Audio RC filter design

[steppenwolf]

Hi!
I'm trying to learn something about passice single pole RC filter designs for audio purposes (that has always been a mystery to me . I'm familiar with the basic concept. You have a resitor, a capacitor and one of those components shunts the signal and depending on what component that is you have a high or low pass filter. I know the basic formulas to calculate the -3 point but what I simply don't get is whrn the source and load impedance comes into play. How do I get those parameters into the equation?
I mean those parameters should change something, or am I wrong? I looked all over the Internet but couldn't find answers or they are not intended for audio use. Does anybody has a link or something where I can learn about this designs?

Thank you very much!
Best,
Stefan

 

[xmvlk]

the source and load impedance comes into play. How do I get those parameters into the equation?

You may compute, what "impedance" see the filter capacitor. {short
voltage sources, disconnect current sources and compute:}
In case of RC lowpass,
put values R', C' in your equation,
where
R'=(Rsource+R)//Rinput.
C'=C//Cinput
C is resistor in RC filter
R is capacitor in RC filter
Cinput is input capacitance of output device
Rsource is output resistance of input device
Rinput is input resistance of output device
+ is addition
// is paralel {A//B=1/(1/A+1/B)}

 

[steppenwolf]

Thank you VERY much!

 

[steppenwolf]

Call me stupid
It's pretty clear now but one thing still bugs me.
Let's say we have an driving device and an receiving device, the receiving device is ac coupled by an 0,1uF cap, no other components involved...

Here the parameters:
Rsource is some 100Ohms, Rinput is 10MOhms couplingC is 0,1uF.
I use this formula to calculate the -3db point:
1/(R'*2*PI*C').

In the case above using your formulas that would give me:
R' = Rsource//Rinput = 100Ohms//10mOhms that gives me 99.99 Ohms
C' = 0,1uF or 0,1*10exp(-6)F

That would give me an -3db point at around 16kHz...But there is no bass roll of noticeable?  ???
Could you help me please?
Thanks again!
Best,
Stefan

 

[xmvlk]

Rsource is some 100Ohms, Rinput is 10MOhms couplingC is 0,1uF.
I use this formula to calculate the -3db point:
1/(R'*2*PI*C').

All right, You want to set-up highpass.
Best way is to put series Cs and after shunt Rs.
You have 100 Ohms source, 1000 Ohm shunt, 10 Meg input.
Resistance, which see the capacitor is R'=(1000//10Meg)+100
If you have small coupling capacitor Cout in the source,
you can compute C'=Cs//Cout
Then compute your eq:
1/(R'*2*PI*C').
If frequency is so high, get shunt resistance larger.

In the case above using your formulas that would give me:
R' = Rsource//Rinput = 100Ohms//10mOhms that gives me 99.99 Ohms
C' = 0,1uF or 0,1*10exp(-6)F

This works only for simple low-pass filter realised as a shunt capacitor.

That would give me an -3db point at around 16kHz...But there is no bass roll of noticeable?  ???

Of course, it is kind of EMI filter.