Yes, you would have to run two sets of calculations from the National "calculator" to find R values for the extremes.
As for my filter, here it is, for what it's worth:
http://www.brianroth.com/projects/m77/hi-pass-filter.pdfThe "wire" appearing in from the far left side is the signal input. As you can see, the input impedance is quite low due to the input pad consisting of the 120 Ohm R in series with the 520 Ohm R. The low values were chosen to ensure a reasonably low input impedance into the filter network. Perhaps a better location for the pad (to match the levels when the filter is switched in and out) would be at the output.
With 27+ years worth of hindsite (I was 23 when I did the original design work):
http://www.brianroth.com/projects/m77/m77.htmlI see other flaws now <g>.
In no particular order:
The opamp was a TDA1024 (now well-known as a NE5534), and I didn't have a full set of docs to work with. That 20 pF cap between pins 2 and 3 was to stabilize the opamp. It should be connected between pins 5 and 8, although the circuit didn't oscillate as drawn.
The 47 uFd output cap is shown as a tantalum. WRONG choice...use a high quality aluminum, and bypass it with a film cap.
The input impedance becomes even lower when the pot is "tuned" to the higher frequencies (plus the opamp stage works into a low impedance as well). If you can stand a cut-off below 350 Hz, then increase the values of the 1K0 Ohm resistors. A better solution would be to rescale the values of the pot(s), series resistors, and 0.68 uF caps to present a higher load impedance through the network.
I don't use tantalum caps any more for power supply bypassing. When they fail, they fail as a short circuit which brings down the entire power supply rail.
Like I said, hindsite is 20-20.....hence my reluctance to tell you "this is IT!" <g>
Bri