What is the importance of a resistor in a high/low pass filter?

[Tuxedo3]

Very simple answer in sure, I just don't understand why there is the need for a resistor when it seems that the cap really does the frequency stopping. Could someone shed some light on this for me? Also how it relates to a tone control.

[solderfumes]

It's all about Ohm's law applied to voltage dividers, and thinking of the capacitor in terms of its frequency-dependent impedance.  The cap doesn't so much "stop" frequencies as provide different impedances for different frequencies, and it's when this impedance forms part of a voltage divider with the resistor that you get rolloff of different frequencies.

[EBRAddict]

The resistor helps you tune the starting frequency of the rolloff.

For example, passive LPF:

R = 1 ohm, C = 100 nF, fc = 1.6Mhz
R = 1k ohm, C = 100 nF, fc = 1.6kHz
R = 1 ohm, C = 100 uF, fc = 1.6kHz

So a few benefits come to mind: precision frequency tuning, options on component values and package sizes, capacitor ESR. You'll have to use very large capacitors with no resistors to LPF in the audio range.

[Tuxedo3]

... thinking of the capacitor in terms of its frequency-dependent impedance.  The cap doesn't so much "stop" frequencies as provide different impedances for different frequencies, and it's when this impedance forms part of a voltage divider with the resistor that you get rolloff of different frequencies.

This makes sense, I still need to ponder it for a bit for it to sink I but I definitely get it. Thank you.

So a few benefits come to mind: precision frequency tuning, options on component values and package sizes, capacitor ESR. You'll have to use very large capacitors with no resistors to LPF in the audio range.

So without a resistor, would the rolloff not be beneficial?

[midwayfair]

It's all about Ohm's law applied to voltage dividers, and thinking of the capacitor in terms of its frequency-dependent impedance.  The cap doesn't so much "stop" frequencies as provide different impedances for different frequencies, and it's when this impedance forms part of a voltage divider with the resistor that you get rolloff of different frequencies.

"Everything useful is a voltage divider" -PRR

[jtn191]

quick question, couldn't you make the resistor in a HP/LP a pot and then have a passive EQ?

[aion]

quick question, couldn't you make the resistor in a HP/LP a pot and then have a passive EQ?

Absolutely - this is the basis of the tone control for the Rat, the OCD and several other circuits. Even the Big Muff is a slightly more complicated version, but it's still just two passive tone stacks with a balance control. Then we get into amps - nearly all of them utilize variable HP/LP filters, including the classic Fender/Marshall/Vox.

It's called a passive tone control, or passive tone stack. The key with it, though, is that it can only attenuate (remove signal from) frequencies, it can't add them. (Although the perception of adding frequencies can be done by cutting everything else and boosting the overall result - this is how the mid control works in the F/M/V tone stack.) If you want to directly boost a frequency then you need an active tone control such as a Baxandall.