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

As far as I can recall my Lionel transformer didn’t hum :-)

Also, while I’m not certain I believe that in at least most of the models made in the 1950s or thereabouts those "transformers" were actually autoformers. Meaning that they just had one winding, rather than separate primary and secondary windings, with a sliding tap on that winding to control the output voltage. The maximum setting being mechanically limited to result in a safe voltage.

Regards,
-- Al

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