so anyway ive been building this 40m direct conversion receiver for about 3 months now, mostly on weekends when i have time. based loosely on the NorCal 40A schematic but i made some changes to the audio section because i wanted more gain and i thought i could improve on the original design. spoiler alert: i probably could not.

the rf side seems fine, i can hear signals and the sensitivity is decent, tuning around i can pick up plenty of SSB and CW. but the audio is this weird bassy muddy sound and theres a constant low hum in the background, not 60hz exactly, more like 120hz so im guessing its power supply ripple getting in somewhere. also when i touch the volume pot the hum changes which is making me think its a grounding thing maybe.

the audio amp section is built around an LM386 which i know isnt the best choice but i had a bunch of them. running it at about 46db gain with the 1.2k and 10uf between pins 1 and 8. bypass cap on pin 7 is 10uf. i did add a 10 ohm resistor and 0.047uf cap on the output like the datasheet suggests to keep it stable but i wonder if my layout is part of the problem since this whole thing is built dead-bug style on a piece of copper clad.

anyone dealt with this kind of audio mess on a homebrew receiver? not sure if i should just tear out the audio section and start over or if theres a tweak i can try first

the LM386 is notorious for picking up garbage if you dont layout the ground plane carefully, and dead bug on copper clad can go either way depending on how you handle the grounds. the 120hz hum is almost certainly power supply, i'd start by putting a bigger filter cap right at the V+ pin of the 386, like 100uf or even 220uf electrolytic with a small ceramic in parallel. also try lifting your audio ground and connecting it at only one point to the main ground — ground loops in homebrews will kill you.

the muddy sound could be the high gain setting tbh. 46db on the 386 is a lot and it saturates pretty easily especially if theres any DC offset coming in from the detector stage. might be worth trying it without the gain boost caps first just to see if the audio cleans up even if its quieter. if it does, thats your problem.

yeah i had almost the exact same thing happen on a DC receiver i built years ago and it drove me crazy for weeks. ended up being that the pot itself was acting like an antenna and picking up mains hum, i had to shield the wiring to it with a short piece of braid. also try rotating the board around if you can, sometimes you just happen to be oriented wrong relative to a noise source and physically moving things helps. sounds dumb but it works sometimes