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We often cite common mode noise as being bad for audio, well it is but how bad is it, where the source comes from, and find a way to work around removing the source. Where noise ends up being audible is in the DAC, that's why this topic is in the DAC section of CA. DACs are very noise sensitive, and their SQ can be greatly improved by trying to remove common mode noise from the signal path and the power supply. There are quite a few good papers to grasp the concept of common mode noise. Here's three that are well written, not too much detail to be lost in the topic, but to find cures needs a little detective work, between all of us, we could find and report and publish the results. From Gryphon and Emerson. From Maxim integrated. From Pulse Electronics. The Maxim paper has drawings of cabling systems that will be familiar to CA users and where common mode noise can cause problems just from cabling. Texas Instruments have produced graphs of a few noise sources we use everyday, and effect of that noise is astounding. The papers are not too arduous to understand, but not all recommendations are accurate.... For instance, the Emerson paper recommends a UPS for common mode noise reduction, but only if there are isolation transformers fitted. Not any transformer will do, as we know, Pulse Electronics mentions on the last two pages, how common mode noise can jump transformers, and how a balanced network can cancel out common mode noise. The drawings are mainly for @Cornan to appreciate why the centre point is earthed (grounded). There's not that much in the way of reducing differential mode noise on AC networks, other than a differential choke which could be worth looking at in some detail. I wish it were easy to measure and apply an xyz and bingo it's gone. Not that simple. The best tool we have is the power of observation and listening.