so ive been wanting to build something from scratch for a while now and figured a crystal set would be the easiest entry point, which it mostly was. grabbed a random coil form from the junk box, wound about 70 turns of 26AWG enameled wire, threw a 365pF variable cap on there and wired up a 1N34A germanium diode i found in a bag of old parts my dad left me. slapped some alligator clips on a piece of cardboard for terminals and hooked it to a long wire out the window.

thing actually worked which honestly surprised me more than i expected it to. got a couple AM stations coming through pretty clearly with some decent earphones, though im in a semi-rural area so not sure how it would do in a city with more stations fighting each other. the selectivity is pretty rough obviously, two of the stronger stations bleed into each other if im tuned anywhere in the middle of the band.

my question is whether adding a second tuned circuit would actually help that much or if im just hitting a fundamental limit with this approach. ive seen some double tuned designs but the coupling between the two coils seems fiddly and i dont want to spend a week tweaking it if the improvement is marginal. anyone built one of these that can give me a realistic expectation of how much better selectivity gets with a second LC stage?

yeah double tuned will help noticeably but youre right that the coupling is a pain to get right. the trick most people use is to wind a separate smaller coil right on top of or right next to the main tank coil and then slide it in and out physically to adjust coupling — loose coupling gives you better selectivity but kills sensitivity, tight coupling is the opposite. most old commercial sets used a thing called critical coupling which is sort of the sweet spot but you basically have to find it by ear. if your two stations are close together in frequency it still might not be enough honestly, depends how strong they are relative to each other.

one thing that helped me a lot on my second build was using a higher inductance coil and a smaller cap range, shifts the Q up a bit. also make sure your ground is actually good, a lot of people skimp on that and then wonder why everything sounds muddy.

i built a double tuned set last winter and the selectivity improvement is real but honestly not dramatic unless the stations are pretty far apart in frequency. what made a bigger differnece for me was experimenting with different diodes, some of the old OA90 types i scored off ebay were noticeably better than the 1N34A for weak signal stuff. also if you havent tried a cats whisker with a chunk of galena crystal thats kind of a fun rabbit hole, gets into the real old school territory. anyway good first build though, getting it to work at all on the first try is more than a lot of people manage