Which SUT? .... for .3mv/10 ohms MC cartridge into 3mv MM input sensitivity

20x would be a good choice for your 0.3mV 10 ohms cartridge, as well outlined previously. If you want a versatile SUT for other MC cartridges, then the Bob’s Devices Sky SUTs come with 2 gain settings. So either a Sky 30x/15x or a 40x/20x would be extremely versatile. I have a Sky 20/10 and Sky 40 myself; they’re exceedingly nice sounding boxes that work equally great with different cartridge makes.

You do have to watch out for MC cartridges that have a high coil impedance relative to a low output level. These can be trickier to match with a SUT since the higher ratios you’d normally use to bring up the low signal level will end up loading the coils down too heavily, which results in a significant loss of signal (i.e. you’re spinning your wheels or worse by going to higher SUT ratios). One example of this would be a Benz Ruby, with 38 ohm coils to 0.35 mV output (3 - 10 ohms is more typical at this output level). If you hook that Benz into a 40x you’ll lose over 7 dB of signal from loading down the coils (20dB * log(29 ohms / (29 + 38 ohms)), where 29 ohms comes from 47,000 / 40 / 40) - in effect, the net output is more like what you’d expect from a 15x SUT, and likely with other significant sonic aberrations to boot. However 40x on a 0.3 mV, 5 ohm Koetsu (which I’ve used) works just great. You could however counteract this issue with an MM stage that allows input loading settings above 47,000 ohms - which unfortunately is probably less common than it should be.

On the other side of the spectrum is the Air Tight / My Sonic Lab cartridges, which have a very low coil impedance relative to their output levels. 

I found this, which I now have some understanding of

https://oswaldsmillaudio.com/consolidated-audio-sut

Consolidated offers three main models-

• Low Impedance / High Ratio (1:20): For lowest output cartridges like SPU, Lyra with an output voltage of less than 0,3 mV

• Medium Impedance / Medium Ratio (1:15): If asked for a universal "one size fits all" step-up transformer we would recommend this configuration for cartridge impedances up to 20 ohms and output levels between 0,2 and 0,5 mV

• High Impedance / Low Ratio (1:10): For cartridges with an output level of more than 0,3 mV like EMT TSD

Enclosure dimensions 4.75" dia x 6" height, weight 6 lbs. Available in black or silver finish.

mulveling

thanks for adding your help here

Long article, interesting chart simplifies recommendations

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

this dealer says SUT into MM sounds better than MC preamps to line


Warren JarrettAudio Note (UK) dealer in SoCal/LA-OCLocation:Fullerton, CA

Long before I started representing Music First Audio and Audio Note I gave up on fully active amplification of the tiny signal from a MC cartridge. I have compared many, many preamps, phono preamps and step-ups. Most step-up transformers do indeed sound compressed. Most fully active MC phono preamps do indeed sound "electronic". But the best transformer step-ups into a MM phono stage have sounded much better, to me, in every way compared to the best active head-amps or MC capable phono preamps.

Now that I carry transformer step-ups from both Music First Audio and Audio Note (and MM-only phono preamps from both), it has been very easy for me to demonstrate this. I have a very nice MC-capable Krell preamp, that sounds better with any of these step-up transformers into its 47000-Ohm MM input. And, I encourage anyone who is interested to borrow one of my demo step-ups, to try in their own home, and find out for themselves.

The answer to your question, in my opinion, is yes. Listen to your MC cartridge, into the MM input with a good step-up transformer that is matched properly to the cartridge. If you tell me what cartridge you have, its output impedance (or coil resistance), and output mV, I can tell you what range of step-up ratio will work well for you.
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later he added

So the most versatile approach is to try a 1:20 transformer which has a provision for adding loading resistors, or is adjustable with built-in resistors. The next best solution is to compare more than one transformer, within a step-up ratio range between 1:30 and 1:20.