so ive been working on this DC receiver for 40m for like the past month and a half, based loosely on the uglywire/manhattan style build i saw on the qrp-labs forum. got it oscillating, got audio out, everything seemed great until i actually tried to use it on the air and realized its basically a noise amplifier with some ham bands mixed in

the LO is running around 7mhz, i built a simple balanced mixer with some 1N4148s in a diode ring config, audio amp after that is just an LM386 which i know everyone hates but its what i had. problem is im getting a ton of local AM broadcast bleedthrough, like i can hear a spanish language station pretty clearly underneath everything and theres this 60hz hum that just wont quit no matter what i do with the power supply filtering

ive got about 30pf across the rf input trying to knock down some of the broadcast stuff but it aint doing much. wondering if anyone has gone through this with a DC design cause i know its kind of a fundamental limitation of the architecture but some guys seem to get pretty clean results with them. do i need a proper bandpass filter on the front end before the mixer or am i missing something more basic

yeah the LM386 is a big part of your problem honestly, that thing has terrible RF rejection and it'll happily amplify anything that makes it past your mixer. but the bigger issue is probably your front end -- a direct conversion without a decent bandpass filter is basically a broadcast receiver by accident. you need at minimum a 2 or 3 element bandpass right before the mixer, something like a couple of toroids wound for 40m. the 1N4148 diode ring will work but it's not gonna have great port isolation, so RF is leaking back and forth in ways you dont want

the 60hz hum is almost certainly your power supply or a ground loop, try running the whole thing off a 9v battery and see if the hum goes away -- if it does thats your answer. and shielding the LO from the audio section helps a lot too, i built a receiver a few years back and just putting a little piece of copper clad between the oscillator section and the audio amp made a noticeable difference

had almost the exact same thing happen with my 20m DC receiver build, the AM broadcast thing drove me crazy for weeks. what helped me was adding a high pass filter after the audio stage to kill anything below like 300hz which got rid of most of the AM garbage, and then i wound a proper toroid bandpass for the front end. used the online filter design stuff from AADE, their LC filter design program is free and makes it pretty easy to figure out the component values for whatever band youre on

also try twisting your power leads together if you havent, sounds dumb but it helped my hum situation quite a bit