Originally Posted by BDP24
I know that (qguy is the one looking), ya big silly. Say, how does Marchand achieve 24dB/octave passively?!
24 dB/octave implies fourth-order response, which can be achieved using two inductors (L's) and two capacitors (C's). For a LPF, you could start with a series L, then C to ground, then another L in series, then a final C to ground. For a HPF swap L's and C's. There are other ways but it has been a while since my filter design class. Choice of filter response is another variable (Butterworth, Bessel, Chebyshev, Elliptical, etc.) that affects amplitude response (roll-off, peaking, ripples in passband and/or stopband, and such) and phase response (linear or not, behavior through pass and stop bands, etc.)
Without op-amps a passive filter potentially has lower noise and distortion, assuming you are far away from saturating the inductors (i.e. they are run at low current, typical for line-level filters), but without active buffers the passive design is more sensitive to the source and load impedances since they will impact the filter's response. A common assumption is a low-impedance source and high-impedance load; a good company will specify the assumed source and load impedances beyond which the filter's response is no longer valid. The passive design will also have more loss, though some "active" designs may use unity-gain input and output buffers with a passive filter network inside (one of the buffers could be a gain stage, natch). It is usually easier to adjust the frequency corner of an active design (depending upon the filter topology you can do it with a single control; passive requires you to adjust L's and C's). Most passive designs I have seen are fixed at a single frequency determined by design.
Active or passive will work fine when properly designed and integrated. I have tended to use active as I appreciate the buffers to isolate source and load and you don't have to worry about the inductor's saturating or ringing. Of course, active circuits can ring too... L's tend to get large at audio frequencies, but passive does need a power supply. You can usually adjust an active design's frequency corner (and sometimes other parameters like gain, filter Q/bandwidth, etc.) though some active designs are fixed-frequency. And so forth, many trades.