so ive been working on this 40m direct conversion receiver for the past few months, finally got it to the point where it actually receives something but theres this persistent 60hz hum underneath everything that just wont go away no matter what i do. ive tried re-routing the audio wiring, added more filter caps on the power supply rail, even tried running it off a battery to rule out the wall wart and the hum is still there, maybe slightly less but still very much present.

the design is pretty basic, NE602 mixer fed by a colpitts VFO i wound myself on a toroid, then into an LM386 for audio. nothing fancy. ive shielded the vfo section in its own little tin enclosure i made from some PCB material and grounded it to the main board at one point only.

my suspicion at this point is the LM386 is just picking up RF from somewhere and rectifying it or something, or maybe my ground plane isnt great, i built it on perf board not a proper PCB. anyone dealt with this kind of thing before? its driving me nuts because the actual reception when there is a signal is really quite good, the selectivity isnt great obviously but for a DC receiver im pretty happy with it.

yeah the LM386 is notorious for being finicky, it'll oscillate or pick up garbage if you're not careful with the bypass caps. the thing that helped me most on a similar build was putting a 0.047uf cap directly across pins 1 and 8, that limits the gain a bit but cuts down a lot of the instability. also 10uf from pin 7 to ground, some people skip that one and it makes a difference.

the other thing i'd check is whether your audio ground and your RF ground are connected at the same single point. on perf board its really easy to accidentally end up with ground loops because you're just running wires wherever they fit. i had a hum on my 80m DC receiver that turned out to be exactly that, had two separate ground returns coming back to the power supply at slightly different points and the difference in potential was enough to cause trouble.

running off battery and still getting hum is interesting though. if it persists on battery id start wondering if its actually 60hz or something else at a similar frequency, like maybe the VFO has a spur or something. worth checking with a scope if you have access to one.

LM386 builds are like 80% dealing with that exact hum lol. I went through the same thing last year on a 20m receiver. ended up being my audio leads were too long and basically acting as an antenna inside the chassis. shortening them and twisting the pair together fixed most of it. also make sure you've got that 250uf cap on the output to speaker or whatever you're feeding into, sometimes people leave that out thinking it doesn't matter.