Often step-up transformers are used to match low source resistances with the optimum source resistance of the active
electronic. For the noise consideration this is a pretty good solution, but all transformers are limited in the band width, since
they cannot transfer DC voltage. Depending on the construction and the transfer ratio of the transformer the lower cut-off
frequency can be in the range of 20Hz. That means a transformer acts always like a high pass filter.
A 1st order high pass with 20Hz cut-off frequency has the following amplitude and phase response:
