion should be measured at least at three widely spaced frequencies.

Quote:
Capacitor in series have a tendency to not share equal amount of charge.
You're probably thinking about series connected electrolytic capacitors in HV supplies, where one needs to use parallel resistors to ensure equal voltage distribution (as otherwise the very variable leakage resistance could cause one capacitor to carry most of the burden). In AC applications, the voltage/charge distortion is well definied (simply by the capacitance ratios).

Samuel
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Old 3rd February 2013, 07:36 AM   #1694
davada is offline davada  Canada
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Quote:
Originally Posted by Samuel Groner View Post
In fact there are several mechanisms. Consider this equivalent circuit:

Click the image to open in full size.

At audio frequencies, only the two R and the basic C element are of importance. Any of these may show a voltage coefficient and thus contribute distortion.

The exact physical explanation why the basic capacitance element exhibits a voltage coefficient is not clear to me either. One possible contribution could be that the electric field contained within the plates of the capacitor may slightly bend/displace the plates, and thus cause capacitance modulation with signal voltage.

The leakage resistance is formed by parasitic elements of not well known chemical composition. These not necessarily follow Ohm's law (which in essence applies to metal conductors only) very well, and may show significant voltage coefficient.

The ESR is at least partially formed by end contact resistance. Bruce Hofer points out in his "Building Analog in the 2010s" slides that, particularly for film capacitors, these may be unreliable and cause some distortion contribution.

Which mechanism dominates depends very much on the circuit impedance, i.e. the corresponding R which forms the effective time constant. At medium impedances (1-10 kOhm), the basic voltage coefficient of the C element seems to be the most significant in my experience. Nonetheless, this discussion shows that you can't fully characterize capacitor distortion at a single frequency. Capacitor distortion should be measured at least at three widely spaced frequencies.



You're probably thinking about series connected electrolytic capacitors in HV supplies, where one needs to use parallel resistors to ensure equal voltage distribution (as otherwise the very variable leakage resistance could cause one capacitor to carry most of the burden). In AC applications, the voltage/charge distortion is well definied (simply by the capacitance ratios).

Samuel
Is there a possibility of dielectric swelling. I seem to have a distant memory of some mention of this from a physics class.
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Old 3rd February 2013, 09:18 AM   #1695
HpW is offline HpW  Switzerland
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Demian,

Quote:
Originally Posted by 1audio View Post
I have my doubts but perhaps its possible. Does anyone else have experience with the L22? It would be a simple solution.
I own a such L22 (PCI-card) , just ask for a specific measurement, keep in mind that this card is/was not cheap

HpW
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Old 3rd February 2013, 09:59 AM   #1696
coluke is online now coluke  Italy
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On C0G/NP0 caps THD: this is from Bateman - the only one I have at hand at the moment:
Click the image to open in full size.

L.
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Old 3rd February 2013, 04:20 PM   #1697
RNMarsh is offline RNMarsh  United States
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Just about anything-including electrolytics- measure well with symetrical waveforms (sine waves). In my 339A I have bipolar types in the signal path. Its a very INsensitive test. But with sine waves to get below -130 takes special care in cap selection and test equipment.
[ BTW- its with asymetrical waveforms (music) where the cap issues show themselves to measure and... sound poor. ]

-Richard

Last edited by RNMarsh; 3rd February 2013 at 04:23 PM.
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Old 3rd February 2013, 04:35 PM   #1698
davada is offline davada  Canada
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Quote:
Originally Posted by coluke View Post
On C0G/NP0 caps THD: this is from Bateman - the only one I have at hand at the moment:
Click the image to open in full size.

L.
Hi Coluke,

Do you have one for a PP just for comparison sake.
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Old 3rd February 2013, 04:55 PM   #1699
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Quote:
Originally Posted by RNMarsh View Post
[ BTW- its with asymetrical waveforms (music) where the cap issues show themselves to measure and... sound poor. ]

-Richard
As you know I don't agree with this anymore than in-harmonic distortion. Please describe an I/V characteristic of a capacitor that would show this behavior. The mechanical effect just quoted has a rectifying property (the force vs voltage is always anti-repellant) and will manifest on a simple sine wave. The extreme case would be the condensor microphone analyzed in detail by B&K.
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Last edited by scott wurcer; 3rd February 2013 at 05:04 PM.
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Old 3rd February 2013, 05:30 PM   #1700
davada is offline davada  Canada
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Quote:
Originally Posted by scott wurcer View Post
As you know I don't agree with this anymore than in-harmonic distortion.
Hi Scott,

I,m not clear about your meaning here.

You don't agree in-harmonic distortion in capacitors?
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