so ive been building this 40m direct conversion receiver on and off for like 3 months now, based loosely on the NorCal 40 design but with a few changes because i couldnt source some of the original parts. anyway got it actually receiving signals last night which was pretty exciting, SSB sounds decent and CW is nice and crisp, but theres this low frequency hum underneath everything that doesnt go away no matter what i do with the volume or the RF gain.

ive already tried a different power supply thinking it was ripple from my bench supply but same hum on a fresh 9v battery so its not that. the LO is a simple colpitts running around 7 mhz and im wondering if its maybe audio pickup from something or maybe my layout is causing issues. ground plane is just a piece of copper clad i etched myself, connections look solid. the hum sounds like its maybe 60hz or close to it, hard to say exactly without a scope probe on the audio stage.

anyone dealt with something like this on a homebrew DC receiver before? im guessing its either my detector stage picking up mains somehow or maybe a ground loop between the audio amp and the detector but im not sure where to even start poking around.

yeah 60hz hum on a DC receiver is almost always one of two things in my experience — either your audio stage is picking up radiated hum from something nearby (fluorescent lights are terrible for this, even LED drivers can cause it) or you've got a ground loop between your detector and audio amp sections. the fact that it doesnt change when you switch power supplies pretty much rules out the supply itself.

i'd try shielding the audio amp section first, just tape some copper foil around it and tie it to ground and see if anything changes. also check if the hum changes when you move the board around or rotate it — if it does, its probably pickup rather than a loop issue. on my 20m receiver i had the exact same thing and it turned out my audio transformer was sitting too close to a toroid i was using for the RF input filter, just rotating it 90 degrees fixed it. weird how much difference that can make.

did you try just shorting the input to your audio amp stage and seeing if the hum is still there? if it goes away with the input shorted then its coming from upstream somewhere, if it stays its in your audio amp itself. thats usually the first thing i do when chasing hum in a homebrew build, saves a lot of time compared to just poking around randomly