I wrote:

The thing to watch is the Q factor,
the detector circuit can be seen asa resistor parallel with the ferrite rod and tuning cap.
----------
|( | |
|( === [ ] Rp
|( | |
---------

The higher Rp, the higher the Q factor, the more signal, the narrower the bandwidth
B = f / Q

http://en.wikipedia.org/wiki/RLC_circuit
scroll down to 'parallel RLC circuit'.

PS
so a high Q gives a narrow bandwidth, better 'selectivity'.
That may be needed if you receive 2 stations close together in frequency.
Especially in the evening on the AM broadcast band you may get very strong signals from far away stations
at the same or right next to the frequency of the one you are listening to,
you then hear both at the same time, or this gives a few kHz interference.

But, too high a Q (hard to obtain [1]) will attenuate the high frequency sidebands of the station you
are listening to, reducing high frequency sound components.


[1] RF feedback can be used in a RF stage to increase Q,

Am stations are spaced 9 kHz or 10kHz apart depending on location,
so all sorts of interference from far away stations exists.
This site is worth reading:
http://en.wikipedia.org/wiki/AM_broadcasting


RR and IF stages with tuned circuits or bandfilters of any kind (mechanical, crystal, piezo, LC, etc) can help
create an accurate bandpass curve.

I think you should perhaps get hold of one of the RSGB radio books,
and start from the
http://www.rsgbshop.org/acatalog/Onl...___CDs_29.html