What I want to do is make a low pass filter using a simple RC circuit (first order), or possibly an LC circuit (which would be second order). Right now, I am in the process of determining the values for the resistance of the resistor and capacitance of the capacitor (I'm currently going with the RC circuit idea). As the equation is f=1/(2*piRC), I understand that if I want a crossover frequency of 200Hz, I will have theoretically infinite choices or R and C. Which values are better to choose? I've heard I should choose C first, because capacitors are more difficult to find in the right denomination. Still I am left with a few hundred possibilities.
Also, is there a best choice of type of capacitor? (electrolytic, etc)
My application, if needed, is that I want to filter out high frequencies of sound from entering my car sub woofer (powered by a 70 W amp), so I'm going to put the low pass filter between the amp and sub woofer (I would put it upstream of the amp, but in this case, the amp and head unit are one in the same device).
Thoughts?
Best Answer
Think about what the input impedance will be to the RC low pass filter when the input signal frequency is much, much higher than 200 Hz - the capacitor can be regarded as a short circuit and this means that the input impedance is simply "R" and, if you decide that you cannot live with an R value lower than (say) 50 ohms, your capacitor has to be no greater than 16 uF.
Next think about what the output impedance will be at frequencies much lower than 200 Hz - clearly, the capacitor is not part of this analysis so, if you chose 50 ohms for "R" previously, the output impedance becomes 50 ohms. Can you live with this impedance? Maybe you could live with an output impedance of 1 kohm? In which case the capacitor value is no greater than 80 nF.
So, decide on input and output impedances and constrain your design to these requirements.
It's likely that a simple RC will not do the job because your sub-woofer will have a loading effect of maybe 8 ohms and this means the resistor value will need to be at least one-tenth of this value. With R at 0.8 ohms, you will have problems when the input frequencies are high because the input impedance becomes 0.8 ohms.
Using an LC circuit is what you need because as input frequencies rise, the inductor impedance rises with them and doesn't create a stupidly low input impedance that your amplifier has to try and drive into. Try this type of circuit: -
With R at 8 ohms (your sub woofer impedance), L at 8.2 mH and C at 82 uF you get this response: -
Source of inter-active tool.
The cut-off frequency (3 dB point) is 216 Hz and has values that are not too far from specific calculators for cross-over units you can find on-line like this one: -