so i finally got my direct conversion receiver for 40m to actually receive something other than static and i have to say i did not expect it to work this well for a first attempt. been messing with electronics for about 15 years but never actually built a radio from scratch before, always just kit stuff or bought commercial gear.

the design is pretty basic, LM386 audio amp, NE602 mixer/oscillator combo, nothing fancy. wound my own coils on a T50-2 toroid which took like three tries to get the inductance anywhere close to where i wanted it. trimmer cap for tuning across the band. whole thing fits on a piece of perf board about the size of a paperback book.

first night i fired it up i heard a couple of ssb signals which honestly just sounded like garbled noise because DC receivers do that whole mirror image thing and both sidebands come through at once. but then i figured out how to tune slightly off center and suddenly i had this guy from ohio coming in clear as anything talking about his antenna situation. sat there for like an hour just listening and feeling unreasonably proud of myself.

sensitivity is decent enough that i can hear stations that my FT-891 also hears, not all of them but enough to know the thing works. selectivity is obviously not great without any real filtering ahead of the mixer but for 40m SSB in the evenings it's workable. thinking about adding a simple bandpass filter on the input and maybe a better audio filter stage. anyone done mods to a basic NE602 design worth talking about?

congrats on getting it working, that first moment when you hear an actual voice come out of something you built from parts is hard to describe. the NE602 is a great starting chip for this kind of thing even if the purists hate on its dynamic range.

for the audio filtering, a state variable active filter centered around 700hz or so makes a huge difference on SSB intelligibility. you can build one with a TL072 and a handful of resistors and caps, and being able to tune the Q a bit helps a lot. i added one to a similar receiver i built a couple years back and it basically transformed the thing from novelty to actually usable.

the image rejection thing you mentioned is just the nature of DC designs, some people add a phasing network to get proper single sideband reception but thats a whole other rabbit hole involving matched RC networks and getting phase shifts right to maybe 1 or 2 degrees which is tedious. might be more fun to just enjoy what you have and then build a superhet next time.

ive been down this exact road, the NE602 plus LM386 combo is practically a rite of passage at this point. one thing i'd watch out for is the LM386 oscillating if your layout isnt tight, had that drive me crazy for a week on one build before i figured out it was just a grounding issue on the perf board. adding a 10 ohm resistor and 0.047uf cap from pin 7 to ground fixed it right away but ymmv.

also dont underestimate how much a good 50 ohm termination on the input helps even before you add any real filtering. just having that impedance right can clean things up noticeably.