SUT - electrical theory and practical experience

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.

As tiny as the signal is from a MC cartridge, it is a, relatively speaking, high current/low voltage source.  The phono amplifier is designed as a voltage amplifier.  The step up transformer converts high current/low voltage to high voltage/low current, which means that the amplifier has more voltage to work with and amplify.  As the name suggests, it transforms current to voltage so it is not actually amplifying the signal--there is no need for an additional source of power and active devices to amplify the signal.  

Is it better to employ active amplification instead of current to voltage transformation as the first stage?  Opinions obviously vary on this.  Some people eschew use of transformers while others prefer it over an additional stage of active amplification.  With a lot of tube phono stages, the use of a transformer for the first stage is preferred because active tube amplification is somewhat noisy, and eliminating the first stage by using the "free" conversion of a transformer means less noise.  I suspect that some people, myself included, like the sound of transformers (i.e., euphonic coloration).  

"euphonic coloration".  Which is to say you believe SUTs introduce a coloration.  Which is one subject of that eternal debate I mentioned.

I personally have never owned a SUT.

Perhaps they do, I don't care about religious arguments over purity or neutrality, etc. because I like many tube phono stages that employ a SUT.  My own, a Viva Fono has a built in SUT.  The Audio Note phonostages are all MM stages with the expectation that an SUT will be used ahead of he stage (they sell some very pricey SUT for that purpose).  Zanden's phono stages employ Jensen SUT.  

SUT's clearly can work. And an argument can be made against them. 

For those designing 'minimalist' vacuum tube stages they are 'free gain'

For others they are a bandwidth limiter. But this could be a good thing depending on your school of thought.