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Vintage threads from the first ten years

RC Filters

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7/11/2000 2:52 AM
Bill	RC Filters Could someone tell me (again) how to calculate the knee in an RC filter. I tried to find Jacks note book, but couldn't. No link to it. Very little of what used to be available there is still available.

7/11/2000 4:25 AM

In the words of the early Texas settlers, the Lord giveth, and the Lord taketh away. There are a number of other places with that particular bit of knowledge, fortunately.

But it's easy enough I never can remember that I need to write it down. A resistor and capacitor are halfway to whatever they will do - highpass or lowpass - when the capacitive reactance equals the resistor. That is, when Xc = 1/(2*pi*Frequency)= R, the filter is at its knee. So with a little elementary algebra, Freq. = 1/(2*pi*R*C).

7/11/2000 12:35 PM
Bill	Thanks RG. You helped me on this one last time too. It's been a while since I modified a circuit so I forgot the formula. Even though it's very simple, I'll write it down this time.

7/11/2000 2:27 PM
Bill	One last question on this subject. What units are we using here. I get the right answer in pf and Kohms. Is this right? Lord, please givith.

7/11/2000 2:31 PM
Bill	I'm sorry. That's F and Kohms.

7/11/2000 2:43 PM
jason	Capacitance in farads and resistance in ohms jason

7/11/2000 4:57 PM
Bill	I read about the technology of a TS-9 and they gave a knee of 720 Hz from a .047uf cap and a 4.7k resistor. Acording to this, F=1(2pi.000047f*4700ohm)=.720. So the outcome is in kHz?

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