EEVblog Electronics Community Forum
Electronics => Beginners => Topic started by: slowaudio on March 11, 2014, 01:33:38 am
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In a lot of AC coupling applications, most notably some audio equipment I've been looking at, the inputs are AC coupled. Now, this is nothing out of the ordinary. However, I've noticed that a lot of the AC coupled inputs are a parallel combination of two capacitors, usually an electrolytic and a film/foil.
I've posted an example. The added capacitor doesn't really add any appreciable significance to the corner point. Why are they doing this? I read/heard something about this once (maybe?) where one capacitor cancels out some non-linearities of the other capacitor. Is there any truth to this?
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It's Hi-Fi, oops, sorry, I'll take off my cynical hat.
Bigger capacitors, were (probably still are) made by wrapping two conductive foil into a spiral. This can give rise to appreciable series inductance, i.e. the capacitor no longer behaves as a capacitor at higher frequencies. The smaller capacitor is there to overcome the manufacturing artefact.
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Keep in mind, by higher frequencies we're probably talking MHz.
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It's Hi-Fi, oops, sorry, I'll take off my cynical hat.
Bigger capacitors, were (probably still are) made by wrapping two conductive foil into a spiral. This can give rise to appreciable series inductance, i.e. the capacitor no longer behaves as a capacitor at higher frequencies. The smaller capacitor is there to overcome the manufacturing artefact.
It's not audiophoolery. These are out of professional pieces (not that section of the industry isn't filled with black magic).
FWIW, I've seen this in other circuits too, not just audio pieces.
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It's not audiophoolery.
May be not, but the reason I gave above still stands. You'll also see this trick applied, perhaps with more validity, across power supply rails.
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In the old days, when electrolytic capacitors were made by winding 2 multi meter long strips of kitchen foil into a cylinder, with the ends used as the tabs, you could have a very high inductance of the resulting capacitor. Not a problem at 100/120Hz for power supplies, but they could have an inductive reactance at audio frequencies, so the bypassing with film capacitors with lower inductance was common. Modern units use a higher gain foil which is shorter, and the tabs are often attached in the middle of the winding, so that they are only going to be inductive over 1MHz. Some have multiple tabs, and a corresponding lower self inductance.
The old design habit of bypassing with a foil is still there, but with modern units it is a question of which capacitor is actually doing the bypass. The modern electrolytic is likely to have a higher self resonant frequency than the film unit.