Low-pass filter with C but no R

[carbonzit]

This is something that's bugged me for some time now.

I hasten to point out that IANAEE (I am not an electronics expert); I know enough to get into trouble, and maybe a little more. Hence my puzzlement on this point.

In another thread there's a discussion of why, when a guy added a series resistor to the input of an amplifier, it degraded the sound quality by what sounded like attenuating high frequencies, the classic effect of a low-pass filter. The theory advanced was that by adding the series resistor, it formed a low-pass filter with an already-existing capacitor at the input of the amp.

This does not make sense to me. Let me explain why.

This shows the classic low-pass filter with both a C and R, but also simply a C:

We know the C-R network forms a low-pass filter. But doesn't a C by itself also form one? It's simply the degenerate case where R=0, right? It will still function to attenuate high frequencies. Look at power supplies that have filter capacitors but no series resistors (some have them, some don't). There's your R-less LPF.

Just to be clear, I am not suggesting that one doesn't ever need an R to make a LPF; it's certainly needed to create the proper response curve, rolloff and cutoff frequency. But if that amplifier already had a C across its input, I don't see how adding an R would suddenly create a LPF where none existed before. It would certainly change the response of the filter, but my guess is that it would have less effect, overall, than the C already has.

So what am I missing here? I'll let the experts here respond.

 

[carbonzit]

Are you implying "shorting" across the power rail with a capacitor? Current flows in and out of a capacitor, it is not a "short" by most people's definition.

Wellll, all I can tell you is that when people do AC analysis of circuits, they speak of capacitors as behaving like a short circuit to AC signals. From one of my electronics textbooks:

In [a figure in the book], the capacitor ideally looks like a short to an ac signal. Because of this, point A is shorted to ground as far as the ac signal is concerned. That is why we have labeled point A as ac ground.

 

[danadak]

AC analysis of the Z of a C :

So Xc is only a short at very high frequency/high C values. But real C's actually model like this
with their imperfections :

So due to ESL primarily XC may rise at high frequencies where the C becomes less like
a C and more like an L.


Regards, Dana.