so ive been messing around with a direct conversion receiver for 40m for about three months now and its been a journey. started with the NE602 mixer and a 7 MHz crystal oscillator, which is pretty standard stuff, but i kept getting this horrible hum on everything and i couldnt figure out if it was the power supply or RF getting into the audio stage. spent two weekends just chasing that down.

turned out the issue was my ground plane situation — i was basically wiring things point to point on a piece of stripboard and the audio return path was crossing over the RF section in a really ugly way. once i rerouted the ground connections and added a proper bypass cap on the NE602 supply pin it cleaned up like 80% of the hum. the other 20% i think is just my shack being noisy, not sure.

anyway the receiver actually works now and im picking up SSB signals fine, sensitivity seems decent though i dont have a signal generator to do a proper measurement. my question is about the audio filtering — right now im just running a simple RC lowpass and it sounds kinda washed out. anybody built a audio cw filter or a decent active lowpass for something like this? i was looking at the TL072 op amp stage but havent commited to anything yet.

yeah ground layout kills more homebrew projects than anything else, i learned that the hard way on a regen i built years ago. glad you got that sorted.

for the audio filtering the TL072 is totally fine for this, i used one in a Sallen-Key lowpass configuration in my DC receiver build and it worked great. two poles gives you enough rolloff to clean things up without adding a ton of phase issues. if youre mainly doing SSB you probably want cutoff somewhere around 2.5 to 3kHz, maybe a bit lower if you find yourself doing more CW. the component math isnt too bad and there are calculators online that do the work for you. just keep the op amp supply clean, another bypass cap close to the pins, that kind of thing. the NE602 can put out some garbage on the supply rail so dont let the audio stage share a filter with it if you can avoid it.

i built almost the exact same thing last year, NE602 plus a LM386 audio amp and a random LC filter i copied out of an old QST article. honestly it worked better than i expected for a first attempt but the audio was pretty rough. one thing i did that helped was add a small ferrite bead on the audio output line before it hit the LM386 input, seemed to help with some of the RF trash getting in through that path. probably not the most scientific fix but it worked for me. the direct conversion thing is really fun to build even if its not the cleanest receiver in the world.