so anyway ive been building this 40m direct conversion receiver for the past couple months, based loosely on the NorCal 40 design but ive been modifying things as i go because i couldnt get some of the original parts and also i just wanted to tinker with the audio stage a bit. the RF front end seems fine, im getting signals, can hear SSB stations coming through which honestly felt like a miracle the first time it happened.

the problem is theres this hum sitting under everything. not a terrible 60hz buzz, more like something in the 120hz range maybe? its there even when i disconnect the antenna so its not pickup from outside. i checked my power supply and it measures clean on the meter but maybe the meter isnt fast enough to catch ripple, i dont know. the LM386 audio amp stage is where i suspect it but ive already bypassed the supply pins with what i thought was adequate filtering. running on a wall wart right now which i know isnt ideal but its what i had.

anyone built something similar and run into this? wondering if its worth just switching to battery power to see if the hum goes away as a diagnostic step or if thats too simplistic

yeah battery is literally the first thing i would try, takes 30 seconds and tells you immediately if its a supply problem. the LM386 is notoriously sensitive to supply noise and that chip will amplify pretty much anything on the Vcc pin right into your audio. 120hz is almost certainly rectifier ripple from a full wave supply in the wall wart, your meter almost certainly cant catch it unless you have a scope.

if switching to battery cleans it up then you know its the supply and you can either add more filtering -- like seriously, people always underestimate how much capacitance you need after that chip, i put 470uf right on the supply pin plus a 10uf tantalum and a 0.1uf ceramic and it made a huge difference on my last build -- or just run on battery all the time which is honestly fine for a receiver. if battery doesnt help then youre probably looking at a ground loop somewhere in the audio wiring or maybe RF getting back into the audio stage. what does your PCB layout look like, are you using manhattan or ugly construction or actual board?

the LM386 thing is real, i had almost the exact same issue on a regen i built last year. turned out one of my bypass caps was just not making good contact on the breadboard i was prototyping on, had me chasing my tail for like a week. also just mentioning -- if you havent already tried putting a ferrite bead in series with the supply lead to the chip that can help sometimes. probably wont fix a serious ripple problem but it doesnt hurt to try while youre diagnosing.