SUT - electrical theory and practical experience

As @intactaudio points out, loading is another aspect of the SUT, and this is where, based on my limited reading, things seem to start to get complicated.  Here is what Rothwell Audio's website says about transformer loading:

"transformer loading
The idea that optimum performance comes from matching the impedance of the load to the cartridge's impedance (shown above to be somewhat hit-and-miss) also gives rise to another fallacy – that of transformer loading. The misguided theory, sometimes advocated on websites and forums, says that a loading resistor on the transformer's secondary winding can be used to “correctly load the cartridge” or to “match the transformer to the cartridge”. This is a very dubious theory indeed, so lets analyse what is really happening. Take as an example the Ortofon Vivo Red cartridge examined above (5 ohm source impedance, 0.5mV output voltage). As has already been determined, a 1:10 transformer will give us the voltage we require for an MM phonostage, but the advocates of “correct loading” may be convinced that the cartridge performs best with a particular load, despite the manufacturer's recommended load being anything over 10 ohms. So what is “the correct load”? Often, it is claimed to be the same as the cartridge's source impedance, hence “matching” is achieved. As shown above, a turns ratio of 1:97 will present a 5 ohm load to the cartridge, but what if such a transformer cannot be found? What if the nearest transformer available is 1:36? Can that be made to “match the cartridge correctly”? The transformer with a normal 47k load would give the cartridge a load of 36 ohms (and produce an output voltage of 15.8mV). In order to make this transformer match the cartridge with a load impedance at the primary of 5 ohms, a load on the secondary of 6480 ohms could be employed instead of the 47k normally found on an MM phonostage. This would not only produce a load impedance for the cartridge of 5 ohms, it would also reduce the output voltage to 9mV. Has the additional loading resistor now made the system optimal? No, it hasn't. The cartridge is now seeing half the minimum impedance which the manufacturer recommends and the signal voltage into the MM phonostage is still enough to reduce its headroom significantly. Clearly, this isn't optimal, but it is a lot better than it was with a 1:36 transformer and no additional loading resistor. Anyone who is taking an empirical approach to optimising their system and experimenting with loading resistors based on the idea of “impedance matching” as advocated on some websites would conclude (understandably) that their system now sounds better because “the cartridge is loaded correctly”. In fact it sounds better because the phonostage is being overdriven less than it was before. It would be better still if a 1:10 transformer was used instead of trying to make a transformer with far too high a turns ratio “match” anything by fudging it with resistors.
The “correct loading” or “load matching” myths are fuelled further by a fortuitous by-product of loading the transformer with an additional resistor – damped ringing, analysed in more detail below.."

For those who don’t mind clicking links, and are interested in reading more, here is the link to the informative, educational page from Rothwell Audio Products, and they do mention use of different materials, such as copper vs silver somewhere on the page, although their experience may differ from others:

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

If the information available from Rothwell answers your questions, that's great.  Just beware of the passages where they compare the relative virtues of active high gain stages to SUTs. Because they sell transformers. Also, if Intact Audio and Rothwell are in conflict, go with the former authority.

@intactaudio has already stated "In fact assuming the frequency response safely covers the audio band, I find core material, dielectric choice and winding wire to be far more dominant factors in the final sound. I find extending bandwidth (particularly at high frequencies) is simply more icing on an already delicious cake".

I have no issues with this Statement, I have had specific type Transformers Hand Produced for owned devices and have been involved in the dialogue between the EE Designer/Builder and their supply chain Tranx producer. There is indelible recollection of use of the construction materials as well as the Math involved.  

"if Intact Audio and Rothwell are in conflict, go with the former authority".

There is more than one road to Rome.

Rothwell are seemingly ubiquitous in their support and have available devices that can be used in conjunction with a Typical Cartridge Design, at reasonable/competitive cost to enable an interested individual to get on board and share in the experience.

Intactaudio are quite different, the device on offer is Bespoke Built and Designed to be an Optimised Interface.

The end user is quite sure about the choices for Cartridge and the supporting ancillaries they choose to use in conjunction to create the Phono Signal Path.

Additionally, the end user is quite sure who's experience they want to assist with producing their Bespoke Built devices.

I get the idea of approaching Intactaudio, it is not a strange approach or a pursuit of a luxury item. The approach is made with a intention to have the best support put in place for the Luxury Items. If one has a TT>Tonearm>Cart'>Phonostage at approximately £$20 000. If a further ancillary is attractive to be used in conjunction with the System, why would a Bespoke Built device not be a consideration. 

I've been having Bespoke Built Devices produced for 25 Years+, where I have relied totally on an EE's Experience and Skills and their supply chain, to bring the idea to a realisation. I do not feel short changed in anyway.

In the earliest days, if I were to also include a Design Service and have a Blingy Encasement produced, I may have started to feel a little out of pocket. There are many nice enclosures 'off the shelf' on offer today, so this is not an issue of concern anymore. 

@drbond 

from rothwell...

In order to make this transformer match the cartridge with a load impedance at the primary of 5 ohms, a load on the secondary of 6480 ohms could be employed instead of the 47k normally found on an MM phonostage.

Lets say for example the goal is to load a cartridge @ its internal impedance.  I agree with Rothwell that heavily loading down the secondary of a SUT to reflect the desired load to the cartridge is a mistake.  However....  One could simply accept the 47kΩ termination on a 1:36 will reflect back 36Ω. In order to reduce the cartridge load to 5Ω, simply parallel a 5.8Ω resistor with the cartridge. (it is kinda cool how 36^3=47K)

I have done this experiment and can emphatically state that in this case 5Ω≠5Ω.  In the case with the load applied through the transformer it seems like a blanket has been thrown over the speaker.  This is the exact "thick" sound people report from this type of loading.  The error all too often made is incorrectly attributing the sound of the reflected load to the cartridge when it is actually the sonic nature of the loaded transformer.  Simply doubling the turns ratio of the SUT and then loading the cartridge at the same 5Ω nets a very different sonic result. Thick, dark and muddy are the last terms I would use as descriptors.  To complete the circle on this I have then taken a transformer loaded Jfet based head amp and compared the same cartridge above loaded at 47kΩ and at its internal impedance and I would say the overall pattern of sonic character followed that of the transformer with the additional load placed on the cartridge directly and the sound of the "traditional" heavily loaded transformer secondary is the outlier.

dave