so anyway i finally got it working after like three weeks of debugging and i'm pretty stoked about it. built a direct conversion receiver for 40m, pretty basic design i pulled from the ARRL handbook plus some stuff i found on the soldersmoke blog. the BFO is just a colpitts oscillator running around 7.1 MHz and i'm mixing that down with a diode ring i wound myself on a binocular core - which honestly took me way longer than it should have because i kept getting the winding directions wrong.

audio is actually pretty decent, little bit of hum i haven't tracked down yet, probably a grounding issue somewhere or maybe my power filtering is garbage. i'm running it off a bench supply right now so that might be part of it. selectivity is basically nonexistent obviously because its just a DC receiver with no IF filtering but for SSB on 40m in the evenings i can actually copy signals which feels kind of amazing for something i built out of junk parts.

anyone else gone down this rabbit hole? curious whether it's worth adding some audio filtering or if i should just move on and try building something with a proper IF strip next. also not sure if my mixer is even working right - how do you actually verify that without a spectrum analyzer

congrats on getting audio, that first moment is honestly one of the best feelings in the hobby. the hum is almost certainly your grounding, DC receivers are super sensitive to that stuff, try lifting the ground on everything except one single point and see if it cleans up. star grounding basically. also your bench supply is probably injecting noise too so yeah try a battery and see if the hum changes or goes away.

as for checking the mixer without a spec an, just hook a second receiver up near it and see if you can hear the LO leaking out around 7.1. if you can hear it then the oscillator is running at least. to actually verify mixing is happening just slowly tune the LO a bit and see if received signals shift in pitch - if they do you've got a working product detector basically. not super scientific but it works. the binocular core ring mixer should be fine if you wound it right, those are pretty forgiving designs. what core did you use, BN-43 or 61 material?

ive been wanting to try something like this for a while. did you use the Hayward or the Wes Hayward designs or just kind of your own thing mashed together? i keep getting intimidated by the oscillator stability part, like i can't figure out how tight the frequency needs to be for just listening. probably overthinking it