Sources of Noise

[One and a half]

2 hours ago, sandyk said:

 

Transformer based AC or DC wallwarts should not be of any real concern.

The DC types are so rare now and limited in power. 

[One and a half]

50 minutes ago, Ralf11 said:

the transformer-only wall warts shouldn't be a problem - the question is whether any of them have active devices inside for regulation, hence my admonition to regard them with suspicion 

Just a clarification here. In the 'past' some wall warts were a classic linear design, with transformer, rectifier caps and a regulation circuit that was typically a simple LM3xx. Due to the size and heat dissipation, the wall wart's capability was small to modest. If anything, the common mode noise 'could' leak through the heatsink and onto ground from the regulator, but that garbage was a mountain less than switch modes with their rotten X capacitor. 

After all, 80% of preamps use the same topology, sometimes with discrete devices but achieving the same purpose of flat DC. 

[One and a half]

8 hours ago, Speedskater said:

It would take a great technical writer to write a understandable & useful to audiophiles description of 'common mode'.  Heck, different electric/electronic fields have very different definitions of 'common mode'.

Perhaps the definitions are lacking or adaptations into different areas are too broad? For example confusion with conducted emissions?

 

For EMC compliance, differential and common mode is readily explained such as this example from Wurth. For common mode noise to complete the circuit, parasitic capacitance forms part of that circuit when the switching device like a transistor. A transistor especially when placed on a grounded (to earth) heatsink can readily transfer common mode noise that exist in other parts of the circuit (or its own). Dependence on the circuit can also cause drama with noise propagation

 

 Differential mode

 

 Common mode

A simpler drawing is from Murata in the pdf. 

Murata Differential and common mode noise.pdf

[One and a half]

40 minutes ago, Ralf11 said:

 

I did the same thing - you "lose" about 5 minutes of effort...

And not gain anything?