so i've been working on a direct conversion receiver for 40m for probably the last three months, mostly following the Solid State Design for the Radio Amateur book with a few mods i found on some blog i can't find anymore. got it mostly working last weekend and i can actually hear SSB which is pretty exciting for a first build.

the problem is there's this persistent hum in the audio that i can't seem to kill. it's there whether the antenna is connected or not and it doesn't really change with the volume control which makes me think it's getting in after the audio amp stage or maybe the power supply is just garbage. i'm running it off a wall wart right now, one of those old switching ones i had lying around. probably the problem right there honestly.

sensitivity seems decent, i can hear plenty of stations but the hum is pretty annoying on weak signals. anyone dealt with this on a DC receiver build? i've tried adding some filter caps across the supply rails but it didn't really help much. wondering if i need to just build a linear regulator board for it or if there's something else i should be checking first.

yeah switching wall warts are basically hum generators, i learned this the hard way on my first NE602 build. even a dead simple 7812 or 7805 linear reg with a decent filter cap before it makes a huge difference. i used a 2200uF before the reg and a 100uF after and the noise floor dropped dramatically. worth trying before you go crazy probing every stage.

also check your ground layout, DC receivers are really sensitive to ground loops because the audio frequencies are the same as the IF so any noise just rides right through. star grounding helps a lot on these builds if you havent already done that.

could also be RF getting into the audio stage and mixing down to audible frequencies, especially if you're near any switching supplies or even LED lighting. i had a similar thing on a regen i built and it turned out the culprit was my monitor's power brick sitting about a foot away. moved it to the other side of the desk and half the hum went away. shielding the audio section with some copper tape helped too, not a perfect fix but definitely reduced it.