SUT With a Manley Steelhead RC?

I don't think it is a dark art. @lewm will correct me if my terminology is wrong, but look up the internal impedance of the cartridge. You need the SUT to present it with a load that is 10X that number. They all "output" at 47k, which is standard MM input. That's the "step up" part.

For example, I had a couple Airtights, very low impedance and set my transformers to the closest setting of 10x their impedance. When I switched to Koetsu stones, I found a slightly higher impedance worked best. Some SUT set ups are more adjustable than others and SS stuff can be tweaked with changes in resistors or boards. (I use tubes for the phono, but the unit, an Allnic, is  adjustable with knobs on the transformers). I have an outboard SUT in my vintage system. Not a terribly expensive unit either and it's good, runs at a setting to match a Franken 103 with a MR stylus and potted into a wooden body by Steve Leung. 

The transformers themselves get attention too. There are some highly regarded older transformers- collectible, and modern high end ones like Slagle produces. 

The SUT doesn’t present a load. The load seen by the cartridge is dependent on that 47K resistor in the phono stage and the square of the turns ratio of the SUT. There is absolutely no reason to believe that you have to fine-tune the turns ratio of the SUT in order to end up with an exact tenfold ratio between the cartridge internal resistance and the input impedance as seen through the SUT. Any ratio greater or equal to 10 works fine. And I would say you can go below 10 as well. Play it by ear.

When I look at SUTs / cartridge pairings, I first consider "are SUT’s good for this cartridge?". For that, you look at the ratio of a cartridge’s output voltage over its coil ohms. This is proportional to how much current the cartridge can supply (in theory). Since a SUT essentially converts current to voltage gain, this is a good indicator of its match to both a SUT, AND to current injection stages (SUT is passive and current injection is active). Of course there’s more to it than that, but this is a good rule of thumb IMO.

To keep nice integer (rounded) numbers, express the output in micro-volts uV. For for example, Koetsu lists 0.3mV = 300uV for 5 ohm coils. That’s a ratio of 300 / 5 = 60. That’s a "very good" ratio for a SUT. Koetsus yield very strong output levels for relatively small coils, and are a known good match to SUT’s.

Benz iron cross models are not quite as efficient, but they still yield 400 uV / 12 ohms = 33. That’s still a good ratio for SUT’s.

Benz ruby core models suck with SUT’s. They have 380 uV / 40 ohms = 9.5
Van den Hul Colibri is another tough one, with very similar specs to the Benz ruby. But again, their lower models’ generators (Crimson, Frog) are more efficient and work great with either SUT or current injection. Why do some cartridge motors have so much less efficiency? Because they’re either doing something weird with the magnet (Colibri positions its coils in a monopole layout rather than the typical dipole) or they’ve made a choice for the coil’s core to be a non-magnetic (or less magnetic) material than the usual iron / permendur. Ruby, air, polymer (IIRC Ortofon said they use these in Anna?) - these all sacrifice output efficiency in order to reduce magnetic flux disruptions during playback conditions (essentially they are reducing the magnetic complexity).

Most LOMC’s will have a ratio somewhere between the Benz iron-cross and Koetsu. A few have ratios even higher than Koetsu: Ortofon Cadenza Red / Blue / Bronze and all My Sonic Labs cartridges (by a LOT there!).

So once you know a cartridge is "good" for a SUT, then what you want to do is match step-up ratio so that the resulting "MM level" is around 5mV. Now I like to go even higher to de-emphasize the MM stage’s noise floor, but at some point you do risk MM stage overload. With tube MM stages though (generally good overload margins), I’ve found anything ~ 10mV and below is perfectly fine. If you’re falling below 4mV then sometimes dynamics and "punch" can suffer. So just match step-up ratio and never worry about loading - if you have a good cart for SUT and a 47K ohm MM input impedance, it always works itself out fine :) I’ve also never found any value in additional loading on the SUT’s primary or secondary.

@lewm- two comments. First, I agree that there is no absolute in matching numbers -impedance of cartridge to primary or input of SUT. I’ve experimented in each case, both on the Allnic and on the outboard unit I use in my vintage system.

Your observation that the SUT does not present a load is intriguing to me. My goal in the above statement was to demystify some of this stuff and simplify it, so it didn’t seem like a dark art. Thus, I used the terminology with a nod toward you.

I do often refer to the setting of the primary or input as the "load" the cartridge sees. I’m also reminded of the occasions where people talk about "loading down" a cartridge as opposed to running it wide open, i.e. 47kOhms.

Thus, this explanation (from a site called "hi fi for lo-lifes"-a moniker that amuses me), speaks in just such terms:

"These passive windings have ratios. These ratios operate as approximations to specify a SUT for matching with specific cartridge impedances. Standard ratios tend to be 1:20 or 1:10 (this is the “gain” usually translated to dB) but numerous ratios exist for differing cartridge specifications, and many SUTs feature multiple winding ratios to add flexibility for different carts (a 1:20 SUT may also include a setting for 1:40 windings). A general rule for MC carts is they prefer a load impedance of 3~6 times their output impedance (for example, a 20-Ohm output impedance cartridge would work well with 60~120 Ohms of input impedance). A 1:10 winding ratio means that the secondary winding has 10 times the number of turns of the primary winding, so the voltage for the secondary is roughly 10 times that of the primary. But, that’s not all the magic the transformer is performing, it is changing the resistive load the cartridge “sees” by squaring the turns ratio divided into 47,000 Ohms (this is the impedance). This translates to the cartridge “seeing” a load impedance of 470 Ohms instead of 47,000 Ohms, a number most MC carts are quite happy with."

I’m comfortable with that explanation. Tell me where I’ve gone astray- not to argue, but to inform. Ty.

Sometimes it's a matter of semantics, but many do speak of SUTs as if they have an impedance of their own.  This is not the case, so I am sensitized to that implication.  As I know you know, any transformer merely converts current to voltage (if you think primary to secondary), and the impedance seen by the cartridge is simply a function of the impedance present on the secondaries.  The SUT adds no impedance. (This is in a perfect world; in reality there is a tiny effect that can be disregarded for loading a cartridge.)  It's obvious that you have a handle on it, so I apologize for my pedantry in correcting your original statement. I just know some others might have been led astray and was trying to prevent that.

For one thing, if you have a cartridge with a high-ish internal resistance, like a Denon or some Benz cartridges, such that it is not an easy match for a SUT, there is no reason you can't replace the 47K resistor at the MM input with a 100K resistor or even higher, to make the cartridge happy with the final impedance it has to drive.