With an active gain stage the input impedance changes the tonal character of the cartridge significantly.
Absolutely! And I would say the majority of LOMC users do not hear the best their cart can deliver because they are limited in the loading they can apply.
Loading via SUT is very different from loading via headamp or active MC stage.
Absolutely! And the loading they can apply with a SUT is very limited.
With a SUT, the primary goal is to step the voltage up to around 5mv - the loading is far less critical to the sound quality.
Sure, the purpose of the SUT is to multiply the output of the LOMC by the turns ratio. So with a nominally 0.28mV Benz LP, the output becomes 2.8mV with a 1:10 SUT.
But I suggest it is a complete non-sequitor to say "the loading is far less critical to the sound quality". The issue with a SUT, IMO, is that there is little ability to get the optimal loading for the cart - so you put up with what you can get.
Sure, if you have a LOMC that really does sound its best at 100 ohms loading ... then with a 47K MM phono stage and a 1:10 SUT, the cart normally sees 470 ohms. But if you put a 12K resistor on the output side of the SUT, this will change the loading seen by the cart to be 96 ohms (47K in parallel with 12K, divided by 100).
But if you have a Benz LP, for instance, there is no way to get it to be loaded at 3K3 (the value which sounds best to me, anyway) with a SUT - unless you replace the 47K default loading in the MM phono stage to 330K (330K / 100 = 3K3). Which will probably have implications for the sound of the phono stage's 1st gain stage.
There is a good explanation of this on the Rothwells site, which explains it better than I can: http://www.rothwellaudioproducts.co.uk/html/mc_step-up_transformers_explai.html.
Mmmm, IMO this is a very biased 'white paper' - IMO full of half-truths. But I'm sure all SUT devotees think it is on a par with one of Moses' tablets - ie. God's truth.
For instance, on the 1st page:
1. More importantly, IM distortion tends to be lower in transformers than it is in transistor amplifiers.
That is an absolutely true statement. But, although I am not an SME in solid-state electronics, my understanding is that IM distortion occurs in Class AB amplifiers - which have a +ve DC rail and a -ve DC rail ... there is none in a single-ended circuit (which has just a +ve DC rail).
So IM distortion is not an issue with a single-ended, JFET-based headamp, for instance.
2. Does the cart's tonal balance change with load impedance? It certainly does if the cart is a MM type but LOMCs are much less sensitive to changes in the load impedance.
That is completely A-about-T, IMO. MM carts are very sensitive to load capacitance ... not so sensitive to load impedance. LOMCs OTOH sound very different at different loadings.
Andy