High Pass Filter for Neve 1272 / 1290

[abbey road d enfer]

Easy. Multiply the RC factor by the inverse of the frequency ratio.
100/250 = 0.4 1/0.4 =2.5   We know we can't very much deviate from 18k for the res, so we modify the caps.
100n * 2.5 = 250nF
So you can use 220nF and increase the R's to 20k

Great!

Is there an equation I can use to plot the frequency response of the overdamped filter?

Thank you.

Unfortunately not. Formulas are quite complex, even for a second-order, worse for third-order.
Use LTSpice.

 

[earthsled]

I'm curious about your mention of API's technique of scaling the RC stage in order to make the filter less damped. Is this something I can employ for my filter?

Thanks!

 

[abbey road d enfer]

They use two C-R sections, the second (10nF/130k) having an impedance 10x higher than the first (100nF/13k).
That wouldn't work as well in your particular case because the input impedance of the second section is not high enough.
Anyway, if you wanted to do it with a 3rd-order filter, the impedance of the third section would need to be about 100 times higher than the first.
You could try, using a smaller factor than 10, maybe factor 2 or 3, and you would need to adjust the third resistor in order to compensate the load presented by the input Z of the second stage.