so ive been building this 40m direct conversion receiver on and off for about 3 months now, based loosely on the NE602 front end design with an LM386 audio stage bolted on the back end. got it receiving signals finally which felt like a huge win but the audio is just terrible. like there's this low frequency hum that kind of pulses and i cant tell if its mains pickup, oscillator leakage getting into the audio, or what. i built it on ugly style construction on a piece of copper clad so shielding isnt really a thing yet.

i did wind my own toroid for the BFO and im not confident the inductance is right. measured it with my cheapo LCR meter and it came out within maybe 10% of where i wanted it. the oscillator does seem stable enough once it warms up, takes like 5 minutes before it settles down though. tuning range is there, im hearing SSB stations so the basic idea is working.

has anyone dealt with hum like this in a similar build, wondering if its worth trying to put the whole thing in a metal enclosure first before chasing other problems, or just keep debugging on the bench as-is?

the enclosure question is actually a real one and i'd honestly say yes, put it in a box first. i built something similar a while back and spent forever chasing a hum that turned out to mostly go away once i had a proper ground plane and lid on the thing. the LM386 is notorious for picking up all kinds of garbage if the layout isnt tight and the supply rail isnt clean. worth throwing a 470uF cap right across the supply pins if you havent already, and maybe a 0.1uF ceramic in parallel with it. also that 10uF cap on pin 7 of the LM386 (the bypass pin) makes a huge difference to the gain and the noise floor, a lot of people leave it off or use the wrong value.

the 5 minute warmup on the oscillator is pretty normal for a free running LC design with no temperature compensation, i wouldnt stress too much about that for a bench receiver.

yeah hum in a DC receiver is just kind of a rite of passage honestly. mine hummed for weeks. ended up being a ground loop between my power supply and the audio amp section, i had two separate ground paths coming back to the same point and it was acting like a loop antenna basically. single point grounding fixed most of it. also are you running it off a wall wart? some of those are just awful, switching supply noise everywhere. i switched to a 7812 regulated supply and a lot of my noise floor problems just disappeared.