Hello Max,
what about the sound of a transformer?
The sound of a transformer is mostly in the metal. Why? That is a kind of difficult question.
Here is one good example, what we refer to as Mu-Metal today was original developed by the Magnetic Shield corporation and is a loose world for anything that has to do with audio transformers, magnetic shields, etc.
Even Mu-Metal is today a registered trademark, there are several manufactures that offer a generic version of it.
The original Mu-Metal composition is
C 0.015
Mn 0.50
P 0.005 max
S 0.001 max
Si 0.30
Cr 0.02 max
Ni 80.20
Mo 4.85
Al 0.01 max
Co 0.02 max
Fe Balance
but most of the so called generic version and alternatives have slightly different trace elements that make the technical specs nearly the same but the overall audible sound is sometimes very different.
In my trials over the years in dealing with audio transformers the smoothness or harshness is given by the type of raw metal that is used for the lamination/transformer.
All additional high mu metal that are available today are based on the 80Ni/15Fe the rest even chromium, molybdenum or cobalt.
What the best alloy is, that is open for debate because the shape of the lamination is the 2nd most important part, where sometimes two types of the same lamination but from two different manufacture (cut different) can sound like day and night.
Best example is the early V72 units made by Maihak, they used Krupp lamination vs. the standard Siemens ones. There is a distinct difference in the upper mid range....
The situation about transformer cores is pretty dark today because there is hardly any manufacture left that offers audio grade "Nickel". Here in the US there is Magnetic Metal and BMI left that still offer off the shelf lamination, but from the 200 or so classic x-former lamination 90% of all are special order and sometimes not available at all.
It is pretty much like the electron tube, a specialty market product....
Best regards,