so ive been building this direct conversion receiver for 40m, based loosely on the old Neophyte design but i swapped out the NE602 for an SA612 because thats what i had in the junk box. got it oscillating, picking up signals, the whole thing. problem is the audio coming out is this horrible low frequency rumble underneath everything, like a washing machine in the background, makes SSB basically unlistenable.

i went back and checked my grounding and it looks fine to me, all the bypass caps are there, i used 0.1uf ceramics at the supply pins. the power supply is a little wall wart 12v which i know isnt ideal but ive run other projects off it without this kind of noise. the LO is pulling a little i think, frequency drifts around when i touch the coil, but i dont think thats causing the audio issue specifically.

anybody dealt with this kind of thing on a direct conversion build? wondering if its an IF issue or just crappy layout on my part. built it ugly style on copper clad.

that rumble is almost certainly hum getting in from the wall wart, direct conversion receivers are brutally sensitive to supply noise because youre basically doing all your amplification at audio frequencies. the 602/612 doesnt help because the carrier null isnt perfect and any ripple on the supply leaks straight into the detector output.

try running it off a 9v battery and see if the rumble goes away, if it does youve found your problem. if you stick with a supply you want serious filtering, like a LM7809 regulator plus maybe 100uf after it and another RC stage before the audio amp section. some guys use two separate regs, one for the oscillator and one for the rest of the circuit. also double check your bypass caps are actually low impedance at 60hz and 120hz, ceramics are fine at RF but for audio hum you sometimes want an electrolytic in parallel too.

yeah the wall wart is probably it but also check if your audio gain stage is just cranked up too high picking up everything. i had a similar thing with a pixie clone i built and it turned out i had a solder bridge i couldnt see that was messing with the bias on the audio amp, took me like two weeks to find it. ugly construction makes that harder to trace but just go over every joint with a magnifier if you can