so ive been working on this 40m direct conversion receiver for the past few weeks, pretty standard NE602 front end into an LM386 audio stage, nothing fancy. got it all wired up on a piece of copper clad with ugly construction and honestly the RF performance is better than i expected — im hearing stations just fine, selectivity is what it is with a DC receiver but thats fine.

the problem is theres this constant 60hz hum in the audio and i cannot for the life of me figure out where its coming from. ive got it running off a wall wart supply that i thought was clean enough, measured the ripple on my scope and its pretty minimal, maybe 20-30mV which shouldnt be causing this. tried running it off a 9v battery and the hum drops way down but doesnt completely go away. so clearly part of it is the supply but something else is going on too.

the audio amp section is probably the culprit honestly, the LM386 is notorious for picking up garbage if you dont lay it out carefully and im not sure my bypass caps are where they should be. i have a 10uf on pin 6 and a .047 on pin 7 but maybe i need more. anyone dealt with this before with the 386

yeah the 386 is a pain in the neck with hum, ive built probably a dozen little receivers and audio gadgets with that chip and layout really does matter a lot more than people think. a few things id check — first make sure your input ground and output ground arent sharing the same path back to the supply, that chip will couple output noise right back into the input if your ground plane isnt thought out. also the bypass on pin 6, 10uf is okay but i usually go higher, like 100uf, and put a small ceramic right next to it too. the datasheet circuit shows a 250uf which seems overkill but its not wrong.

also and this is the thing people miss — if youre picking up hum even on battery there might be a ground loop from your antenna connection. if the coax shield is connected to anything else in your shack thats also connected to mains ground youre gonna get 60hz no matter what your supply is doing. worth lifting the antenna ground and seeing what happens.

i had the exact same issue with a receiver i built last year, drove me crazy for like two weekends. turned out the problem for me was i had the input and output wiring of the 386 running parallel to each other for a few inches and it was just oscillating at a low frequency and presenting as hum. rerouted the wires and it cleaned right up. might not be your problem but worth looking at the physical layout before you go changing component values.