My name is Ryan Mintz and the ATX Linear Power Supply is my design that I have been working on with my engineers now for close to 8 months. It has gone through SEVERAL revisions in that time and is a pretty neat device.
Kijanki is correct, it's not possible to build a linear power supply into a tiny ATX spec box -- sort of.
In our ATX Linear supplies we divide it into two separate boxes. The external box is larger and houses the transformers, AC Filtration, rectifiers, and bulk capacitance (720kuF in the S2).
The internal chassis fits in a standard ATX slot, screws right in and has a standard ATX wire harness for connecting to a motherboard.
The internal board handles the logic circuitry (PSON, PowerGood, Overvoltage, undervoltage, over current, over temperature protection) and is a 9-rail linear power supply. -12V, 3.3V, 5V, 5Vsby, 12V1, 12V2, 12V3, 12V4, 12V5. It uses 12 transistors capable of sourcing a 25A load each. At low current loads the fan doesn't run as the heatsinking and power sharing between transistors is adequate to dissipate the power.
The design uses a supervisory circuit for power management, and that is linked through a logic inverter to enable our regulation circuitry. The regulators are our same design used in the New (as of the end of April) Kalos power supplies with very fast transient response, wide bandwidth, and less than 3uV output noise (you can see the pictures of the Kora on our website).
Special focus was given to the 3.3V rails in terms of current and noise floor. The 3.3V rail is responsible for powering PCIe/AGP, logic circuitry on the motherboard, low voltage CPU regulators, and the chipsets -- this means the 3.3V rail (not the 12V rail) is by far the most important to your sound quality.
We do very high-end Mac Mini systems as well and they sound very good. The biggest limitation here is the DC-DC conversion on the logic board itself. Our logic board shielding helps to absorb much of the EMI here, but that's still going to be the limiting factor if you're looking for ultra high-end performance. The Mojo supplies are good, though not a fan of the LT1083 he still uses in his latest design, it's too slow and noisy. Regardless, the power supplies are still a great upgrade, you can't go wrong with any of the power supplies that are popping up on the market these days. They're all tremendous upgrades for the Mac Mini, though some better than others of course.