so ive been working on this DC receiver for about two months now, based loosely on the old Neophyte design but i changed out a few parts because i couldnt source the exact transistors. the thing actually receives, which honestly surprised me after the first power-on smoke test (lost a 7805 but whatever). im getting signal, SSB is kinda intelligible, CW is decent actually.

the problem is this low frequency hum that gets into everything. its not 60hz exactly, sounds lower maybe, and it pulses kind of rhythmically. i checked my power supply filtering and added another 1000uf cap across the rails which helped a tiny bit but its still there. the LO is running off a NE602 with a colpitts oscillator and i wonder if the oscillator is coupling into the audio stage somehow. my layout is pretty rough, just point to point on some perf board.

anyone dealt with this before? im not sure if its RF getting rectified somewhere or if its just my grounding being a mess. probably the grounding honestly.

yeah that rhythmic pulsing thing is almost always a ground loop or you've got your audio return path sharing a wire with something noisy. on perf board builds the ground scheme matters way more than people think and its tempting to just daisy chain everything together but it bites you every time.

what i'd do is trace where your audio ground goes back to the main filter cap and make sure the NE602 ground and the audio amp ground are both going to the same star point directly. if theyre sharing a long trace or wire with the oscillator circuit you'll pick up all kinds of garbage. also the 602 output pins can couple LO harmonics into the following stage if theres any impedance on those lines, some guys put a small RC low pass right at pin 4 and 5 before hitting the audio amp, something like 10k and 0.1uf, cheap fix worth trying.

the 1000uf cap you added helps with ripple but its not gonna fix coupling issues inside the circuit.

could be your colpitts oscillator loading up when signals hit the mixer, ive seen the 602 do weird stuff when the input isnt properly terminated. whats your antenna situation, are you running it straight into the RF input or do you have any filtering ahead of it? sometimes a big broadcast station nearby will basically overdrive the whole front end and you get all kinds of intermod junk that sounds like hum at audio frequencies. i built something similar last winter and had a really similar symptom, turned out i had an AM station about 2 miles away just killing my dynamic range. threw a simple bandpass filter in front and it cleaned up a lot.