========================================================================
 Sallen-Key LOW-PASS (one stage)  |  fc = 2500 Hz
========================================================================
Part                    Ideal       E-series      Error
------------------------------------------------------------------------
R1 (=R2)             3.001 kΩ           3 kΩ     -0.04%
R2                   3.001 kΩ           3 kΩ     -0.04%
C1                      15 nF          15 nF     +0.00%
C2 (=2·C1)              30 nF          30 nF     +0.00%
------------------------------------------------------------------------
 LR4 note: cascade TWO identical stages of the above for 24 dB/oct.
 Op-amp: OPA2134 (or equiv. FET-input audio op-amp), unity gain.

========================================================================
 Sallen-Key HIGH-PASS (one stage)  |  fc = 2500 Hz
========================================================================
Part                    Ideal       E-series      Error
------------------------------------------------------------------------
C1 (=C2)               6.8 nF         6.8 nF     +0.00%
C2                     6.8 nF         6.8 nF     +0.00%
R1                    6.62 kΩ         6.8 kΩ     +2.72%
R2 (=2·R1)           13.24 kΩ          13 kΩ     -1.81%
------------------------------------------------------------------------
 LR4 note: cascade TWO identical stages of the above for 24 dB/oct.
 Op-amp: OPA2134 (or equiv. FET-input audio op-amp), unity gain.

Tip: for a 3-way system pass two frequencies, e.g.:
  python calculator.py 300 3000