Modular synth fault finding

Suggestions harvested from muffwiggler:
 * you need a quiet power supply. low ripple. upgrade the power supply, upgrade the bus board.
 * you need low impedance from the power supply to every module.
 * you need to have a ground the does not move at all. by move, I mean the voltage at ground is no different from ground anywhere else in the system. differences in electrical potential, especially differences that change rapidly over time, are the source of the noise.
 * there are two kinds of noise, common mode, and differential mode. anything can happen in a synthesizer. the voltage ref is also a power rail.
 * read all of the posts on power supplies from Graham Hinton.

Background / theory

 * Differential (Normal) Mode Noise and Common Mode Noise－Causes and Measures