Whenever I claim a transformer has "no impedance", I make every effort to ALWAYS include "inherent". it has no inherent impedance. In other words you cannot measure transformer impedance with a meter.
As said, real world transformers do have measurable impedance (mainly DCR + inductance). This inherent impedance shunts connected and reflected circuit impedances and is the reason for the magnetizing current, which flows with and without load. It is essential that this inherent transformer impedance is considerably higher than connected and reflected impedances at all frequencies to be transferred. So a 100k to 50k transformer will need to be constructed different from an 8 Ohm to 4 Ohm transformer.
so it would seem to me any potential red plating would be due to DC conditions, not demands of the signal path.
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