so i've been messing around with a simple direct conversion rx for 40m for the past few weeks and i finally got it to a point where im actually receiving stations on it, thought id share some thoughts for anyone thinking about going down this road

started with the NE602 based design from the ARRL handbook, pretty standard stuff. the BFO/VFO combo using the onboard oscillator in the 602 works okay but man is it sensitive to hand capacitance, i ended up shielding the whole front end in a little tin box i cut from an altoids tin and that helped a lot more than i expected honestly

audio side is just an LM386 with the gain cap on pins 1 and 8, nothing fancy. getting some hum that im still chasing down, probably a grounding issue or maybe the layout — i just used ugly construction on a piece of copper clad so the ground plane situation is a little chaotic in spots. pulls in SSB just fine though, heard a few dx stations last night which felt like a win considering this thing cost maybe 8 bucks in parts

anybody else dealt with persistent 60hz hum on the LM386 stage? wondering if its worth just swapping to a TDA2822 or something

yeah the hum thing with the 386 is pretty classic, ive fought that battle more than once. usually for me its been the power supply — even if you think youve got clean DC feeding it, sometimes a little ferrite bead right at the Vcc pin on the chip makes a suprising difference. also check that your bypass cap on pin 7 is actually a good low ESR cap and not some ancient electrolytic that's mostly dead inside

the layout thing on ugly construction can bite you with the 386 specifically because any stray coupling from the RF side into the audio stage just gets amplified like crazy. i'd try keeping the audio board as physically far from the NE602 as you can get away with and see if it helps before swapping chips. the TDA2822 is fine but the 386 should work if the layout is clean enough

nice work getting it on the air, 8 bucks is pretty good. i built something similar last year except i used a toriodal wound VFO instead of the onboard osc and that helped the hand capacitance thing a lot, but it was obviously way more work to get stable. whats your frequency coverage like, can you tune the whole band or just a chunk of it