With one SUT you can't use all MC cartridges, such phono stage is not universal for all MC. It will work for some of them nicely, but if your cartridge impedance is very low (2 Ohm) or very high (40 Ohm) you can't use one SUT for both of them.
I'll differ from Ralph and agree with chakster 100% on this one. The thing everyone misses in this discussion is the transformer needs a specific amount of inductance to assure low end extension based on cartridge internal impedance. If you take a 2Ω cart and use a SUT designed for 40Ω, the LF extension will be down into and past record warp and tonearm resonance and the low impedance drive will cause an underdamped top end (HF peak in response). Depending how close to the audio band this happens, the results can be problematic. Going in the other direction and using a 40Ω cartridge for a SUT designed for a 2Ω source typically will not have a full bandwidth bottom end and an over-damped top end. The general solution to this is to design with enough inductance for the highest impedance cart and rely on a network on the secondary to fix all the other issues that arise from any mismatch. The other typical solution is to use a dual or tapped primary that allows for different turn ratios to better suit a wider range of cartridges.
For given source and load characteristics a wide bandwidth transformer can be wound to match the situatu
dave