so anyway i finally got it working after like three weeks of banging my head against it. built a direct conversion receiver for 40m, based loosely on the Sudden design but i kinda went off script with the LO section because i had a VXO setup already from another project and figured why not. pulls in SSB pretty well actually, CW is great, but theres this low frequency hum that i cannot shake no matter what i do. tried different bypass caps on the supply rail, tried a separate regulated supply, even tried running it off batteries to rule out the wall wart and the hum is still there just a tiny bit quieter on batteries so maybe its not totally a supply issue?

the audio amp is just an LM386 which i know people have opinions about but its what i had in the junk box. wondering if the hum is getting in through the input stage somehow or if its the mixer, its a diode ring i wound myself on a binocular core. RF shielding is minimal right now, just an open pcb on the bench. i dunno, any ideas appreciated because it drives me nuts to sit and listen to 60hz on top of everything

LM386 is almost always the culprit for that kind of hum in my experience, not because its a bad chip but because you really have to baby it. the input pins are super sensitive and if you have any ground loops or even just a longish wire to your volume pot thats basically an antenna for picking up crud. try putting a 0.047uF cap from pin 7 to ground and also make sure pins 1 and 8 dont have anything across them unless you want the extra gain, the gain is 200x with the cap there and it just amplifies everything including your hum. also worth checking if your ground plane is solid or if youre doing point to point because a sketchy ground return path will do exactly what youre describing.

the diode ring mixer could also be injecting some LO leakage back into the audio path depending on how close everything is physically. shielding the LO from the audio section helps a lot. but id start with the 386 stuff first, its the usual suspect.

yeah the hum being quieter on battery but not gone is a classic sign its not just the supply, something else is radiating into your circuit. i had almost the exact same problem with a regen i built last year, turned out my audio wiring was running parallel to the coil leads for like two inches and that was enough. moved the wires and it dropped like 15dB. worth just poking around physically and seeing if moving wires changes anything