so i finally finished building a direct conversion receiver for 40m, been working on it for about three weeks now. used a NE602 as the mixer and a LM386 for the audio amp stage, pretty standard stuff. the thing actually works which honestly surprised me a bit because my soldering is not exactly museum quality.

problem is theres this weird hum coming through that i cant seem to kill. its not 60hz, sounds more like 120hz so im thinking rectified AC getting in somewhere but ive got it running off a 9v wall wart and i even tried batteries and the hum is still there. less on batteries but still noticeable. also the audio is kind of muddy, like stations that my commercial rig pulls in clean sound like theyre underwater on the homebrew. the sensitivity seems okay though, im hearing things i didnt expect to on 40m at night.

anybody built one of these and run into the hum issue? im wondering if its RF getting into the audio stage or if my ground plane is just a disaster. built it on perfboard which was probably a mistake but i wanted to see if it even worked before committing to a real PCB layout.

the 120hz hum with batteries is interesting, that does sort of rule out the power supply as the main culprit. i had the same exact thing on a receiver i built a few years back and it turned out to be RF from the oscillator getting into the audio amp. the LM386 is notoriously sensitive to that kind of thing. try putting a small cap, like 0.1uF, right across pins 1 and 8 on the 386 and also bypass pins 6 and 7 to ground real close to the chip. the muddy audio is probably a bandwidth thing, the 602 mixer output needs some low pass filtering before it hits the audio stage or you get all kinds of garbage in there. what does your filter between the mixer output and the amp input look like? if its just a coupling cap with no LC or RC lowpass youre gonna have a bad time on a perfboard layout especially.

yeah perfboard bites you every time with RF stuff, the ground paths are all over the place. i went through like four versions of a similar receiver before i gave up and just did ugly construction style, dead bug on a piece of copper clad. way better results honestly. the NE602 is a great chip but it does oscillator injection into everything if you're not careful about shielding. did you put the osc section anywhere near the audio? that'd do it. also 40m at night is rough for a direct conversion because of all the broadcast hash, you might just be hearing the band and blaming the radio lol