A second order low pass filter (LPF) can be made by cascading two active LPF (an RC network + opamp.) What happens if, instead of immediately cascaded, an active LPF acts at the input (buffer) of a circuit and another one at the output of the same circuit? Say, the input and output of another intermediate audio circuit. Would the 40dB slope of a second order filter also be attained, or does the effect of a second order LPF (40dB cutoff slope) only result if two first order filters are immediately cascaded?

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    \$\begingroup\$ Whereas two buffered single pole filters cascaded will have the same ultimate 40 dB slope in the stopband, their passband corner will not be as sharp as a 2nd order filter. That is, the -3dB frequency of the two filters will be a -6dB point for the whole filter, so more slopey than a properly designed 2nd order Butterworth or Cheby filter. \$\endgroup\$
    – Neil_UK
    May 21 at 13:38

1 Answer 1


Would the 40dB slope of a 2nd order filter also be attained?

Overall yes but, for the intermediate circuit between the two filters, only 20 dB per decade slope would be present at its input. This part of my answer also assumes that the intermediate circuit does not introduce any extra filter slopes i.e. it is a flat-gain amplifier for example.

Or the effect of a 2nd order LPF (40dB cut-off slope) only results if two 1st order filters are immediately cascaded?

No, 40 dB per decade can be obtained this way providing the intermediate circuit doesn't introduce another slope.

  • \$\begingroup\$ The intermediary circuit is indeed a variable state filter (PEQ), so curves are added there. But all of them within the audible range, whereas the LPF in discussion would be well above it (at 48kHz). Should I be aware of any problems like interactions or so? Thanks a lot in advance. \$\endgroup\$
    – Domingo
    May 21 at 16:46
  • \$\begingroup\$ @Domingo looking at your previous set of questions you asked I note that you may not be aware of protocol on this site so, please take the 2 minute tour and consider accepting answers to previous questions you have raised (where applicable). \$\endgroup\$
    – Andy aka
    May 21 at 17:20
  • \$\begingroup\$ @Domingo there should be no interaction between the filter system in the intermediate section and the two external filters. All three can be expected to perform to their own characteristics. \$\endgroup\$
    – Andy aka
    May 21 at 22:59

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