so ive been building this 40m DC receiver for the past couple months, basically a pretty standard NE602 front end into an LM386 audio amp, nothing fancy. been using ugly construction on a piece of copper clad which honestly i think looks great compared to my earlier stuff.

the problem is im getting this really strange artifact, hard to describe but its like a low frequency warbling underneath everything, maybe 3-4 hz, and it gets worse when i touch the coil form. at first i thought it was LO leakage or maybe microphonics in the coil but ive tried rewinding it twice and it didnt really change much. the warbling is there even when theres no antenna connected which is the part thats really got me scratching my head.

the NE602 is running off a 78L08 regulator so its not raw battery voltage, and i've got 100uf and 0.1uf bypassing pretty close to the pin 8 supply. ground plane seems solid. i dont know, maybe the LM386 is oscillating somehow and im hearing it get mixed back into the audio path? or could it be RF getting into the power supply somehow. any ideas appreciated because im kind of stuck

that warbling with nothing connected and it being sensitive to touching the coil — my first guess would be the LM386 is oscillating at a low frequency. those chips are notorious for being finicky, especially if you dont have that 10 ohm resistor and 0.047uf cap in series from pin 7 to ground. a lot of people skip that because it seems optional in the datasheet but it really isnt if you want stable gain. also check if you have any feedback path between your output and input ground, even a tiny bit of common impedance in the ground wiring can cause exactly what you're describing. try lifting the LM386 section completely and seeing if the warbling stops — if it does thats your culprit for sure.

yeah the LM386 thing is a good call but also — and i know this sounds dumb — are you sure your 78L08 has enough input headroom? if youre running it off like a 9v battery thats starting to sag a bit the regulator can drop out of regulation and do all kinds of weird stuff. had almost exactly this problem on a 80m regen i built last winter and it turned out the battery was just getting tired. the oscillation frequency i was seeing was similar to what you describe. stuck a 12v supply on it and the problem just disappeared. might be worth trying before going too deep into the circuit diagnostics