CR Filter Theory Question

[robrob]

I've been working on a How the TMB Tone Stack Works webpage so filter theory has been on my mind. Over on an amp facebook page I was told that RC low pass filters work by bleeding high freqs to ground through the capacitor that's connected to ground. That makes intuitive sense.

I was also told that CR high pass filters do not bleed low frequencies to ground through the resistor connected to ground. The cap just blocks the low freqs the same way they block DC.

Well, here's a CR high pass filter thought experiment. With an RC low pass filter we do say that the filtered frequencies are "bled to ground" because the cap is connected to ground. So why does a CR high pass filter not bleed frequencies to ground through the resistor?

We know a CR high pass filter cutoff frequency is inversely proportional to the resistance of the R. If you use a zero ohm resistor (a short) the cutoff frequency goes infinitely high and no frequencies make it through the high pass filter.

Would you agree the frequencies are being "bled to ground" through the short and not blocked by the cap? What about a 1 ohm resistor? 10? 1k? 100k? When does the resistor stop "bleeding low frequencies to ground" and the cap start blocking them?

Where is my logic going wrong?

Thanks in advance for helping me out here.

[eschertron]

My Quick-n-dirty response would be that in the CR filter, ALL frequencies are bled to ground through the resistor. The cap makes less of the low frequency energy available at the output node.

[catalin gramada]

Just look at CR network as a potential divider in AC mode, means you use a reactive element- the AC"resistance "is not fixed but variable in respect with frequency it pass.Translate the cap value in what is call "reactance" that means effective "resistance" of capacitor at specific frequency. For instance 100nF cap present 15.9K at 100Hz but 1.59K at 1000Hz. Just use a reactance calculator for frequency you ask. no high pass filter blocks low frequencies but the atenuation is bigger cause reactance of series element (capacitor) is bigger as go to low frequencies
If you reverse and the cap becomes shunt to ground it is low pass cause the shunt will be smaller as frequency raise and attenuation will be bigger for high frequencies

[R.G.]

Hats off to eschertron and catlin. Good answers, both!

[Mike Sulzer]

The problem here is the phrase "bled to ground": it has no definite meaning. It is a common explanation for how the capacitor in a guitar tone circuit works, but it fails to shed much light on anything there. But cg's divider explanation can easily be extended to put a resonant circuit in the shunt path and then in parallel with that, a variable resistor and a C in series.

[MarkusBass]

That's right - this term has no meaning in electronics. It's a common way to explain how tone control pot works in guitar to people that do not have any electronics knowledge. I'm sure that you wouldn't like to put such a bullshit on your web site (which at the moment is not that bad). I wouldn't put there any information copied from facebook. Instead, I advise to read some books that cover basic terms like: impedance, reactance. If you don't have access to such basic books, I advise to visit and read these sites:
Series R, L, and C : Reactance And Impedance -- R, L, And C - Electronics Textbook
AC Capacitor Circuits : Reactance And Impedance -- Capacitive - Electronics Textbook
Capacitive Reactance - The Reactance of Capacitors
Capacitive Reactance
https://en.wikipedia.org/wiki/Electrical_reactance

Regarding your question about 1 Ohm resistor, you should learn a little bit about output and input impedance of circuits. If the signal source has a very low output impedance (e.g. 0.0001 Ohm), adding 1 Ohm resistor will not change much the output signal. But if the output resistance of the voltage source is high (e.g. 100 Ohms), adding 1 Ohm resistor on the output will form a voltage divider and result in very low signal on the output. Additionally, the reactance of the capacitor depends on frequency so it will form additional voltage divider which is frequency dependent. This is described nicely on the Web sites listed above.

Mark

[robrob]

I understand reactance and impedance at the textbook level, I'm trying to understand audio filters at the AC vibrating electron level. If the back and forth movement of high audio frequency AC electrons aren't sent to ground then what in a CR filter stops their movement?

Do you guys have a problem with saying that an RC low pass filter bleeds frequencies to ground through the capacitor? It seems intuitive to me that the unlimited electrons at ground feeding the cap are what dampens the frequencies.

I want to make sure you guys know I'm not trying to be combative because I know you guys know a crap ton more about electronics than I do. I really appreciate that you are taking the time to educate me. I'm just trying to understand this stuff at a fundamental level.

[MarkusBass]

I understand reactance and impedance at the textbook level, I'm trying to understand audio filters at the AC vibrating electron level. If the back and forth movement of high audio frequency AC electrons aren't sent to ground then what in a CR filter stops their movement?
Do you guys have a problem with saying that an RC low pass filter bleeds frequencies to ground through the capacitor? It seems intuitive to me that the unlimited electrons at ground feeding the cap are what dampens the frequencies.

No electronics text book explains audio filters "at the AC vibrating electron level". Simply, such a term is not used by people who know electronics. But if you want to know what stops electrons from flowing to the ground, it's impedance and capacitor reactance. This is quite good explained in the first link I posted previously.
I don't have a problem with "bleeding frequencies to the ground" but, as I said previously, the term is only used by persons who do not know electronics (like luthiers) who want to explain how the tone control potentiometer works in guitar to other persons that also do not know anything about electronics. You can do better than that .

Mark

[pdf64]

How about something along the lines of viewing these simple filters as being akin to regular resistive potential dividers, but with the upper or lower resistor replaced by a cap, depending on whether we want a high pass or low pass function?
Everyone kinda gets volume controls, right?

[g1]

I don't seem to have the same aversion to the term 'bleed to ground' so I'll try this. Electrons will follow the path of least resistance.
Capacitive reactance is in loose terms the equivalent of resistance but changing according to frequency.
So depending on the frequency in question, the cap to ground may or may not be the path of least resistance for the electrons to follow.
When it is the path of least resistance, I'll accept the terminology of signal being 'bled to ground'.

[robrob]

pdf64, that's a very good point. I don't assume that voltage is "bled to ground" in a resistive voltage divider so why should an audio filter "bleed to ground." Hmmm.

By the way, I have removed all "bled to ground" references from the TMB Tone Stack page. I'll look for more on the website and root them all out eventually.

[J M Fahey]

Bleed to ground makes perfect sense if you think current, not voltage.
And of course, in any voltage divider IF we have voltage acros impedances, we have current flowing.

[MarkusBass]

By the way, I have removed all "bleed to ground" references from the TMB Tone Stack page. I'll look for more on the website and root them all out eventually.

I don't want to draw you away from the terms you use. Everything depends on to whom you address your Web site. If you want to explain how tone stack works to guitarists, the term "bleed to ground" is OK. If you want to explain tone stack to technicians who know electronics, it's not that good but maybe still acceptable. But if electronics engineer is going to read it, he may only smile . So the basic question is to whom you address your Web site - don't remove the terms to quickly.

Mark

[catalin gramada]

I learned a lot from electricity looking by flowing water in pipes and hydraulic mecanisms shows in popularisation books from 60-70 which use analogies to explain a electric fact. I bet even teachers in schools use those kind of comparisons

[Malcolm Irving]

... . I don't assume that voltage is "bled to ground" in a resistive voltage divider so why should an audio filter "bleed to ground." Hmmm.
... .

Yes. Keep in mind that in a guitar amp it is voltage that represents the signal (until we get to the power stage where currents become equally important).