so ive been building a direct conversion receiver for 40m, pretty basic stuff, NE602 mixer feeding an LM386 audio amp and it mostly works but the audio is just... weird. like i get signals fine, SSB is intelligible if i crank the volume but theres this constant low frequency hum underneath everything and when strong signals come in the whole thing kind of motorboats for a second before settling down.

ive bypassed everything i can think of, put a 10uf cap across the 386 supply pins, tried different values on the gain pins (pins 1 and 8), even wrapped the thing in foil which helped a tiny bit with the hum but not much. the oscillator is running off a separate little 78L05 regulator so i dont think its bleeding into the audio stage from there. board is ugly point to point on perfboard but ive kept the RF and audio sections on opposite ends.

anyone built one of these and dealt with this kind of thing? im wondering if the 386 is just a bad choice here and i should go with an op-amp audio stage instead, or if theres something obvious im missing. its my first homebrew receiver so probably something dumb

the LM386 is notorious for this honestly. it wants to oscillate at the drop of a hat especially when you have any gain set on pins 1 and 8. what value do you have across those pins right now? if youre running the full 200x gain thats almost certainly your problem with the motorboating. try pulling whatever is between pins 1 and 8 and just leave them floating for the 20dB gain setting, itll be quieter even if a bit low on volume.

the hum though, in my experience with direct conversion builds, is almost always either the VFO leaking or just plain power supply noise. the 78L05 is fine in theory but whats upstream of it? if youre running off a wall wart without much filtering that hum is going to get in everywhere. ive had luck putting a big electrolytic like 1000uf right at the power input and then another 100uf after any regulators. also make sure your ground returns are all going back to one point rather than daisy chaining across the board, ground loops on perfboard builds will kill you.

yeah the 386 thing is real, i went through like three of them on a pixie clone before i gave up and just used a TL072 with a little feedback network. way less drama. sounds better too IMO.

one thing nobody mentioned to me when i was starting out with DC receivers is that 60hz hum can also come in through the antenna if youre picking up mains radiation from house wiring, so even a perfect power supply wont help if your antenna is running near the house. not saying thats your issue but worth ruling out by just trying a dummy load and see if the hum changes.