so ive been building this 40m direct conversion receiver from scratch, mostly just following a design i found in an old QST article from like 1994 and some stuff i cobbled together from online schematics. the VFO is a colpitts oscillator running around 7 MHz and the mixer is just a pair of diodes, pretty basic stuff. it actually receives signals which honestly surprised me because my PCB layout is kind of a disaster.

the problem is the audio coming out of it sounds like garbage. not like weak or anything, i can hear stations, but theres this really annoying hum and also kind of a motorboating sound when i turn the volume up past like halfway. i put a 100uF cap on the power supply rail to the audio stage which i thought would help but it really didnt do much. using an LM386 for the final audio amp stage.

also the whole thing is really microphonic, like if i tap the bench the audio freaks out. im guessing thats the VFO being sensitive but not sure. anyone dealt with this kind of thing before? its my first homebrew receiver so i might just be in over my head here

the motorboating is almost definitely a power supply decoupling issue, 100uF alone isnt gonna cut it on an LM386 circuit. what you want is a combination of a bigger electrolytic AND a small ceramic right at the supply pin of the chip, like a 10uF or 47uF electrolytic plus a 0.1uF ceramic bypassed together. the LM386 is notorious for this. also check pin 7, theres a bypass cap that goes there too that a lot of people miss when they build these from scratch.

the microphonics on the VFO are just part of life with a colpitts unless you do something about it. you can try potting the coil in wax or just enclosing the oscillator section in a small tin box, even an altoids tin works. definitely shield it from the rest of the circuit too because the mixer diodes can inject noise back into it. direct conversion receivers are kind of sensitive to all of this stuff, its not just you, they take some fiddling to get right.

yeah the LM386 hum thing got me too on my first build, took me forever to figure out it was a ground loop thing more than the power supply. try running a dedicated ground wire straight from the audio stage back to your main ground point instead of relying on the PCB trace. star grounding basically. made a huge difference on mine. also how are you powering it, if its a wall wart that thing could be injecting all kinds of noise