I'm posting DIY stuff here so I don't clutter up the Pictures thread with it. Feel free to post your Homebrew/DIY projects here, too. I'm mostly just posting my findings here in case they help somebody else.
I modified my Soul Sonic mixer pretty heavily (added transformers galore, simplified the cueing section, made a new faceplate with fewer channels and bigger knobs) and realized that, for me, mods and clone-building were a stepping stone, ultimately, to designing my own mixer from scratch.
Arlo is my 6-month old son
It will be 4U high with most of the "performance" controls as 2" knobs. I find that this unusually-large size really does offer a precision and "feel" advantage. Ergonomics studies have shown this as well: "Optimum knob crowding" Bradley, James V. 1969
Knob spacing and Knob size are significant (but more expensive of course).
Originally I was going to go with transformer-based summing since it's only 3 channels, but apparently there are a lot of crosstalk issues and other weirdness that can crop up. A smart old guy said that there was a reason they stopped doing it this way. The lower dB loss of TX-based mixing isn't worth the tradeoffs. So, plain old resistor-based mixing it is.
The core of the mixer is the Jensen 990 Discrete Op-Amp, which is definitely worth Googling for the history lesson, if nothing else. It was basically the precursor to a modern "open-source" project, as its plans were released in 1979 by Jensen and anybody was encouraged to make one. I'm building (9) of them from the recently-released Hairball kits. The 990 is basically a small power amplifier; in terms of real power mixed with fidelity, it blows virtually any line-level output stage away. It easily matches the original Bozak output stages for instance. I won't say that it surpasses it, because that would be blasphemy.
I looked into the newest op-amps, a variety of passive and discrete designs, and even tube designs and the 990 won out. I am adding a pair of UREI 1620 output transformers to the Master Out though, just because I like their style and the "American Thing" they do to the sound.
The EQ/Isolator section is unique. It's built around some old White inductors I ran across. White, along with his friend Dr. Boner (haha), were the pioneers in installing room-correction EQ. Before commercial EQ's existed, they would wire a bunch of these "inductor cans", along with resistors and capacitors in certain values, to correct room sound issues in a completely passive (power-free) fashion:
I had (6) of the Inductors but I needed (8). After scouring the internet and not finding any, I learned that Boner and Associates had an office down the street from me in Austin, TX, in an old house just north of the UT Campus. (It was about a mile from my old house in Austin, actually).
Anyway, I ended up talking to the deceased Dr. Boner's partner, I told him what I was doing, and he said he had some inductors laying around that I could have!
The EQ/Isolator is entirely passive and cut-only (unpowered), relying on the 990's enormous makeup gain to restore any signal losses. It consists of a few 6dB/octave, LR, lowpass and highpass filters in parallel that are re-summed. Phase distortion and frequency errors are minimal, considering. The crossover points are adjustable without using any servos or other circuitry, as the old-school White Inductor values were carefully chosen to make such things easy to do with just potentiometers.
All the knobs are ALPS RK27's, of course. There are some Quad knobs used, for the Crossover selection and for the FX Wet-Dry. Power supply is an internal switching unit by Mean Well. I'm running the 990's at 24V, for greater gain and headroom. There aren't any Capacitors in the signal path; Telco-grade matching TXes are used for noise and DC rejection instead.
More as it comes. I'm awaiting the 4U case from Par-Metal and then I'm going to stuff the circuits inside it. I've found that the casework/mounting is actually much trickier than designing the circuits, at least for me. I still want to get really good at the metalwork part.