so ive been working on this DC receiver for 40m for like the past two months, based loosely on the Sudden receiver design but with a few mods i found on various sites. got the VFO stable enough finally after fighting that for way too long, and i can actually hear stations now which feels huge after nothing but noise for weeks.

problem is the audio quality is just... bad. not like bad SNR bad, more like there's this low frequency hum that kind of wobbles in and out and the whole thing sounds like im listening through a tin can. ive double checked the power supply decoupling and added a few extra caps across the rails which helped a tiny bit but not much. the audio amp stage is just a basic LM386 circuit, nothing fancy, gain set to about 46dB with a cap across pins 1 and 8.

my gut says its either a grounding issue or maybe some RF getting into the audio stage but i really dont know where to start. the layout is kind of a mess on the breadboard honestly. anyone been through something like this with a similar build?

yeah the LM386 is notorious for picking up junk when the layout isnt tight. at 46dB gain that thing will amplify basically everything including stuff you dont want. first thing id try is keeping the input lead as short as absolutely possible and make sure you've got a cap right at pin 6 to ground, like physically right there on the chip not a few inches away. also a 10 ohm resistor in series between pin 5 and your output cap is kind of a standard trick to tame some of the instability, lot of the app notes mention it but people skip it.

the wobbling hum you're describing actually sounds like it could be microphonics on the breadboard if you're on spring contacts. those things are terrible for RF builds, any vibration or even air movement can modulate stuff in weird ways. if you can solder even a rough ugly deadbug version of the audio stage and compare that tells you a lot.

the Sudden is a great starting point, built one years ago. one thing i ran into was RF getting back into the audio through the volume pot, had to add a small cap from the wiper to ground to kill it. might not be exactly your problem but worth a shot, took me forever to figure that one out and it was like a 5 cent fix.