so i've been working on a direct conversion receiver for 40m for the past few months, mostly following the Minima design but i kind of went off-script on the audio stage because i had some TL072s laying around instead of the specified opamps. got the whole thing assembled last weekend and it actually receives signals which honestly surprised me a bit.

the problem is there's this low frequency hum in the audio that gets worse when i touch the enclosure, and also i'm getting some weird heterodyne-type squealing when i tune near the bottom of the band. the BFO is running around 7.0 MHz and i built it on ugly-style construction on a piece of copper clad. my shielding is basically just a cookie tin i found in the kitchen which is maybe not ideal.

anybody dealt with this kind of thing before? im wondering if the hum is a grounding issue or if its RF getting into the audio stage somehow. the VFO is built on a separate board about 3 inches away from the audio section, no shielding between them. should i be worried about that or is 3 inches fine for this kind of build

the hum responding to touch is almost always a ground loop or a floating ground somewhere, i'd start by checking every ground connection with an ohmmeter and make sure your audio commons are all tied to the same point. star grounding matters a lot on these DC receivers because you dont have IF transformers doing the isolation work for you.

the squealing near the bottom of the band sounds like LO feedthrough or maybe the VFO is getting into the audio input — 3 inches on ugly construction with no shielding between the oscillator and a high gain audio stage is honestly asking for trouble. TL072s arent the worst choice but they can be a bit noisy, though i wouldnt blame them for what youre describing. try running a piece of copper clad or even just aluminum foil taped to cardboard as a divider between the VFO and audio boards and see if the squealing changes. also make sure your audio gain isnt cranked way up before you need it, those DC designs can have massive gain and even a little RF gets amplified into something horrible.

i had almost the exact same thing happen with a DC receiver i built last year, turned out one of my bypass caps on the audio supply rail was not making good contact, just barely touching the pad. drove me crazy for like two weeks before i found it. not saying thats your problem but worth reflowing everything on the power supply section first before you go nuts chasing RF issues