In order to check if the noise from the mixer originated from the +/-12Vdc ATX computer supply, I decided to build a standalone ultra low noise PSU that could fit in the VSTiBox case. Of course pre-build solutions were available from ebay&Ali, but they did not exactly fit my requirements and size restrictions.
I wanted the PSU to be a slave of the ATX computer supply, and added a relay which is powered by 5V from the ATX.
The 230Vac input is filtered by 3 varistors, a common mode choke and some class Y caps. I choose a 5VA toriod transformer with 2x12Vac output. The output is rectified by very fast super low Vf schottky's with 100nF in parallel, which acts as a snubber to prevent HF ringing. Another single HF snubber is placed after the bridge rectifier. The TPS7A4700 linear regulators are simply the best I could find in terms of noise. The output stage has several different caps, with one big 4700uF buffer.
Because this is a 230Vac circuit, I made sure to separate the low voltage domain by at least 4mm, and the high voltage signals at least 2mm.
This design is a first time right, with no modifications needed. I tried to measure it with a high-end scope with the probe set to 1:1 and AC input, but could not find any output noise.
When I powered the mixer with this ultra linear PSU instead of the ATX, the noise (which was already quite low) only slightly decreased. I think it now compares to other instruments like my Nord. Overall I'm pretty pleased with the result, though this ultra linear is an absolute overkill. If I were to design the power supply again specifically for the mixer, I would have chosen the same toroid and filtering, but a simple 2x LM317 will do just fine.