Question about choosing R (or C) in RC filter

XNOR

Hi All,

(Sorry this question is not specifically about Parallax, but I'm not sure where to ask this.)

I have a basic question about RC filters.

If I want to make a high-pass audio filter (for example, to remove low-frequency rumble),
I see that I can use the equation f = 1 / 2PiRC to get the component values for R and C.

My question is how and why do I pick a specific value for R or C in order to calculate the other value?

For example:

Let's say I choose f = 40 Hz to remove "rumble" noise.

Then, if I choose R = 100 ohm, I get C = 39.8 uF.
But, if I choose R = 1K ohm, I get C = 3.98 uF.
Or, if R = 2K ohm, C = 2 uF, etc.

So, why does one choose 100 ohm, or 1K ohm, or 2K ohm, etc?
(Or, conversely, why choose 39.8 uF, or 3.98 uF, or 2 uF, etc?)

Does choosing a higher R value introduce more "white" noise?

Thanks for any help!

Duane C. Johnson

Hi XNOR;

XNOR wrote: »

Hi All,

(Sorry this question is not specifically about Parallax, but I'm not sure where to ask this.)

I have a basic question about RC filters.

If I want to make a high-pass audio filter (for example, to remove low-frequency rumble),
I see that I can use the equation f = 1 / 2PiRC to get the component values for R and C.

My question is how and why do I pick a specific value for R or C in order to calculate the other value?

For example:

Let's say I choose f = 40 Hz to remove "rumble" noise.

Then, if I choose R = 100 ohm, I get C = 39.8 uF.
But, if I choose R = 1K ohm, I get C = 3.98 uF.
Or, if R = 2K ohm, C = 2 uF, etc.

So, why does one choose 100 ohm, or 1K ohm, or 2K ohm, etc?
(Or, conversely, why choose 39.8 uF, or 3.98 uF, or 2 uF, etc?)

Does choosing a higher R value introduce more "white" noise?

Thanks for any help!

It really has to do with the general impedance of the circuit around the filter.
Some circuits can't drive low impedance filters because the signal would be distorted.
Lets say you have an OpAmp that is capable of delivering 1mA at 1V. This would be able to accommodate a 1KΩ load but not 100Ω.

Also, the load impedance affects the filter. Generally the load impedance should be at least 10 time greater than the filter impedance or a lot higher.

That is a good start.

Duane J

$WMc%

Yep
'
I'm assuming your wanting to filter the output from the amp to the speaker.
'
Take a look at the speaker spec.'s
'
This will fill in the math.
'
Take a look at crossover's crossovernetworks and 6db over 12db roll off

LoopyByteloose

This site has a lot of good audio info that applies. Also sells nice cheap OPamp boards to build an active filter.. if that choice seems optimal.

sound.westhost.com

XNOR

Hi All,

Thanks for responses.

Duane:

Why does a low impedance cause distortion? (Is the impedence the R part in the RC filter?)

How do you get the value of 1K ohm (load) for the 1mA @ 1V OpAmp example?

(Sorry for all the questions: I seem to be okay at understanding DC circuits, but AC circuits are difficult for me.)

Thanks again for the help,

XNOR

Beau Schwabe

Think of impedance as riding a bike .... If your impedance does not match, it's like pedaling in the wrong gear. If you don't have the proper drive strength, a high impedance circuit will have a problems driving a low impedance load. The same can be true in the reverse, if you have too much drive strength, but often not as much of a problem. It depends on the circuit and if improperly matched you can have over saturation. Yes "R" is part of the impedance but with an AC circuit, "R" is a ratio of the voltage to the current and possesses both magnitude and phase where as a DC circuit only possesses magnitude.

http://en.wikipedia.org/wiki/Electrical_impedance