so i've been building this direct conversion receiver for 40m over the past few weeks, based loosely on the old Neophyte design but with a few mods. got the VFO stable enough, mixer seems to be doing its thing, and i can actually hear stations now which is more than i could say two weeks ago.

the problem is the audio is really muddy and there's this low frequency rumble underneath everything, like a hum almost but not 60hz exactly, closer to maybe 120 or so. i've got a simple audio amp stage after the mixer using a LM386 and i know those things are noisy but this seems worse than it should be. decoupled the power supply pins with a 250uf cap and a 0.047 going to ground and it helped a little but not enough.

also getting a lot of what i think is AF feedback, when i turn the volume up past about half the whole thing kind of oscillates. i built it ugly style on a piece of copper clad and the layout is probably not great. anyone dealt with this kind of thing with the LM386, is it just a garbage chip or am i missing something obvious

the LM386 is kind of notorious for this honestly. that oscillation at higher gain settings is really common if you dont have the right bypass on pin 7, thats the bypass pin and most people skip it but you really need a cap there, something like 10uf to ground. also check how close your input wiring is to your output wiring because with ugly construction that coupling can just kill you.

the 120hz hum sounds like rectifier ripple getting in somewhere, either through the supply or through a ground loop. i'd try running it off a battery and see if the hum goes away, if it does you know its a supply issue and not a layout thing. if it stays then you've got a ground loop somewhere and that's more annoying to chase down.

fwiw i built a DC receiver a few years back and spent like three weekends fighting audio issues, ended up redoing the whole audio section with an LM380 and it was night and day. the 386 can be made to work but it needs more babysitting than the datasheet implies.

yeah the layout thing is probably a big part of it. i had almost the exact same problem on a regen i built last year, ugly style on copper clad, and the audio was oscillating and just generally bad. ended up putting a small shield between the RF and audio sections made from some scrap copper clad and it helped a lot. not a perfect fix but enough to make it usable.

also are you shielding the LM386 circuit at all or is it just out in the open? those things pick up all kinds of garbage if they're exposed.