so anyway i've been building this direct conversion receiver for 40m for the past couple months, just a simple affair with a NE602 mixer and an LM386 for audio. got the basic audio chain working fine a few weeks ago but the RF front end has been driving me crazy ever since i added the bandpass filter and preamp stage.

the problem is i get this weird overload thing happening whenever i tune anywhere near the broadcast band bleeding in, even though im on 40m. i thought the BPF was supposed to kill that. i wound the toroid myself, T50-2 cores, tried to get the turns ratio right but honestly i may have miscounted at some point. it actually receives, like it does pull in signals, SSB sounds decent, but every few minutes there's just this low rumble that kinda swamps everything and i'm pretty sure its AM broadcast intermod.

the preamp is just a single 2N3904 common emitter stage with maybe 15dB gain, wonder if that's just way too much gain before the mixer and making the whole thing more sensitive to junk. thinking about pulling it out entirely and going straight into the 602 through the filter. anyone done something similar or had this kind of issue with NE602 based builds?

yeah the NE602 is notorious for being easy to overdrive, its got maybe -10 or -15 dBm input IP3 which sounds like enough until you've got a 50kw blowtorch AM station 2MHz away. i'd definitely yank that preamp, at least temporarily, and see if the problem goes away. direct conversion receivers on 40m are kind of a tough case because the band is crowded and you're mixing right down to audio so any low frequency garbage gets in there pretty easily.

also double check your BPF insertion loss and whether the thing is even tuned right, a mis-wound toroid can shift the center freq enough that you're not actually getting the attenuation you think you are. if you have a way to inject a signal and measure, even just a rough check with an SDR dongle as a spectrum analyzer, it helps a lot to actually see where your filter is cutting. i've had builds where i was 100% sure i wound it right and the filter was way off, embarrassing but it happens.

dont underestimate how much junk 40m sucks up at night, even with a perfect frontend you're gonna hear broadcast intermod sometimes especially if you're anywhere near the eastern seaboard. that said if its happening constantly and not just at night then yeah something is off with the filter. the 2N3904 preamp idea is fine but 15dB is a lot before a mixer with that kind of IP3, id try 6 or 8dB at most if you want any preamp at all.