SUT - electrical theory and practical experience

I can only speak a little to the theory.  A step up transformer works on the same principle of voltage transformation used for AC power products.  For instance, when you buy gear designed for US voltages but want to use it in Japan.  Transformers can convert one voltage to another, but the power on either side of the transformer is roughly the same (minus losses). 

That is, a 100V to 120V transformer will still have the same power (current x voltage) on either side, 

Like Eric says, a transformer is a device that transforms the voltage to current ratio of a power source, in either direction, where Power, which is a product of voltage times current, is a constant. So when V goes up, current (I) must go down in direct proportion to the increase in V, and vice-versa. While that happens, there is also an effect on impedance, from one side to the other of a transformer. In a SUT, the output V of a cartridge is increased in direct relation to the turns ratio of the SUT (as in all other transformers). For example, a 1:10 SUT will increase cartridge output V by 10-fold. Meantime, the current output of the cartridge is likewise reduced by 10-fold. The effect on impedance, on the other hand, is related to the square of the turns ratio. So, for a 1:10 SUT, the impedance "seen" by the cartridge on the primary side of the transformer will be reduced by 100-fold in relation to the input impedance of the phono stage. A 47K phono load resistor on a typical MM phono stage will look like 470 ohms to the MC cartridge. As to the rest of it, that is the subject of eternal debate among audiophiles, and I won’t go there.