fala,
The filter is a 2-pole low pass filter. This filter topology is know as the Sallen Key topology. If you derive the circuit transfer function you will find that the denominator is a 2nd order polynomial of the form.
K/(s2 + 2ZetasW0 + w0^2).
Where Zeta is the damping factor (=.707 for a Butterworth filter), W0 is the undamped natural frequency.
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You can, indeead connect pin 6 directly to pin 2 to obtain a unity DC gain. One reason for using a gain > 1 is that it allows you to select C1 = C2. With a gain of 1, setting C1 = C2 results in a negative value for R2. Radio Shack does not stock many negative resistors. If you are willing to use unequal value capacitors, then setting the gain = 1 is not a problem. The usual practice is to select one of the capacitors to be a standard value, and calculate the other capacitor, which will almost always turn out to be a non-standard value.
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See any book on anmalog filter design for more information, or Google on "active filter".
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Regards,
Kral