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Würth Elektronik ANP125 - Capacitors don’t cause any appreciable signal distortion

Heine

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Würth Elektronik’s new Application Note ANP125 publishes the results of a study “The acoustic effects of harmonic distortion of aluminum electrolytic capacitors” on harmonic distortion of electrolytic capacitors. The result: Capacitors don’t cause any appreciable signal distortion.

The discussion is ongoing in the audio technology world about what circuit elements affect the sound quality of amplifiers. The app note by Würth Elektronik provides empirical evidence to add to this discussion and answers questions that most audio engineers have.


Application Note ANP125 is the result of international research collaboration between R&D teams at production sites in Asia and the Würth Elektronik Competence Center in Berlin. The text begins with an introduction to human hearing and psychoacoustics and goes on to examine harmonic distortions in capacitors. Furthermore, results from model calculations are presented in order to check the plausibility of the measured results. The measurements show no appreciable distortion of signals caused by capacitors.

Material variations also tested

Dr. René Kalbitz, Product Manager in the Capacitors & Resistors Division at Würth Elektronik eiSos and author of the study, explains:

- The investigations indicate that material variations have a negligible influence on distortions, and these are below the hearing threshold. Electrolytic capacitors do not add any appreciable harmonics to the fundamental frequencies in signal transmission, so, to a good approximation, they can be considered as linear components. It is likely that other voltage-independent capacitor types and passive components, as a rule, generate similarly low distortion amplitudes compared to the audibility threshold. Consequently, the choice of non-linear components such as operational amplifiers and diodes has a greater distortion impact on the audio quality of the amplifier, i.e., the overall distortion characteristics, than the choice of electrolytic capacitor.

APN125
 

DVDdoug

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Yeah, I'm not afraid of capacitors, electrolytic or otherwise.

But I think bipolar electrolytics are a bit of "hack" (I've only seen them used in speaker crossovers.) I've never heard any distortion but maybe there is measurable distortion?
 

MaxwellsEq

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Excellent, thank you. I think figure 6 is particularly interesting, since it demonstrates that there are artefacts (i.e. capacitors are not "perfect"), but these artefacts are lower than humans can perceive.
 

Ratterbass

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Caps do matter. Especially in the lower frequencies. I recently ran some tests when testing capacitive voltage dividers (as opposed to resistive) and there most definitely are differences.

As expected, ceramics perform pretty bad in this regard while quality foil caps and electrolytics are usually fine for the levels we're dealing with in audio applications.

There's also some paper on this by TI, which goes into more detail.




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solderdude

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IM distortion is not researched though.
Only a 470uF electrolytic was tested. In audio only used as coupling or power supply reservoir/filter capacitor.
Also it isn't clear which load impedances were involved and how much bias voltage was used and how the cap performs without bias voltage. (think high value cap in feedback circuit of amp in an attempt to lower DC gain).

I agree with @DVDdoug regarding this aspect (appreciable = sound quality degrading).
The paper is written for the company by the company and thus a result can be steered towards.

I prefer to look at independent measurements (like a few fellow members already did at ASR).

Me... I am not worried at all about caps when it comes to audible aspects. It does start to matter in the SINAD race though but only for the numbers...
With filters in the audible band where voltages can fall across a component it will matter.
For DC blocking caps that are well dimensioned not so much.
 
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