SUT - electrical theory and practical experience

People try to lump all kinds of technical truths into simple rules of thumb which then become the norm. After being accepted, the underlying truths these norms are built upon tend to get lost.  The three main things that need to be considered when designing / implementing a SUT are Impedance (source and load), gain and loading.  While all three of these items are interrelated by math, they all need to be addressed (juggled) individually.

The source / load impedances are what determine the required primary inductance and the winding geometry required to get the desired bandwidth.  What needs to be done for a 2Ω cartridge is different than what is needed for a 40Ω cartridge.

The turns ratio is set to elevate the cartridge output to the desired input of the phono stage it is driving.  A 0.25mV output 40Ω Denon 103 roughly fits the same impedance numbers as a 24Ω 1.0mV EMT but requires a different turns ratio.

Loading is the the part that gets way too much emphasis and really should be only a minor part of the consideration. The focus on cartridge loading as a primary concern causes much confusion and turns what should be a very simple process into a black art.  There are two aspects to the reflected load, the resistive part and the capacitive part.  The resistive part sets the actual reduction of gain from the ideal. Attention must also be given to the load capacitance as the turns ratio increases since that value can quickly truncate the top end.  I always consider the reflected load to be a liability and feel that using a transformer to reflect a desired load is misguided at best.  People tend to consider the transformer as an ideal device that will perfectly reflect the terminating load to the primary.  In reality this is far from being the case. More often than not, adjusting the secondary load to reflect back a desired value to the cartridge will have both a measured and a perceived effect on the performance of the SUT. It is this change that people incorrectly attributed to the load the cartridge is seeing. All transformers will reflect a minimum load based on the math and the goal should be to keep that number as high as possible. Increasing the 47kΩ input resistor values is a good place to start.  Once the minimum load is determined, if additional loading is deemed necessary it should then be added to the cartridge directly where it will have a minimal effect on the behavior of the SUT but can have a dramatic effect on the tracking behavior (ie sonics) of the cartridge.

dave

Dear @holmz  : " There are all sorts of hypothesis as to why the metal choice might impart a different sound, but most are a bit light and fact and oversubscribed in magic.
It is possible that the dielectric is as important as the metal. "

Dielectric, yes I think with out facts its importance as a wire insulator and with out facts too the qualitlevel of the of the cooper/silver wire.

@intactaudio  audio that were posting in the thread can put some " ligth " about because he manufacture SUT either: cooper or silver and normally his customers prefer silver material over cooper.

So, Dave hopefully you can share your know-how and experiences about.

R.

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.