3.1 A bit of theory...

A step-up transformer is a device which we use to increase a given voltage.

As you can see in the schematics, there are two distinct circuits which are called primary and secondary, respectively. There is, in fact, no direct electrical connection between the two circuits, but each circuit contains a coil which links it inductively to the other. In real transformers, the two coils are wound onto the same iron core.
The purpose of the iron core is to channel the magnetic flux generated by the current flowing around the primary coil, so that as much of it as possible also links the secondary coil. The common magnetic flux, linking the two coils is conventionally denoted in circuit diagrams by a number of parallel straight lines drawn between the coils (see pic).
This type of transformer has more turnings on the secondary coil than on the primary and, therefore, the voltage induced in the secondary coil is larger than the primary coil voltage. For example, if there are 200 turnings on the primary and 2,000 turnings on the secondary, then the voltage induced in the secondary coil is ten times larger than the primary coil voltage. In this case, we say that our transformer has a gain of 10:1. Finally, please note that a real world transformer of this kind can very well have multiple primary circuits: 5:1 (14 dB), 10:1 (20 dB) and 20:1 (26 dB) are among the common gain values used by transformer manufacturers. Thus, their customers can choose the value that best matches their cartridge output vs total system gain. Just to be practical – and in the case of a phono stage of about 45 dB gain - I would recommend using 26 to 30dB (1:32) gain for cartridges of 0.1mV to 0.25mV, 20dB for 0.25mV to 0.75mV, and 14dB for 0.75mV to 1.5mV. Cartridges with higher outputs do not require a transformer.