so ive been lurking on this forum for about a year now and finally pulled the trigger on building something from scratch. went with a basic direct conversion receiver for 40m, mostly following the EMRFD designs with a few mods i found on various blogs. got it mostly working last weekend which felt pretty good honestly.

the problem im running into is theres this low frequency hum that shows up whenever i touch the enclosure or get close to it. not 60hz exactly, more like a warbling kind of thing. i grounded everything i could think of including the chassis to my station ground but its still there. the audio stage is built on manhattan style pads and im wondering if maybe im picking up stray RF from my antenna feedline which runs pretty close to the bench. or maybe its the LO leaking back somewhere, i dunno.

also separate question — the BFO injection level, how much do you guys typically run into the mixer for a diode ring type setup? ive been reading anywhere from 7dBm to like 17dBm depending on the source and im not sure what to believe. my signal gen only goes to about 10dBm output so thats what im using right now and it seems okay but the sensitivity feels a bit low compared to what people describe.

anyway the whole thing is crammed into an old Altoids tin for now which probably isnt helping anything RF-wise but i wanted to get it on the air before doing a proper enclosure. any thoughts appreciated

the warbling hum thing sounds like it could be your LO getting into the audio path, especially if youre running the VFO and audio stages close together on the same board. direct conversion receivers are notoriously sensitive to that kind of stuff. the fact that it changes when you touch the enclosure makes me think theres a ground loop somewhere or maybe the bypass caps on your audio amp power rails are insufficient. id try stuffing a 100uF right at the chip supply pins if you havent already, and make sure all your grounds are star-grounded back to one point rather than daisy chained.

on the BFO injection question — for a standard SBL-1 or similar diode ring mixer you generally want around 7 to 10dBm at the LO port, going higher doesnt help much and can actually start to degrade things depending on the diodes. 10dBm should be fine. if sensitivity feels low id check your RF preamp gain first, or just try adding a simple one or two transistor amp stage ahead of the mixer. sometimes that makes a huge difference.

altoids tin is definitely not helping lol. i built a similar rx a few years back and had so many weird grounding issues until i moved it to a proper aluminum chassis. the stray capacitance from the lid alone can do weird things. that said ive seen people make altoids builds work so its not impossible, just harder to debug.

cant really comment on the specific hum without knowing more about your audio amp schematic but the touching the case thing sounds classic ground loop to me. does it go away at all if you disconnect the antenna?