so i've been working on this 40m direct conversion receiver for a few months now, mostly following the NorCal 40A schematic but modified a bit because i couldnt get a couple of the original parts. got it mostly working last weekend and i can actually hear signals on 40m which is pretty exciting for a first build.

the problem is there's this persistent 60hz hum in the audio that i cant seem to kill. i've tried running it off a battery instead of my bench supply and the hum drops a lot but doesnt go away completely. so im thinking its either a ground loop somewhere in my audio chain or maybe my layout on the ugly construction board is picking up something. the LO is running at 7.1 MHz using a colpitts oscillator and the audio comes out through an LM386 amp stage.

has anyone dealt with this with a DC receiver? i know they're inherently sensitive to low frequency stuff but this seems worse than what people describe online. also curious if shielding the audio section separately from the RF would help or if that usually just makes things worse with more ground paths to worry about.

yeah the LM386 is a notorious hum magnet if you dont bypass it properly. pin 7 to ground with like a 10uf cap, and also check that you have a cap from pin 6 (Vs) to ground right at the chip — and i mean right at the chip, not somewhere else on the board. also DC receivers are really sensitive to any leakage from the LO getting into the audio path, so even a tiny bit of coupling through shared ground impedance shows up as hum or buzz. if you haven't tried a ground star topology for the audio section that might help more than shielding. shielding can help but usually only after you've sorted the ground stuff first.

i built something similar a couple years ago and the hum drove me insane for weeks. turned out one of my audio wires was running parallel to a piece of wire carrying the LO signal, only about an inch apart, and that was enough to couple into everything downstream. moved it and like 80% of the hum just vanished. sometimes its not an electrical fix, just the physical layout being sloppy. not saying yours is sloppy but worth looking at with fresh eyes, sometimes you stop seeing whats right in front of you after staring at it too long.