Built my first active mixer this weekend and it just works

Built pretty much the same thing last winter for my little side case, except I used a TL072 and wired it point to point instead of perfboard. Took me two attempts because the first one hummed like crazy til I realized I'd run the input wires right past the power ribbon. Rerouted them along the panel edge and it went dead quiet. That shallow clearance you mention is no joke, I had about the same space and ended up angling the pots through the faceplate with the board hanging below them at a tilt.

One thing I learned the hard way: put a resistor from the summing node to ground even if the datasheet example doesn't show it clearly. Mine drifted and thumped every time I plugged a cable in until I added that. Also throw a socket in for the chip if you can, I fried one half of my first one from a soldering slip and desoldering it from perfboard was miserable.

The inverted output is such a good call. I use mine constantly for phase flipping when I'm layering two oscillators and they're cancelling in the low end. Cost me almost nothing too, just jacks and a chip from the parts bin, and now I keep meaning to build a second one for the effects chain.

Funny reading this because I went the complete opposite direction with my mixer last year and skipped the active part entirely. Built a simple resistor-based passive unit in an old tin box, four inputs into one output, nothing powered at all. Signal loss was real with long cables and quiet sources, but running it into a pedal with input gain on the receiving end made up for most of that. Cost me maybe six bucks and an afternoon.

The passive route does have one advantage nobody sells you on: it will never pick up hum from its own supply because it has none. My powered modules live in a rack that shares a wall wart with two other things, and that shared supply is where all my noise problems have come from over the years, not from the audio wiring. That said, I do get the appeal of what you built. I eventually added a powered boost stage after the passive mixer using a single op-amp board from a kit supplier, and the two togehter do the same job as the modules I coud never justify buying. Four knobs and six jacks sounds like the exact right size too. Mine has three channels and I regret the third less than I regret not having a fifth.

Mine lives on the same trick of grabbing whatever's in the drawer, except I used an old quad chip so I could run two extra channels without another board. The part that surprised me was the pots. I wired them with long leads so the board could sit wherever it fit, and expected noise problems, but keeping the pot wipers short and letting the long runs be the input side made it totally fine. Now I'm curious about your faceplate setup. Did you print the jacks into the plate itself or just screw them in after? I keep breaking thin prints around jack holes and haven't found a fix I like.

The long pot leads thing checks out in my expirience too. Mine has maybe 15cm of wire between the board and the knobs, and I braced for noise but got none. What did bite me was the ground wiring. I'd daisy chained the jack grounds thru the sleeve connections and got this faint buzz that followed the cable routing around. Soldered one solid wire from the board straight to the first jack ground and daisy chained from there instead, and it cleaned right up. Took me an evening of head scratching to find it.

Mine gets daily use and the one thing I'd change now is a clip LED on the output. I kept wondering why hot sources sounded gritty till I finally scoped it and the chip was slamming into the rails way sooner than I expected. A little indicator off the output would've saved me the head scratching. Also, do you find four channels is enough? I hit the limit constantly with drums and ended up chaining a second strip into it, works fine but the level juggling gets old fast.