Impedance and frequency response
Ribbon microphone: -



Firstly, if you load the transformer output too much the high frequency response will be reduced because of parasitic inductances in series with each winding. But there's a more fundamental thing that happens.

The ribbon is moved by changes in air pressure i.e. it is an electrical generator hence too much load current from the output mechanically dampens the ribbon and alters its frequency response.

Information: Ribbon Microphones