so ive been working on a simple direct conversion SSB transmitter for 40m, nothing fancy, based loosely on the BITX design but with some mods because i couldnt source a few of the original parts locally and ended up substituting. the RF output looks okay on the scope but when i pipe audio in from my mic preamp the output sounds really muddy, almost like theres a low frequency oscillation riding on top of everything. ive checked the balanced modulator and the carrier null looks good, maybe 40dB down. the audio stage itself sounds clean going in.

im thinking maybe its the VFO coupling or possibly something in the driver stage picking up the audio line, i ran the wiring kind of sloppy on the first build. anyone dealt with something like this on a homebrew rig where the audio path is getting into the RF chain in a weird way? i cant figure out if its feedback through the power supply rail or actual physical coupling. the whole thing is point to point on a piece of copper clad, no ground plane under the audio section which in retrospect was probably a dumb idea

yeah that sounds like a grounding issue to me, almost certain. on my first BITX-ish build i had almost exactly the same thing and it was because the audio ground and the RF ground were sharing a long trace back to the supply. any AC on the supply rail gets into everything when you do it that way. what i ended up doing was running a dedicated ground bus in heavy gauge wire straight back to the main filter cap, kept audio completely separate from the RF side. the muddy sound went away almost immediately after i did that.

also worth checking if your mic preamp has enough rejection of RF getting back into it — sometimes the transmitter couples back into the preamp and you get this weird feedback loop that shows up exactly how you described. a small bypass cap right at the preamp output before it hits the modulator can help a lot with that.

could also just be the VFO. if its not well shielded the audio wiring running nearby can FM modulate it slightly and that'll mess up the output in a way that doesnt really look like classic hum, more like that muddy thing you're describing. i had a manhattan style build years ago where the VFO was totally open and my mic cable was only a couple inches away, took me forever to figure out why it sounded bad on SSB. stuck a little tin can shield around the VFO coil and it cleaned right up.