so ive been working on this 40m direct conversion receiver for about three weeks now, following the NorCal 40A schematic more or less but i kind of deviated on the audio section because i had some TL072s on hand instead of whatever they called for. got the thing mostly working last night and i can hear stations but theres this really annoying hum that seems to follow the signal level, like it gets louder when im tuned to a strong station and then quiets down a bit when im between signals. doesnt totally go away even with the antenna disconnected though which is weird.

my power supply is a wall wart 12v switching type, i know i know, probably should have used a linear supply or at least added more filtering. i do have a big 2200uf cap across the rails and a couple of .1uf ceramics scattered around. the hum is around 120hz i think, i tapped it through my soundcard and looked at it in fldigi and it looks like a really dirty 120hz spike, so definitely some rectification artifact getting in somehow.

has anyone dealt with this on a homebrew DC receiver specifically, the part that confuses me is why it gets worse with strong signals. i half wonder if the switching noise is somehow getting coupled back in through the antenna but that seems backwards. anyway its been bugging me for like 4 days and im starting to question all my decoupling choices

the switching supply is almost certainly your problem, those things are garbage for receiver work, i learned that the hard way on a similar build a few years back. but the thing about it getting louder with strong signals is actually pretty explainable -- if your AF gain stage has any DC offset or your op-amp rails are getting pulled around by the supply noise, a strong signal coming through is going to modulate that noise floor in kind of a weird way. basically the supply cant keep up cleanly and you hear it.

TL072s are fine btw, thats not your issue. what i would try first is just temporarily running the thing off a 9v battery or a decent bench supply and see if the hum goes away. if it does, you have your answer and you can work on filtering from there. a 7812 regulator with some decent filtering ahead of it goes a long way. if the hum persists on battery power then you got a grounding or layout problem and thats a whole different conversation

yeah what he said about the battery test, do that first before anything else. i spent a whole weekend re-routing traces on a VFO build chasing hum and it turned out to be my bench supply the whole time, felt pretty dumb about it honestly.

one other thing to check -- are your audio grounds and your RF grounds all coming back to a single point? on DC receivers especially ive seen people run into trouble when the audio return path is kind of floating around looking for ground and it picks up all kinds of junk. star grounding on these things makes a real difference even when the layout looks fine on paper