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.

 

[schmitt trigger]

As Diver correctly mentioned, the series diode prevents simple linear analysis.
In this instance, the capacitor is behaving as an energy storage unit. It is a completely different analysis.
And let’s not forget, every single real-world transformer has an effective series resistance. As a first order calculation, it is the sum of the secondary winding’s resistance plus the primary winding resistance multiplied by the square of the turns ratio.

 

[Nigel Goodwin]

Yes, decreases, thanks.
Sorry to keep coming back atcha, but then how does this work?

View attachment 146766

(Virtually) no R here, except the driving impedance, which must be quite low, no? And the filter frequency has to be less than 50 or 60 Hz in order to eliminate ripple. So how does that square with your statement about filter R and frequency?

It's not an LPF, it's a rectifier feeding a reservoir capacitor - the capacitor basically acts as a battery.

 

[carbonzit]

So is calling it a "filter capacitor" something of a misnomer? or is that OK, since that is what it's doing, functionally speaking if not technically?