OP
this is my SUT, the one lewm referred to:

PASS is so you can run a MM cartridge (or HOMC) thru it bypassing the internal transformers, so there is no need to plug/unplug any cables.
Finally I realized the designations 3; 10; 30; 100 are as he said, for you to select based on your coil’s internal impedance:
i.e. for my AT33PTG/II with a 10 ohm coil, I use the 10 ohm setting. Inside, that rotary switch selects one of the 4 various windings of the transformer, which corresponds to a signal boost,( which I call x factor) of 20.68. (equal to +26 db) My signal is 0.3mv x 20.68 = 6.2 mV (now a strong enough signal to go to a MM Phono Input which will perform RIAA Equalization, and further signal boost, up to Line Level.
My unit, the impedance load that will be shown to the MM Phono Input is RESULTANT, and for that 10 ohm/20.86x signal boost is 110 ohms. That is ideal for my cartridge as it’s coil impedance is 10 ohms.
Your unit, the load and the gain are SEPARATELY controlled
Gain settings: 40, 45, 50, 55 or 60 dB
Loading: 100, 200, 475, 1k or 47kOhms
and the RIAA EQ also occurs within your unit, the total gain is adjustable
Using a SUT into a MM Phono Input (where RIAA EQ occurs), the total gain is:
SUT gain PLUS MM Phono Gain
I do not know how much gain in db my MM Phono stage adds (to my SUT’s +26db gain). My Preamp’s Output Level is 2.5V (unless attenuated, by the volume control).
