So when is a SUT really needed?

Yes, the back emf opposes the motion of the cartridge, but it's very small compared to the generated voltage- which is due to the conversion of mechanical energy to electrical energy as described above- and essentially can be ignored in calculating the dynamics of the cartridge arm system.

The above reference from Wyn Palmer touches on one subtlety that often gets missed.  Vinyl playback is not a level playing field when it comes to frequency.  The low frequencies are the easiest to track due to the low velocities afforded by the RIAA curve.   As frequency goes up, so does velocity and accurate tracking becomes more challenging.  When the velocities get too high, mis-tracing occurs.  I suspect it is at these high velocities where the increasing back EMF from aggressive loading or trans-Z stages can actually play an audible role.  

sorry for the slight drift off course.  This is related since SUTs, particularly ones with high ratios tend to reflect back loads that can be small multiples of the internal cartridge impedance.  

dave

OP

short answer to your question, do you need a SUT if you have a good enough phono preamp, and I answer would be No. your good enough preamp should be good enough. 

for me, having a SUT means you know what you are doing with it. certain SUT works well with certain cartridges and others may be a wash. I have a Bobs SUT, and I have a Para sound Jc3+ as my Phono stage. When I was using the Hana Blue, this SUT shined, but the moment I switched to Songbird, it was a wash. like it did nothing to the sound. both the Songbird and Hana Blue are MC

so not all SUT will be an effective requirement if using a MC cart

YMMV

@lewm 

"Meantime, don’t you think this argument is off topic?"

Let's get back to SUTs then!

@intactaudio The quote used suggests the condition being referred to is Linear, as the Author of the quote states their is the option to calculate out the opposing motion caused by electrical to mechanical conversion.

There is a known equation usable to negate this condition as being an impediment.

Does this mean all materials selected for the design for the Cart'. Where selected parts are intended to be enablers for a conversion of Mechanical energy to electrical energy are to remain optimised for their role, through undertaking such equations as referred to by the author of the quote? Hence up to the point an electrical signal is sent, a Cart' is totally predictable through the equations undertaken and attention to the accuracy of the assembly.

If the above is correct, does not the ancillaries such as a SUT - Head Amp - Phonostage benefit from this. Is it not the consistency and accuracies produced for  the Cartridge's Function prior to conversion, resulting in electrical energy sent, that is the optimal condition present for the added Gain and reverse RIAA conversion occurring to the sent signal by said ancillaries. 

The confusion is old: ''contradictio in terminis''. Logic is not abut ''terminis'' or 

expressions  but about statements which can be true or fals (tertum non datur)

but more important is the relation between assumtions or premise with deduction

from them. because from fals assumptions no true atatement  can follow.

What than are ''opinions''?