Ill try it and let you know.
Capacitor in series have a tendency to not share equal amount of charge.
I have no idea what effect this may have in a circuit like an integrator.
There has been discussion that capacitors do create distortion but I'm not clear
about the mechanism that causes this. If there is no information on cause then all we can do is study the effect.
In the absence of facts, I'll opine that low ESR and low dielectric absorption are the two most important characteristics for lowest distortion. Polystyrene or polypropylene seem best suited, and my trials of various dielectric materials in my version of Bob Cordell's SVF oscillator support this opinion.
Ceramics are generally the worst; tantalums not so great; then aluminum electrolytics (haven't tried the new "solid" types); them films, with mylar OK, but the two mentioned above the best. I tried these at 100Hz and/or 1kHz, depending on available capacitance values.
Once the polypropylenes proved good, I haven't looked back -- they are my choice due to wide availability and low cost. If polystyrenes of the right values can be had, then they are first choice. You guys in the EU might have better access to styrenes than we in North America.
Ceramics are generally the worst; tantalums not so great; then aluminum electrolytics (haven't tried the new "solid" types); them films, with mylar OK, but the two mentioned above the best. I tried these at 100Hz and/or 1kHz, depending on available capacitance values.
Once the polypropylenes proved good, I haven't looked back -- they are my choice due to wide availability and low cost. If polystyrenes of the right values can be had, then they are first choice. You guys in the EU might have better access to styrenes than we in North America.
Except for C0G/NP0 types - in my experience they're almost as good as polystyrene and polypropylene ones.
L.
The volume cap users have changed over to polyprop because of size and max temp rating. This has caused PS type mfr to be dropped by more and more cap companies due to low volume.... After that the PS film makers stopped producing the film. Now there is one mfr in China still making them to a high standard. REL-CAP in Calif uses their film source as it is highest quality film. But REL-CAP is a custom cap maker and does not stock caps of any type-- built to customers order. Some audiophile distibutors exist that have ordered and stock such parts from REL. The best is always hard to come by as they cost more. But if not enough useage, all will be lost. Just as happened with some super good transistors and jFETs..... no longer available. Get PS if you can and while you can... otherwise use a high quality made PP. Thx-RNMarsh
This gent seems to be able to measure THD to -130+dB with a Lynx L22 in loopback : http://www.diyaudio.com/forums/anal...ch-preamplifier-part-ii-3407.html#post3352626 . I have my doubts but perhaps its possible. Does anyone else have experience with the L22? It would be a simple solution.
Hi Demian,
What he's showing there is impossible.
This card isn't much better than the QA400.
I think he has a gross mis-calibration.
ARTA is showing a 1Vrms FS input signal.
L22 specs:
Analog In Performance(Analog performance specs measured at 44.1 kHz sample rate, 24-bit, card installed in computer)
Frequency Response
20 - 20 kHz, ± 0.05 dB at 44.1 kHz sample rate
Dynamic Range
117 dB, A-wtd.
Signal-to-Noise
116 dB, A-wtd.
Input THD+N
-108 dB (0.0004%) @ -1 dBFS
-104 dB (0.0006%) @ -8 dBFS
1 kHz signal, 22Hz - 22kHz BW
Well the best way to find out is to stick them in one our oscillators and see what happens.
Any volunteers?
In fact there are several mechanisms. Consider this equivalent circuit:
An externally hosted image should be here but it was not working when we last tested it.
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.
Demian,
I own a such L22 (PCI-card) , just ask for a specific measurement, keep in mind that this card is/was not cheap
HpW
I own a such L22 (PCI-card) , just ask for a specific measurement, keep in mind that this card is/was not cheap
HpW
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
[ BTW- its with asymetrical waveforms (music) where the cap issues show themselves to measure and... sound poor. ]
-Richard
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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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