so i've been building this 40m direct conversion receiver from scratch, mostly following the W7ZOI designs from Experimental Methods but adapting a few things because i couldn't source the exact toroids and ended up winding my own on T50-2 cores. anyway the thing actually receives signals now which is honestly more than i expected given how many times i reflowed the NE602 pads.

the problem is theres this hum riding on everything. not horrible but noticeable enough that its annoying on weak signals. im pretty sure its 60hz or maybe 120hz, i havent scoped it yet but it kind of pulses at that rate. the power supply is a little wall wart i had laying around, regulated 12v but who knows how clean it really is. audio chain is just the NE602 into an LM386 with a cap on pin 7 to knock down the gain a bit.

anyone dealt with this on a DC receiver before? i added a 100uf across the supply rails already but im wondering if i need to do more filtering or if the issue is more likely ground loops or something in the AF stage. the whole thing is built ugly style on a chunk of copper clad so grounding should be decent but maybe not.

yeah that sounds like supply noise getting into the AF stage, the LM386 is pretty sensitive to that. the cap on pin 7 helps but id also try a RC filter right before the chip's supply pin, something like 10 ohm resistor and then 470uf to ground. makes a huge difference. also check if the hum changes when you touch different parts of the board, sometimes just moving a wire or adding a small ferrite bead on the supply lead to the 602 cleans things up a lot.

the wall wart thing is real too. some of them are genuinely terrible, i had one that was supposedly regulated but was putting out like 80mv of ripple. grabbed an old linear supply i had on the bench and the hum just disappeared. worth ruling out before you chase your tail on the board layout.

DC receivers are kind of notorious for this honestly. part of it is just the nature of the beast, you're basically amplifying audio all the way from the detector so any garbage in the supply chain gets amplified right along with your signal. i built a similar thing a few years back and spent like two weekends chasing a hum that turned out to be a ground loop between the audio board and the BFO section, had them on separate chunks of copper clad and the connection between them was creating a little antenna. ended up just mashing it all onto one board and most of it went away.

also worth checking if the hum is actually 60 or 120hz, 60 suggests magnetic pickup (like from a transformer sitting too close or even just the orientation of your toroids relative to AC wiring in the room), 120hz is more classicly rectifier ripple. if you can borrow an oscilloscope even briefly itll tell you a lot fast.