so i've been working on this dc receiver for about two months now, started from scratch on a piece of copper clad using the ugly construction method. got the basics working — DBM mixer with a couple of 1N4148s, a colpitts vfo running around 7mhz, and ive been picking up stations which honestly still feels like magic every time it happens.

the problem im running into is the audio. i've got a TL072 opamp stage after the mixer but theres this really annoying hum in the output and i cant quite pin down where its coming from. its definitely 60hz related, goes away if i touch the ground plane in certain spots which makes me think its a grounding issue but ive already redone the ground connections twice. also getting some microphonics when i tap the board near the vfo area.

using a wall wart for power which i know probably isnt helping. might switch to a battery to see if the hum clears up. anyone dealt with this kind of thing with TL072 audio stages? or should i just bite the bullet and use something like an LM386 instead, i know its noisier but maybe simpler to get working.

the wall wart is almost certainly your problem or at least a big part of it. switching to a battery will tell you pretty fast whether its a power supply issue. i built a similar rig a few years back and had the exact same hum, switched to a 9v battery and it dropped way down, then added a simple LC filter on the supply rail and it went away entirely.

the touch-the-ground thing is classic ground loop stuff. with ugly construction you can end up with little antenna loops in your ground path without realizing it. try to make sure all your grounds return to one common point rather than daisy chaining them across the board. also worth checking if your audio output wire is running close to the vfo section — that can couple noise in ways that are really hard to trace.

id stick with the TL072 honestly, the LM386 has more gain than most people need and its not actually easier to stabilize, just more forgiving of bad bypassing. make sure you've got a 10uf and a 0.1uf right at the opamp supply pins, not a few inches away.

microphonics near the vfo is probably the capacitors in your tank circuit. ceramic caps are notorious for that, especially the smaller values. if you havent already, swap anything in the vfo tuned circuit for NPO/C0G ceramics or silver mica if you have them, makes a huge difference. i learned this the hard way on a 20m cw transmitter i built last year, was picking up every vibration on the desk until i swapped out those caps.