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In Reply to: RE: 9400 pF in HRSCF? posted by JLSmith on March 05, 2009 at 04:09:33
A series R-C network looks like the capacitor at lower frequencies, with impedance inversely proportional to frequency, and the resistor at higher frequencies, with impedance independent of frequency.
The frequency where the capacitor impedance magnitude is equal to the resistance of the resistor is called the corner frequency. It is the frequency where R equals 1/(2*Pi*F*C), or,
F = 1/(2*Pi*R*C), where '*' indicates multiplication, 'Pi' is 3.14159..., R is in ohms, and C is in farads.
Follow Ups:
So the corner frequency at 10ohm and 10000uf is 1.6hz?
" " " " " 10ohm and 20uf is 796.2hz?
What does this mean that the filter is doing when placed across the speaker wires? What does the corner frequency have to do with filtering out the bad high frequencies? Did I miscalculate?
Edits: 03/05/09
I just calculated the frequency for 39pF (a value I have) and got 408 MHz. Did you convert to farads? This converter is helpful:
Ok I didn't convert from pico to micro before using the calculator I found on the internet.
What corner frequencies are we aiming for with these filters?
For R of 10 ohms and C of 0.01 microfarads (the Walker Audio High Definition Links recipe), the time constant is 0.1 microseconds. Invert this, divide by 2 and divide again by Pi.
The answer is 1.59 MHz.
I'd stay above half of this number to keep the filter out of the audio band.
What is the range of the audio band? 0-20kHz? These corner freqs are in Mhz.
1.59Mhz, 159Mhz and 1591Mhz for 0.01 uF, 0.001uF and 0.00001uF capacitors. or 10000pF, 100pF, and 10 pF, right?
What would cut off say from 300khz and up? What am I missing
What is the range of the audio band? 0-20kHz? These corner freqs are in Mhz.
1.59Mhz, 159Mhz and 1591Mhz for 0.01 uF, 0.001uF and 0.00001uF capacitors. or 10000pF, 100pF, and 10 pF, right?
What would cut off say from 300khz and up? What am I missing
With a first-order filter like the simple R-C, there is still some influence on the audio band with a filter corner two octaves above (80 KHz).
Remember the formula: corner frequency Fc = 1/(2*Pi*R*C).
You can use a larger capacitor with the ten ohm resistor to get to a 300 KHz corner: 0.053 microfarads, or use a combination of a larger resistor with a larger standard cap value, such as 11 ohms with a 0.047 microfarad cap.
So, at 10 ohm and 9900pF I have about 1.6 Mhz. Is that low enough? Can I go 15 ohm (1.07Mhz) on this leg but 10 ohm on the next two? If I need to keep the resitance the same, would 530Mhz then be high enough with 15 ohm and a 20pF cap? BillF
I've gone to 20 ohms without any trouble. You don't need to make all the stages have equal corner frequencies.
nt
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