so i've been building this 40m direct conversion receiver on and off for probably four months now, based loosely on the Neophyte design but with a few mods i found scattered across various blogs and one old QST article from like 2003. got it to the point where it's actually pulling in signals tonight which felt like a minor miracle honestly.

the problem is there's this really annoying hum that i cant seem to kill. it's there whether the antenna is connected or not so i dont think its RF pickup. power supply is a regulated 12v wallwart which i know isn't ideal but its what i have on the bench right now. i tried adding more bypass caps around the audio amp stage and it helped maybe 20% but the hum is still pretty obvious. also getting some microphonics when i tap the bench near the receiver, not sure if thats a separate issue or related.

the LO is an NE602 based design running around 7.1 mhz. i didnt do anything fancy with shielding yet, figured id get the basic circuit working first. anyway just curious if anyone has been down this road and has thoughts on the hum before i start ripping things apart again

the hum is almost certainly your wallwart. those switching supplies are noisy as anything and a simple regulator on the output doesnt fix the RF garbage they put on the rail. try running it off a battery temporarily and see if the hum goes away, if it does you have your answer. i went through the exact same thing with a 80m receiver i built last year, swapped to a 7ah gel cell and it was night and day.

the microphonics could be a few things, loose component leads, a ceramic cap that doesnt like vibration, sometimes its even the NE602 itself if you're running it anywhere near its max supply voltage. worth checking your bypass situation around pin 8 on the chip too if you havent already.

yeah the neophyte is a good starting point, built one myself a while back. the hum thing drove me nuts too. one thing i havent seen mentioned much is the ground plane situation, like if youre building on perfboard or whatever and your ground traces are long and shared between the audio and RF sections you can get all kinds of weirdness. i ended up running a dedicated heavy ground wire back to the power entry point from the audio amp and that helped a lot. also seconding the battery suggestion, that will tell you fast whether its supply noise or something structural in the circuit