Question about loading with an SUT

I have a DL-103R feeding into a Cinemag 3440 with switchable taps for 1:18 (150 ohm) or 1:36 (37.5 ohm). I use 1:18 and it sounds lovely. I also have a Rothwell MCL with 1:20 (100 ohm) and I honestly can't hear a difference between these two SUTs. I've switched back and forth many times and settled on the Cinemag. 

The 1:36 ratio with the Denon cartridge gives you an impedance that the Denon cannot drive efficiently. About 50% of its signal voltage output will be lost to ground. So doubling the turns ratio from 1:18 to 1:36 gives you nothing in terms of added gain. In addition at the higher ratio the treble may roll off due to the effect of capacitance. My point being that with a SUT, impedance as affected by turns ratio is every bit as important as voltage gain, and it’s usually a trade off.

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.