Has anyone tried resistor loading the primary of a MC SUT ?

Loading on the primary side would have the effect of reducing the net output resistance of the cartridge, theoretically making it more capable of driving a low impedance on the secondary side.   So, if you have an LOMC with a high-ish internal resistance but low signal voltage output (so it needs a relatively high turns ratio), like the Denon DL103, then it might make some sense to load on the primary side.  I don't know how or whether that might color the sound, apart from the impedance matching effect. I think there is a good white paper on loading the SUT to be found on the Jensen transformer website.

There is little point to loading the input side.

The reason is that most of the loading issues that you will hear are caused by the transformer itself and not what is driving it. The loading will be different depending on the cartridge used, but the point here with the loading is to prevent the transformer from ringing.

The cartridge won't ring at audio frequencies so there's no point in loading it. It does ring in ultrasonic or radio frequencies, but the transformer is likely blocking that so loading the output of the transformer is really the effective thing to do.

If the loading is too light (value too high) the transformer will ring, if too heavy (loading value too low) the transformer will roll off the highs. This is why the correct value is known as 'critical damping'.

Ralph, whether it is only the phonostage that needs loading or also the cartridge is debatable. The general terminology is "cartridge loading". So adding resistors to primary should load and damp the cartridge. 

Pani,
You are wrong about Ralphs comments.
Loading the primary can induce ringing in the transformer. Loading the secondary "dampens" both the transformer and cartridge. The results of loading are quite variable and unpredictable - depending on cartridge attributes, transformer resonances that will vary with secondary loading from both the phono stage and or secondary loading if additional loading is applied. Jensen transformers use a zobel network to tame ringing, but this is included in the design of the transformers to achieve extended response in phase without ringing.
Loading of transformers is a minefield. From my own experiments with both vintage and Jensen transformers (playing with the zobel network ) I prefer transformers unloaded with minimal gain required to achieve a good signal to noise ratio. This minimises the "distortions" introduced by the transformers. If you really want to get serious with transformers you need to scope the output from the transformer and minimise ringing by adjusting a zobel network for the specific cartridge/transformer combination.
Personally I prefer a high gain active mc stage.

 

Ralph, whether it is only the phonostage that needs loading or also the cartridge is debatable. The general terminology is "cartridge loading". So adding resistors to primary should load and damp the cartridge.

Actually, regarding whether its the cartridge or the phonostage, there is no debate at all. Its just physics.

I know this as at one time I also thought low output moving coil cartridges needed loading, and to take the mystery out of the process, I began researching a device that could sort out the right value.

What I found was that it was not the cartridge that the loading was affecting. Instead the loading affects the Q of the resonance caused by the inductance of the cartridge and the capacitance of the cable. Reduce the Q enough (with proper loading) and the resonant circuit can't oscillate.

With SUTs, you don't get the same issues, mostly because the SUT can't pass the Radio Frequencies (RF) that are typically generated. But the SUT is an inductive device and does ring if improperly loaded.

LOMCs are inductive too of course, but they don't ring at audio frequencies (quite unlike MM cartridges BTW). I was a bit incredulous at first but you can 'ring' a LOMC cartridge with a square wave and its very good at passing it right through. IOW they don't ring at audio frequencies!

I also prefer a high gain MC stage as well. If executed properly they are more transparent than using a transformer.