So when is a SUT really needed?

Funny, the choice you made for a definition of "diatribe".  Here is mine, also from Merriam Webster:

https://www.merriam-webster.com/dictionary/diatribe

Your definition for diatribe as a "prolonged discourse" is mentioned as a third option and noted to be "archaic" in Webster’s.  I delivered no diatribe by the modern definition in Webster’s (see above) or as defined in the Oxford English dictionary, for that matter. Meantime, don’t you think this argument is off topic?

Nandric, after more than a decade, you might have learned to spell the past tense of the verb "to do".  It’s "did", not "deed".  But what indeed did I do that brought me to your attention? 

The main thing you "need" one for is when you have a LOMC cartridge and your phono stage only has an MM option.  Everything else is subjective.

I recently dipped my toes into the world of SUTs for a specific reason.  I have two turntables and a lot of records.  One table has a very expensive (at least to me) cartridge and I'd like to use it for my best audiophile records only and extend its life. 

My Herron Audio VTPH-2A has one MM and one MC inputs. I've tried four different SUTs recently, using a ZU Denon DL-103 on the second table (a vintage JVC QL-Y66F) using the MM input and achieved sound quality that approaches the sound quality of my main table. 

Somewhat by chance, I tried one of them with my Hana SL, which up to that point I had found somewhat laid back and reserved, and was quite surprised at how the SUT transformed it. It sounded better than the Denon had and has more dynamics and detail than I remember hearing from it in the past.  The SUT I'm using is a Jena Labs / Echo Step It Up, which is the one of the four I'll be keeping.  My main table is still better, but I can be content playing the many less than perfect recordings I have, not to mention the convenience of the JVC being fully automatic. 

The only way you'll really know if a SUT is for you, is to try different things in your own system.  The great thing is that there are lots of pre-owned SUTs and you can buy one, try it, and flip it if it doesn't work in your system without losing much, or anything.

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!