so ive been working on this direct conversion receiver for 40m for the past couple months, mostly following the EMRFD designs but ive been kind of winging it on the audio stage because i couldnt get the exact parts the book calls for. anyway got it on the bench last night and it actually receives something which honestly surprised me given how messy my Manhattan style construction got toward the end.

the problem is theres this really annoying hum on everything, like a 60hz thing underneath whatever im trying to listen to. ive already tried lifting the ground on the audio amp section and that didnt do much. power supply is a regulated 12v bench supply so i dont think its coming from there but who knows. also the image rejection is pretty much nonexistent which i expected but its worse than i thought it would be, stations on the wrong side of zero beat are almost as loud as the ones i actually want.

not really sure which problem to tackle first. the hum is more annoying but the image thing might point to something being wrong with the phasing in the detector. anyone dealt with this with a simple diode ring type detector

the hum on DC receivers is almost always one of two things — either ground loops somewhere in the audio chain or RF getting into the audio and rectifying on something. the second one is sneaky because it looks exactly like power supply hum but its actually coming in through the antenna or just radiated pickup. try wrapping your audio board in some copper tape and grounding it, and make sure your audio leads arent running parallel to anything carrying RF. also double check that your detector diodes are all matched, mismatched diodes in a ring detector kill your balance and thats what lets hum in through the LO port.

the image thing in a DC receiver without phasing is just kind of the nature of the beast unless you went with a phasing design. if you used a simple diode ring without the 90 degree hybrid stuff you basically have a DSB receiver and both sidebands come through equally, thats not really a flaw its just how it works. you can add a phasing network after the fact but honestly a lot of people just live with it on 40m homebrew stuff.

Manhattan style construction and 40m DC receivers, that combo can get weird with layout. i had the same hum issue on one i built a few years back and it ended up being that i had the audio stage physically too close to the LO circuitry. moving them apart by like an inch and a half on the board made a huge difference. sounds dumb but RF coupling into the audio input transistor was the whole problem.