Hey all. First post here. I have found many calculators online that tell you how to choose resistor / capacitor values when making hi/lo pass filters. It appears to me that you can achieve the same cutoff frequency with completely different values of capacitors / resistors. Here is an example:
.01uf capacitor with a 100ohm resistor will give you a 159.23 cutoff for a passive highpass filter.
same as a 100pf capacitor and a 10M ohm resistor will also give you a 159.23 cutoff.
So what logic do you use to choose your values? is higher resistance and lower capacitance better?
I was using this calculator High Pass Filter Calculator "RC High Pass Filter Calculator"
Thank you.
.01uf capacitor with a 100ohm resistor will give you a 159.23 cutoff for a passive highpass filter.
same as a 100pf capacitor and a 10M ohm resistor will also give you a 159.23 cutoff.
So what logic do you use to choose your values? is higher resistance and lower capacitance better?
I was using this calculator High Pass Filter Calculator "RC High Pass Filter Calculator"
Thank you.
> use to choose your values?
What is around it? What is the effect?
Yes, 0.01u and 100r makes 160Hz... but can your Source drive the 100 Ohm load? Typical chip opamps strain below 2K or 500r.
Yes, 100pf capacitor and a 10M ohm is 160Hz.... but now connect the 10K input of a mixer, sound-card, CD-recorder, or most other modern goodies-- the filter is now not 100p+10Meg, but 100p+10K (9.99K) and up near 160KHz! (No audio at all!)
In cave-man days, if we didn't know better, we scratched "10K" in the dirt and worked from there. Most circuits can drive 10K easy. If we fear the destination may not be >>10K, we buffered, and an in-box buffer can easily be >1Meg loading.
What is around it? What is the effect?
Yes, 0.01u and 100r makes 160Hz... but can your Source drive the 100 Ohm load? Typical chip opamps strain below 2K or 500r.
Yes, 100pf capacitor and a 10M ohm is 160Hz.... but now connect the 10K input of a mixer, sound-card, CD-recorder, or most other modern goodies-- the filter is now not 100p+10Meg, but 100p+10K (9.99K) and up near 160KHz! (No audio at all!)
In cave-man days, if we didn't know better, we scratched "10K" in the dirt and worked from there. Most circuits can drive 10K easy. If we fear the destination may not be >>10K, we buffered, and an in-box buffer can easily be >1Meg loading.
I am working on making a set of high-pass and low-pass filters for an eq that i want to create. So i been bread-boarding them using different values to see how they work / sound. I never took into consideration that ability to drive the circuit via line level. I am still pretty new to this, but not know to programming and audio.
Thank you for the direction though. The load is definitely a reason to use higher value resisters. I am going to do some more tests this evening.
Thank you for the direction though. The load is definitely a reason to use higher value resisters. I am going to do some more tests this evening.
You need to know the impedance of your circuit. In audio it could be anything from 8ohms (speaker crossover) to a few K (line level, or within a solid state circuit) to 1M (valve input).
You need to think about what frequencies you are filtering, so you know whether component parasitics need to be considered. A 0.01uF capacitor is a small capacitance at audio frequencies but if you are trying to filter out mobile phone interference at around 1GHz then the 'capacitor' is actually a small inductor; 33pF might still be a capacitor.
You need to think about what frequencies you are filtering, so you know whether component parasitics need to be considered. A 0.01uF capacitor is a small capacitance at audio frequencies but if you are trying to filter out mobile phone interference at around 1GHz then the 'capacitor' is actually a small inductor; 33pF might still be a capacitor.
Quality effects are known. 100 uf up caps leak electrically , more so after some years. Some capacitors have self inductance that limit frequency response. in the older days, carbon composition resistors were known to raise in value faster 100k and over.
10 M resistors can short out due to humidity or salt in the air, or dirt deposited on them from the air. The frequency of occurance of the effect drops around 470 K. Some systems will never sell near the ocean, others go to sea as a primary market.
Then there is the cost factor. .33 uf caps can cost 5x what .033 uf caps do. Spend some time with selector tables of distributors after you maka a trial component selection.
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