ok so ive been putting this off for like two years because i kept convincing myself it was gonna be too hard but i finally sat down last weekend and built a simple direct conversion receiver for 40m based loosely on the NorCal 40A design, except i swapped out a few parts because i couldnt source the exact caps they called for and used whatever i had in the junk box

honestly expected it to be a boat anchor but i fired it up last night around 9pm and immediately heard a guy in ohio working somebody in texas, audio was a little rough and theres some hum i need to track down, probably a grounding issue somewhere in the audio chain but overall it decoded signals just fine and the VFO stability is way better than i thought itd be considering i just wound the coil on a pvc chunk i had laying around

the LO is running around 7.1 mhz with a colpitts oscillator and the mixer is just a double-balanced arrangement with a handful of 1n4148s, nothing fancy. got maybe 8 or 9 dollars in parts into the whole thing not counting the enclosure which i robbed from an old CB i had

anyone else done a 40m DC receiver lately? curious how people are handling the audio filtering, mine sounds a little wide and i feel like im hearing too much of the band at once if that makes sense

yeah the audio filtering on DC receivers is kind of the thing people forget about until they actually fire it up and realize they're hearing everything from here to Timbuktu at once. what i've done on a couple of mine is add a simple active lowpass after the first audio stage, just an op-amp in a sallen-key config rolling off around 2.5khz or so, cleans it up a lot. some guys go as low as 1.8khz for CW only but if you're doing SSB you want a little more headroom.

the hum is almost certainly your power supply bleeding through, DC receivers are pretty notorious for that. try running it off a battery temporarily just to confirm thats where its coming from, if the hum disappears youll know to start adding more filtering on the supply rail. a ferrite bead right at the board power input plus a big bulk cap does wonders, ive seen 10-20dB of improvement from that alone.

nice build though, 40m DC receivers are a great starting point and honestly a colpitts on pvc isnt as bad as people make it sound if the coil is wound tight and the core isnt moving around on you

i did basically the same thing like 8 months ago except mine was for 80m which in hindsight was maybe not the best choice for a first DC receiver because that band is an absolute zoo at night lol, the audio filtering thing is a real problem. i ended up just sticking some headphones on it and calling it a day, ears do a decent job of filtering honestly

whats your audio stage look like? im running a LM386 and it works but i keep reading people complain about its noise floor, havent really compared it to anything else though so i dunno if mine is actually bad or if thats just how it is