so i've been working on this DC receiver for 40m for the past couple months, been using the NE602 as the mixer and a standard LM386 for audio amp, pretty standard stuff you see all over the place. got it mostly working but the audio is really muddy and i'm getting a ton of hum even when i touch the volume pot. i've gone over my grounding like three times and i think the layout is okay but something is clearly off.

the RF side seems fine honestly, i can tune across the band and hear stations so the oscillator is working, the BFO is stable, whatever. it's just this audio stage that's killing me. i read somewhere that the LM386 is kinda sensitive to layout and you really need that bypass cap between pins 1 and 8 if you want any decent gain without oscillating but i added that and it didnt really help. also tried a 100uf on pin 7 to ground and still getting this low frequency hum.

my power supply is a wall wart so thats probably part of it but ive built stuff with wall warts before and it wasnt this bad. anyone dealt with this or have thoughts on what to look at next

the LM386 hum issue is almost always either a ground loop or RF getting into the audio stage, and with a direct conversion receiver those two problems show up constantly because your IF is basically audio frequency so any 60hz noise has nowhere to hide. the wall wart is suspect for sure but before you swap it try wrapping your audio wiring, especially anything going to the volume pot, in some shielded wire or at least keep the leads really short and close to the board. i spent probably two weekends debugging an almost identical build a few years back and it turned out my pot wires were running right next to the oscillator section and picking up all kinds of garbage.

also worth checking: whats your decoupling look like on the VCC line going into the 386? i usually put a 100ohm resistor in series with the supply to that chip followed by a 100uf and a 0.1uf to ground right at the pin, makes a huge difference. the datasheet kind of glosses over how much the 386 cares about clean power.

yeah the wall wart is probably your problem tbh. i gave up on them for any audio work, just run everything off a 12v gel cell now and the difference is pretty dramatic. you could also try one of those 78xx regulators after the wall wart if you dont want to deal with a battery, at least you'd know if thats the source of the hum before going further.