myths and facts on electrolytic capacitors in audio circuits

[krachkiste]

There already are a few threads on electrolytic capacitors

This one arises from a discussion in the thread " Dx Blame ST - Builder's thread - post pictures, reviews and comments here please." around the posts #333 to #357

The statement was that electrolytic caps work best for audio when they are operated at a DC Voltage bias of at least 10% of its rating.
Or something like that. The auther of this statement did not yet give a more precise version of what that is actually supposed to mean.

I suggested to spin off this topic to a seperate thread, to keep the other thread clean, and it got approved by one member, so here we go

Feel free to copy the relevant posts of the Blame ST thread and repost them here, or link them, so everyone has the chance to get up to date as easily as possible.

My intention is, to have a fair discussion, and to stick to facts, reality, i.e references to repeatable experiments and results.

This thread is supposed to clear up myths that come across from time to time, like the one mention above. It is not meant to propagate and create new myths, as this is already happening enough anyway.

Have a good time!
krachkiste

[Andrew Eckhardt]

I really know very little about capacitors, which are truly complex as any other part. But I would imagine a polar electrolytic aluminum capacitor has an oxide on only one side of the plates, and to maintain originally specified performance for a long length of time the capacitor should always or often have some voltage on it of proper polarity. If you operate such a capacitor for long with ac loads and no bias there will be all sorts of strange things developing in there, maybe even quickly enough to change an audio signal enough to hear. Also I suppose that keeping the capacitor polarized would help some distortions be more even order. Actually, this effect would probably exist for non-polar capacitors as well. If the bias is very high voltage compared to the signal, voltage dependent characteristic change would certainly be minimized for the signal. (possibly helping tube circuits sound better) What effect voltage bias has on current dependent characteristics is well beyond my knowledge, though it seems to me it would be a larger effect in electrolytic types.

[riktw]

well if you keep caps at 10% of there max voltage they have a long life.
but i don't know if they perform worse at a really low voltage.

[alexcp]

"Capacitor Sound" by Cyril Bateman, published in the Electronics World in July 2002 - January 2003, is the authoritative work on capacitor distortion. Electrolytics are discussed in the last installment of the series. I recommend to read that work, available at Scribd; it does clear myths.

[lineup]

I have never heard or read anything like that.
That a capacitor should not work well at low voltage.

When it comes to film capacitors,
they are generally electrically better the higher the voltage rating.
And I think this goes for electrolytics, too.

There might be very big lyts unsuitable for certain things in audio,
but this is mainly because of physical size.
And of course because lyts are several levels below film caps in electrical quality.

[krachkiste]

Thanks alexcp.
The article looks promising, and I'm looking forward to reading it.

[Andrew Eckhardt]

Wish I had one of those test rigs lying around.

[homemodder]

Now we need the author of that statement to come explain why the 10 percent or how it was arrived at, has anyone else come accross this recommendation.

[Andrew Eckhardt]

Interesting to note that my assumptions about 2nd harmonic and increased bias voltage were essentially wrong in an opposite way...

[SoNic_real_one]

That article is about solid dielectric capacitors, not electrolytic ones. Conclusions CANNOT be extrapolated from one type to another since there is a major difference.

The electrolytic capacitors have the dielectric maintained based on the DC voltage across them. That DC is necessary and I can see why a value close to the nominal rated one could be beneficial, enhancing the electro-chemical oxidation insulation layer.

That is different from a solid dielectric capacitor, that is "stressed" by the DC component and suffer from hysteresis and short-term remnant polarization.