so ive been building this direct conversion receiver for 40m based loosely on the NorCal 40A design but with some changes i made to the BFO section because i couldnt source the exact crystal. got it on the air last night for the first time and its actually pulling in signals pretty well, better than i expected honestly. the sensitivity seems decent and i can hear CW stations no problem.

problem is theres this low hum riding on top of everything. its not 60hz exactly, sounds more like 120hz to me but hard to say without putting a scope on it. ive got the whole thing running off a 12v wall wart right now which is probably the issue but ive tried two different supplies and both do the same thing. the filtering on my voltage regulator is just a couple caps nothing fancy. thinking i need to add more filtering or maybe just switch to battery power for now to rule stuff out.

anyone dealt with this kind of thing on a homebrew DC receiver? i know they can be finnicky with power supply noise. also wondering if it could be ground loops since i have the audio amp section kind of floating from the RF board right now, wired them up with some pretty long leads between the boards which was dumb in retrospect.

yeah almost certainly your power supply or grounding, classic DC receiver headache. the long leads between boards is definitely suspect, ground loops on those things are a nightmare. i built a similar rig a few years back and had the exact same 120hz hum, turned out to be a shared ground return path that was picking up rectifier ripple. star grounding fixed it almost completely.

try the battery first before you go down the filter rabbit hole, if the hum goes away youre chasing a supply problem and can work from there. if it doesnt go away on battery power then youre dealing with RF getting rectified in your audio stage or a layout issue. also how are you decoupling between the LO and the audio amp? if that section isnt well isolated you can get all kinds of weird stuff coupling through.

long leads between boards will do it every time, i learned that the hard way on my first scratch build. even an inch of wire can act like an antenna for interference in those low level audio stages. i'd just run it on a battery and see what happens before you tear into anything else, saves a lot of chasing your tail