Thanks for keying me onto this. I think I have figured out a path to try.
First, I believe I have found a resource that agrees with my findings. My Dynavector 20x2, 5 ohm 2.58mv cart, doesnt sound as good with a 280 ohm load with a transformer vs. being loaded at 100 and 50 ohm load using my phono preamp. I hear positive things, but over all, its not the best setting.
Here is the resource - "A typical MC cartridge will sound somewhat “lightweight” when it is loaded too lightly (load impedance is too high) and will sound somewhat dull when the load impedance is too low."http://www.kandkaudio.com/applications-guide/
That said, I reached out to Andrew at Rothwell and he confirmed my computations and gave me some information that allowed me to understand the MC-1 much better. I used three primary resources to figure out my next move.
I need to get the SUT to present the right load to the cartridge.
I studied up on the science and formulas using Rothwell's site - http://www.rothwellaudioproducts.co.uk/html/mc_step-up_transformers_explai.html
K&K provides a schematic how to load the SUT so that it presents my desired load to the cartridge.https://i.imgur.com/kf86Pryl.jpg
I reverse calculated the way my MC-1 is built so that I can compute the load resistor I am going to use in the schematic from K&K using MH-Audio's SUT Guide - http://mh-audio.nl/Calculators/StepUpTransformer.html#c
I arrived at the following conclusion. The MC-1 presents a 280 ohm load with a 20 and 258 turns of wire on the primary and the secondary. Or thereabouts, the exact ratio is 12.9.
The nonlinearity of the impedance transformation gets a little complicated. The impedance differs by the square of the turns ratio. For a 1:12.9 (22dB) step-up, the impedance is transformed by a factor of 167. That is, with the secondary loaded by 47k ohms, the primary reflects what appears to be 280 ohms to the cartridge.
The question now is what resistor should you put in parallel with the 47k phono stage to change the loading to 50 ohms? I used the following formula:
First get the SUT impedance presented to the phono preamp with the MC on the SUT - (Transformer Ratio^2 * Desired Loading) = (12.9^2 * 50) = 8320 ohms
From there, you can get the Resistor load using this formula 1 / (1 / Desired Loading - 1 / Phonostage) whereby 1 / ( 1/8320 ohms - 1/47Kohms) = 10.1 Kohms
I verified my calculations using this site - http://www.hagtech.com/loading.html
Soldering a 10.1 Kohm resistor to the output of my SUT, with the phono preamp set at 47Kohm will present a 50 ohm load to my cartridge.
You can also put the parallel resistor on the primary side, but then the value is the above divided by the turns ratio squared.
Ive got a few resistors incoming to modify my SUT. I am hoping I am able to keep the sweetness of the SUT's sound, while retaining the brilliance of the cartridge.
Man, I havent done this much analysis and math since school