This mod. of the E-Bay xtal receiver worked so well (even pulling in a 738 hetrodyne audible with 740 KCBS - Tahiti and Japan), that in early May 2007, I tried out a portable loop antenna I had built in 1996 to improve mediumwave and longwave DX reception in England and Ireland, by installing a 1N34a detector diode and a 1/8-inch audio-output jack that could be used to patch audio to an outboard speaker-amplifier. In this fashion, I not only have a portable loop antenna like the commercially available (and larger sized) "Selectantenna," but a crystal radio too. (I should note here that crystal radio purists do not use any active amplification to receive signals but prefer sensitive, matched headphones. Also, gorgeous receiver designs abound. It is fascinating to do a Google search for "crystal radio" and see what you come up with! This is a fine tutorial-page on a great design by Steve, VE7SL in British Columbia. By comparison, my own dabblings into this cool realm of receiving is crude, but I was experimenting...)
My homebrew portable loop already had a 2-turn coupling loop to its 10-turn main-loop coil, and accompanying wire-input binding-post, so I decided to chain another tuned circuit in front of the loop receiver, and sure enough, a real "DX-machine" has emerged - very high-Q, capable of separating BCB stations within 20 kHz pretty well if one tweaks the tuning knobs carefully and in tandem. This system pulls in stations very strongly using a 60 ft. long inverted-L wire up 25 feet.
Right below is a recording of 660 KTNN Window Rock, AZ made May 2007 in the evening using this dual-tuned-circuit 1N34a receiver, outputted into a speaker-amplifier employing an LM386 audio chip. KTNN is over 500 miles distant and is the premiere Navajo Nation AM/MW BCB radio station with 50 kw output to a two-tower cardioid-pattern array at night, with a potential ERP of about 150 kW in my direction to the west. Supposedly clear-channel KTNN occasionally suffers nighttime interference from an ill-allocated station in Santa Maria, California, but in this recording, KTNN is hugely strong with a wonderful Indian-style identification.
660 KTNN on the 1n34a germanium-diode "crystal" receiver
By Stephen P. McGreevy, N6NKS