As I thought, multi-pole elliptic filters of high "Q." They are the "special
circumstance" I mentioned above. Fading SW, AM is not high quality audio
under any definition. These filters are used to "clean up" an already very
poor audio signal and are appropriate for that purpose. Whether they work
very well or not is another matter. They ain't HI-FI and should NEVER be
used in your stereo for the reasons I mentioned! These filters are designed
to be used with speakers at 8ohms but they have some inconvenient, large
components. It is possible to do these filters at low level using op-amps as
active components with R's and C's, no inductors. That may be more
convenient, simpler and a lot cheaper.
Ah, yes, I think the OP's posting name, "AM DX," gives a clue. ;-)
As Bob says, implement them with active filters. In fact, if you do,
you can re-tune them relatively easily and evaluate which works best
for you without having to rewind big coils.
Even better, if you can, implement your filter as an FIR filter in a
processor. FIR filters can easily have linear phase, at least, though
they likely will have some "ringing" to get the sharp cutoff you
probably want. An additional advantage is that it's then easy to
change the filter cutoff frequency to suit your needs.
If the stations you listen to are channelized, that is with uniform
frequency spacing, it's probably worthwhile to have a filter with a
deep null at the channel spacing frequency. For example, a filter
with a 10kHz notch is worthwhile for listening to US medium-wave AM
broadcast signals, because it kills the "whistle" from adjacent-