SUT - electrical theory and practical experience

To answer the OP's question re: what does the SUT do to the sound quality...

For what it's worth,I have several phono stages and several active step up devices/ SUTs and generally, the SUTs are quieter and more open (when properly matched etc...)

I currently run a Sunvalley Audio SV-EQ1616D phono stage and ZYX cartridge. The Sunvalley has a built in " FET head amplifier that adds another 25–32dB gain with fixed 50 ohm loading for use with low-output moving-coil cartridges". The MM stage is good, the FET is NO Match for a good SUT. (n.b. the proper load for the cart is +100 ohm). I am using a copper wound SUT in custom housing and sheilding. Best combo I've had since the 70's.

Elliot, in operating principle there is absolutely no difference between your step down transformer and a SUT. The functional difference is only in the respective turns ratios, and also the fact that a SUT has to handle a wide bandwidth whereas the step down always works at one fixed frequency.

lewm

I wanted OP to understand:

cartridge SUTs are PASSIVE, no need to plug them in. And that they boost signal strength by themselves, i.e. boost, not ’enhance’ the signal.

Step Up or Step Down NEED to be plugged in to wall outlets. They also boost/cut voltages and do not enhance signals.

And a Cartridge SUT is working on a phono cartridge signal that goes into the preamp/amp/speaker system

A Step Up/ Down Voltage converter has nothing to do with the audio signal, just proper voltage to the motor.

Of course to alter AC line voltage a step down transformer of the kind you own must be inserted into the AC line. That’s precisely analogous to inserting a SUT between a cartridge and an MM phono stage in order for it to do its work. Both are passive devices. You implied that your step down is per se dependent on AC for power. I hope the difference is clear.

Dear @drbond  : " Does the SUT theoretically enhance or degrade the sound quality?  What does the SUT actually do to the sound quality?  "

First any device where the signal must pass degrades the sound quality, so the SUT degrades the signal quality and there is no way no matters what that that SUT could enhance the audio signal.

Today SUT's exist because there are tubes phono stages and even that exist 2-3 all tube high gain active units those are not the best solution for LOMC cartridges.

A SUT is not a passive device because any audio signal passing trhough those transformers makes that the " hundreds " of meters on each transformer react to that signal and starts the degradation and you have to think that the signal has a " long trip " inside each transformer wires and at each mm. the audio signal is degrading by that SUT. 

In the other side a good SS active high gain design say ith bipolar  active devices the signal must pass only trhough a matched pair of transistors in that first critical gain stage and degradation is at minimum way lower that in a SUT.

Advantage of a SUT: NONE. SUT develops or could be contaminated with some kind of " noise/colorations " and yes its frequency response is way limited and not only that: everything the same the badwindth, noise levels, THD, deeper bass, etc, etc belongs to the SS active high gain.

You can ask for measured frequency response and the like to any today builder to confim all what I mentioned.

I owned and still own SUTs and the only one that I listen time to time is the Denon 

AU-1000 modified by me where that SUT only sees a connector when in the MM phono stage. This Denon measured FR is: 5hz to 200khz, that's exceptional for any SUT and I admit that my modified unit puts the level degradation ower that any other SUT I experienced but not even my SS Essential 3180 phonolinepreamp.

There are several SS active high gain phono stages and normally are expensive ones as FM Acoustics or CH or the Channel D Seta L20 but exist very low price as 2.5K only SS units that makes a formidable job. Here one of those unit reviewed by MF and compared against Dartzeel and other way $$$$$ phono stages:

Today top LOMC cartridges are really expensive and makes no sense at at all try to been mated with a SUT that as I said and no matter what will degrades that valuable cartridge signal.

For me that I'm a MUSIC lover the only  path is a SS active high gain phono stage but only an opinion.

Regards and enjoy the MUSIC,

R.