so ive been building this 40m direct conversion receiver based loosely on the Norcal design, not a straight copy but similar topology, NE602 mixer into an LM386 audio stage. got it on the air last weekend and it actually receives stuff which honestly surprised me because my PCB layout is kind of a mess.

problem is theres this low frequency hum riding on everything, not quite 60hz but close, and it gets worse when i touch the case or move the audio pot. ive got the whole thing powered off a 9v wall wart through a 7808 regulator so i dont think its coming from the supply side but who knows. grounding on these simple receivers is always where i screw up.

also the LM386 is oscillating a little bit at high gain settings, ive got the 10uf cap on pin 7 already and the 0.047uf bypass on the output but it still wants to sing to itself on certain frequencies. anyone dealt with this before on a homebrew audio chain, i feel like im missing something obvious

the hum thing is almost certainly a ground loop somewhere, especially if it responds to touching the case. what are you using for an enclosure, metal or plastic? if its plastic thats probably your issue right there, no chassis ground reference for anything. even just running a wire from your circuit common to a metal panel screw can make a huge difference on these sensitive audio stages.

on the 386 oscillation, the bypass on pin 7 is good but you also want a snubber on the output, like a 10 ohm resistor in series with a 0.1uf cap from output to ground. that tames the high frequency instability on almost every LM386 circuit ive ever built. also check your layout around pins 1 and 8 if youve got any gain-setting resistor in there, those traces pick up everything if theyre too long.

yeah the 386 is notorious for that. i had the same thing on a little CW monitor i built a while back and spent way too long chasing it. ended up being that i had too much lead length between the chip and the bypass caps, moved them right up against the pins on the underside of the board and it mostly went away. also dont run any audio traces parallel to anything carrying RF, even a short run near your antenna input can couple enough to make it misbehave. the NE602 is already pretty microphonic on its own so any pickup gets amplified all the way through