Question about loading with an SUT

I have a Zesto SUT with adjustable ratios and loadings going into a EAT eglo petite also with adjustable gain and loadings for an Aphelion2 cart (and previously a Soundsmith Zephyr MIMC*). In both cases the recommended loadings sounded best to my ears. Differences between settings are extraordinarily subtle to zero, so only major mismatches made a clear difference. In your case, the recommended and available values are very close, so as others have said, don't worry about it. 

I just realized the 103R has an internal resistance of 14 ohms, as compared to the standard DL103 with R = ~30 ohms. Therefore I have to temper my earlier advice. Setting the SUT ratio at 1:36 would still be suboptimal but not as bad as I indicated. Ideally you want the output impedance, in this case the internal R, to be at least one tenth of the input impedance of in this case the phono stage. A higher ratio is fine. At lower ratios more and more of the cartridge output is lost to ground. When the two impedances are equal, 50% of the signal is lost.

I’m wondering if there would really be much of a difference if I had an SUT with the exact ratio and loading that is specified rather than "getting close?"

@adam8179 Transformers will ’ring’ (distort) if insufficiently loaded and will roll off if loaded too much. So the answer is yes. You can hear it and its easily measured.

The correct loading value varies with the source impedance (the cartridge).

Transformers transform impedance; they do not isolate impedance, so the output impedance of the transformer varies with the source impedance.

Don’t worry about what is ’ideal’ for any LOMC cartridge- loading such cartridges for sound quality is a myth. Your main concern is making sure the transformer isn’t modifying the sound by being incorrectly loaded (although all SUTs make distortion and limit bandwidth even if loaded correctly).

This is tricky since the capacitance of the tonearm cable and that of the cable between the transformer and phono section plays a role. Quite literally this should be done with a square wave generator, an oscilloscope and a variable resistance. With the cartridge in place the square wave (of very low amplitude) is applied to the input side of the transformer and the output observed on the ’scope. The variable resistance is placed across the output (which is connected to the phono section) and varied until the ’scope shows something that looks as close to the square wave as possible. If this does not make sense to you find a technician to do it for you.

This value of resistance is called ’critical damping’.

Obviously this is tricky (in most cases will have to be done by a technician) and is one reason I prefer using a phono section that does not need an SUT; one competently designed so I don’t have to worry about ’cartridge loading’ (which is for the benefit of the phono section, not the cartridge); just plug and play.

As others have well pointed out, you should never stress about differences like 18x versus 20x. It truly doesn’t matter. If you swap from one brand/model of SUT to another, the inherent sonic differnces between those SUTs - due to materials choices, core size, lamination quality, wire windings, etc - will absolutley swamp out anything caused by small differences to loading and/or step-up ratios.

When playing with SUTs, what you need to get a sense of is the "bounds" you’re working in:

• If you load a cartidge down too heavily, you’ll start to lose increasingly more than 1dB of signal level, due to your phono stage’s input being on the "wrong side" of a voltage divider. This is a problem when you use too high of a SUT ration for a cartridge based on its coils’ DC impedance. You would’t want to use a 40x SUT with 38 ohm coils, beacause you will be losing 6dB of signal! This can also cause frequency response aberrations, besides the bulk signal loss.
• You can calculate out the losses, and you probably want to stay lower than 2dB loss in all cases. You’re fine with what you’re using here - well under 1dB of losses.The "load your cartridge at 10x its coil impedance" isn’t a bad rough guide. You can always go higher, no problem. Going lower, I find 5x - 6x to be the cutoff - lower than this is almost always bad.
• If you choose too high a SUT ratio - even if the loading works out OK and the signal losses are acceptable - you run the risk of encroaching overload margins on your phono stage. At first this can cause the sound to seem "dynamic, punchy" but as you push it the sound can become harsh, gritty. As a rule you need to keep the signal hitting your phono stage input under 10mV as calculated by the cartrdge’s rated output level. E.g. a Shelter rated at 0.5mV with a 20x SUT = 10mV (it will actually be a little less due to loading losses described above). It’s generally best to target 5mV, but up to 7.5mV is fine.
• Avoid cartridges with "inefficient" magnetic generators like Benz Ruby / Gullwing / LPS (Ebony / Wood / Glider series is fine), Dynavector Karat 17D, there are a few others. These cartridges have high coil DC resistances paired with relatively low output levels - they are NOT a good match to SUTs or "transimpedance" head-amps. The ruby plate armature (as opposed to iron / permendur) is at fault here. Ortofon has started using some non-magnetic armature materials in its top cartidges, but thy reign it in (hybrid) just enough to still allow use with carefully chosen SUT ratios.
• "Transimpedance" head-amps are the active analogue to a SUT (passive) - sort of. The loading (as close to 0 ohms as possible) is really weird and totally different. You can buy a Hagerman Piccolo Zero for cheap ($270), and it’s a lot of fun. Its musicality competes with much more expensive SUTs, at the expense of a higher noise floor. It’s almost silly how well this device matches with certain Ortofon SPUs (Classic GM Mk II spherical; Meister Silver) - this has actually been my favorite combo of the past couple months.

You left out “head amps” aka “pre-preamps” which have been around since the early 70s. These devices are active gain stages that boost cartridge output voltage ahead of the phono stage. They do not correct for RIAA. Sutherland and Hagerman make good ones but there are many other choices. When MC cartridges hit the US market in the 70s, most audiophiles owned only MM phono stages with insufficient gain to accommodate the MCs. Pre-preamps were the cure for that, even before there were many SUT choices. The good news is that a prepreamp will work with any MC, regardless of its internal resistance. So there are 3 viable options.