so ive been building a simple direct conversion receiver for 40m, nothing fancy just a NE602 mixer with a VFO i wound myself and a basic audio amp stage after it. got it mostly working which honestly surprised me but theres this low frequency hum sitting under everything that just wont go away. doesnt matter if i touch the antenna or disconnect it, hum is still there. ive got the whole thing running off a 9v wall wart right now and im wondering if thats the issue but before i go tear apart the power supply i wanted to see if anyone else ran into this with NE602 builds.

the bypass caps are all there, i followed the application note pretty closely. VFO seems stable enough on the scope. its really just this hum, maybe 60hz or a harmonic of it, hard to tell without a proper audio analyzer. could be ground loops i guess but i dont have a ton of experience diagnosing that kind of thing on homebrew stuff.

almost certainly your wall wart. those switching supplies are terrible for sensitive receiver front ends, even the regulated ones. the NE602 is actually pretty sensitive to supply noise and a cheap wall wart is going to dump ripple right into the audio. try a battery, just clip a fresh 9v on there and see if the hum disappears. if it does, you know exactly where to look. you can build a simple LC filter on the supply rail if you want to stick with the wall wart eventually, or even just a 78L08 regulator with good bypass caps will clean things up a lot.

also worth checking if the hum changes when you pick up the board or move it around — sometimes its just a ground loop between the audio stage and the input side and rearranging the layout helps. direct conversion receivers are notoriously picky about layout.

yeah i had almost the exact same thing happen on my first NE602 build. for me it turned out to be the audio amp stage picking up mains through a long ground trace that looped around the board. switched to a battery like someone probably already told you and it got better but didnt fully fix it. ended up having to reroute some traces and add a ferrite bead on the audio input. kind of a pain to troubleshoot honestly but once it clears up these little receivers actually work pretty well for what they are