so anyway i've been building this 40m direct conversion receiver for the past couple months, based loosely on the old Neophyte design but ive been modifying it quite a bit. got the VFO stable enough finally after switching to a JFET buffer stage, that took way longer than it should have.

the problem now is the audio stage. im getting a ton of hum and also this weird low frequency rumble that kind of pulses, not sure if its 60hz related or something oscillating. ive got a TL072 in there as the audio amp and the supply is a 12v wall wart which i know probably isnt helping. tried adding some filter caps on the supply rail and it helped a little but not much. the demodulated audio actually sounds decent when i plug in a battery instead so yeah its definitely the supply but even with a decent filtered supply i still get some residual hum.

also picking up a lot of broadcast AM bleedthrough which i expected but its worse than i thought. dont have an RF prefilter on there yet, that's probably next. just wondering if anyone else has gone down this road and has tips for the audio chain specifically. grounding is kind of a rats nest right now i'll admit.

the grounding rats nest is almost certainly your main problem, or at least a big chunk of it. direct conversion receivers are super sensitive to layout because the signal levels at audio are so low and you've got the LO running right there on the same board. i built a dc receiver a few years back and spent honestly more time on grounding and shielding than the actual circuit. what helped me was going to a single point ground, running everything back to one spot on the ground plane rather than letting the return currents wander around. also make sure your audio wiring isnt running parallel to anything carrying RF, even a little bit of coupling there can cause all kinds of weird artifacts.

the broadcast bleed is going to be rough without some filtering up front. a simple low pass or bandpass before the mixer makes a huge difference, even just a couple toroids and caps. the TL072 is fine for this application but it does have some noise so if you're still getting hum after sorting the layout you might try a NE5532 or even just a single LM386 to test with.

yeah wall warts are terrible for this stuff, i learned that the hard way on a receiver i built. even the switching ones that claim to be filtered are just awful. i ended up winding a little power supply with a 7812 and a proper transformer and it made a night and day difference. also +1 on the single point grounding thing, that fixed a similar hum issue i had on a bitx build i was messing with. the pulsy low frequency rumble you're describing sounds like it could be a ground loop to me but hard to say without seeing the layout.