so i've been building this 40m direct conversion receiver on and off for like 3 months now, kept putting it down and coming back to it. finally got it oscillating and i can hear signals which is great but the audio sounds... wrong somehow. like there's this low frequency rumble underneath everything and voices sound kind of hollow and phasey. the LO is a colpitts running around 7.1 MHz, mixing straight into an NE602 then into an LM386 audio stage.

i think it might be RF getting into the audio section but ive shielded pretty much everything i can think of. or maybe its an issue with the product detector stage, im not totally sure. the rumble in particular is bugging me, its almost like 60hz hum but not quite. anyone built something similar and run into this? im pretty sure the power supply isnt the issue because it measures clean on my scope but what do i know lol

yeah that hollow phasey sound is classic direct conversion stuff, its just the nature of the beast honestly. youre hearing both sidebands at once so audio kinda folds back on itself. not much you can do about that without going phasing or weaver architecture. the rumble though, that does sound like a grounding issue or maybe the LM386 oscillating at low freq. those chips are notorious for that, you got a 10uF from pin 7 to ground? and a 0.047uF from pin 1 to 8 if you want more gain? if those bypass caps arent there it will do exactly what you're describing.

also check how close your audio wiring runs to the oscillator section. even a centimeter or two of unshielded wire near the LO can inject hum harmonics into the audio path in a weird way that doesnt look like hum on the scope but sure sounds like it.

NE602 into LM386 is a pretty standard combo, i built almost the exact same thing for 20m last year. the 386 is just kind of a noisy chip in my experience, some people swap it out for an LM380 or even just an op-amp audio stage and say it helps. but honestly for 40m direct conversion that phasey audio might just be what you have to live with unless you want to add a phasing network. the rumble is weird though, what are you running it off of? wall wart or battery? if its a wall wart i'd try a battery just to rule out the supply even if your scope says its clean, scope isnt always gonna catch everything.