so ive been working on this 40m direct conversion receiver for about three weeks now, started from a design i found in an old QST from like 2003 or so. built the VFO around a hartley oscillator, nothing fancy, just whatever toroids i had in the junk box and a 2N3904 for the active part. the product detector is an NE602 which i know some people hate but it works fine for me usually.

the problem is the audio out is super noisy, like not just band noise which i'd expect, but this kind of low hiss/buzz that doesnt change no matter what band conditions are. grounded the input, still there. tried a different audio amp stage (LM386 originally, swapped to a little op-amp config i pulled from somewhere) and the noise profile changed a tiny bit but its still there. power supply is a 12v wall wart which yeah i know could be the issue but ive got a cap on the supply rail right at the board.

not sure if its oscillator leakage getting into the audio path or just RF getting in somewhere. layout is pretty much dead-bug on a piece of copper clad which probably doesnt help. anyone dealt with this before

the LM386 is notorious for that kind of noise honestly, those things pick up everything. but if you already swapped it out and it's still there then yeah my first suspicion would be the wall wart. a single cap isn't always enough, try an LC filter or even just a 78L09 to drop it down and regulate it properly, that alone has fixed similar stuff for me more than once. also with dead bug on bare copper clad you really want to think about where your ground returns are going — if the audio ground and the oscillator ground are sharing a long trace back to the supply you'll get all kinds of garbage coupling in. star grounding is kind of a pain to retrofit but worth it.

what value cap do you have on the rail right now and where physically is it relative to the NE602?

yeah dead bug and wall warts are basically a noise party waiting to happen lol. i built something similar a while back, took me forever to figure out it was the switching supply radiating into my input transformer. just grabbed a 9v battery to test and half the noise went away immediately. worth trying before you go tearing anything apart