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

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.

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'.