so i've been building this 40m direct conversion receiver from scratch, been at it for probably 3 weekends now. based loosely on the Sudden design but i changed a few things because i didnt have all the parts. anyway got it to the point where im actually hearing signals which is exciting but the audio is kind of... flangy? like everything sounds like its going through a phaser pedal or something. both sidebands are coming through at once i think which i know is a thing with DC receivers but this seems worse than what people describe online.

the BFO is running a colpitts oscillator around 7.1 MHz, NE602 as the mixer, then im running the audio into an LM386 amp. the problem might be my audio filtering, i basically just slapped a cap across the feedback resistor on the 386 and called it a day. i have a bunch of op amps in my junk box and i was thinking about adding a proper audio bandpass filter stage somewhere around 300-3000hz but not sure if that'll actually fix the flangy thing or if its something else going on upstream.

anyone built something similar and run into this? pictures of my ugly construction available if anyone wants to see what a mess looks like haha

yeah the phaser sound is almost certainly image rejection, or lack of it. direct conversion receivers pickup both the upper and lower sideband simultaneously and they mix together in the audio stage. if the two signals are close in frequency you get that weird comb/flange effect. its not a filter problem downstream, its just how DC receivers work unless you go quadrature and do phasing or use a Weaver method to cancel one sideband. some guys just live with it for CW since CW is symmetric anyway, but for SSB its pretty rough.

the LM386 thing probably isnt helping either, those things oscillate if you look at them wrong. the standard 10 ohm resistor in series with a 0.047uF cap from pin 7 to ground helps a lot, also bypass pin 6 to ground right at the chip. but honestly fix the image issue first or the audio filtering wont matter much.

i built almost the exact same thing last year, NE602 and LM386, and had the flangy issue too. what fixed it for me was paying more attention to the local oscillator leakage. my BFO was bleeding into places it shouldnt have been and causing all kinds of weirdness. shielding the oscillator section made a noticeable difference, i just used some copper clad board scraps soldered around it. might not be your problem but worth checking before you go rebuilding the whole audio chain.