SUT - electrical theory and practical experience

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).  

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.  

antinn,

A friend has the Tron and it is indeed a good pono stage.  You can tailor the sound to your specific system and taste by trying different tubes.  This is a component worth going that extra mile to optimize.

As to the issue of built in or separate SUT, I have mixed feelings.  A built in SUT of good quality will save one the expense and trouble with finding the right interconnect.  Also, outboard SUT tend to be more prone to picking up noise and hum from external electrical fields.  On the other hand, one can, with separate SUTs, one can find the best match for your choice of cartridge and one can move the SUT around to minimize hum and other noise issues, whereas, if the built in SUT is the source of such noise, one cannot change its location and orientation.  As to the optimizing of the choice of SUT.  I agree with what Art Dudley said many years ago.  He noted that even when a particular SUT supposedly has non-optimal gain or other characteristics for a particular cartridge, he noted that they usually still sound very good.  He said that he never found a SUT that he didn't like.

What Dover posted above about proper gain structure being the main objective is shared by quite a few experts.  A good long explanation of why loading, of either the primary side or secondary side is not very important, but, preventing overloading of the phonostage by selecting the proper gain IS important is described in a "white paper" posted on the Rothwell site:

http://www.rothwellaudioproducts.co.uk/html/mc_step-up_transformers_explai.html

I believe others, like Jonathan Carr of Lyra, believe in utilizing very little loading (i.e., HIGH value loading resistor or no added resistor) unless the phonostage is prone to overloading from the ultrasonic peak of the cartridge or RFI breaks through.

I utilize a tube phono stage that is not prone to ultrasonic overloading, and I agree that very little loading usually works the best.  My phono stage has a built in SUT, so I cannot fool around with gain, but, I do know that gain is a bit low for my particular cartridge (Transfiguration Orpheus L), but, given that I have zero noise breakthrough, I don't worry about cranking the volume up a little bit to compensate.