The step-up transformer in the phono stage is used to optimize the impedance mismatch between a low output moving coil cartridge and the phone stage itself. It is a passive device. The phono circuit is an active device and has enough gain for moving magnet cartridges because they are high output devices. Very low output moving coils may only have 1/10 of the output of a moving magnet cartridge, and therefore need a signal 'boost' in order to be amplified properly by the phone stage. The typical scenario is you turned the volume control past 2 o'clock and you still can't hear the music very well and now you have a lot more noise. An interesting fact; when you listen to vinyl you are using not only the phone stage, but the line stage as well.
why is a step up transformer used
I was wondering what the purpose was of a step up transformer for a phono stage? I have a Krell KPE but it does not seem to have enough gain using the preanmp as a power supply. Any suggestions?