Which SUT? .... for .3mv/10 ohms MC cartridge into 3mv MM input sensitivity

Q1) No, 3 mv is the input voltage for the phono inputs which will result in the main output of the preamp providing its rated output (3 volts, assuming that spec is the same for the MX110Z as for the MX110), when the volume control is at max.

3 volts/3 mv = 1000x = 60 db overall gain

Also, since the sensitivity of the Aux (line-level) input is 300 mv we can deduce that the preamp’s phono stage itself provides a gain of 300 mv/3 mv = 100x = 40 db, which is typical of the gains provided by MM phono stages. The line stage provides the other 20 db.

Q2) Correct, but keep in mind that the choice of SUT gain will also affect the load impedance that is applied to the cartridge (see below).

Q3) Not correct. Doubling of gain corresponds (to a very close approximation) to a 6 db increase. So doubling the gain from 10x to 20x corresponds to 20 db + 6 db = 26 db.

db = 20 x log(Vout/Vin), where "log" is the base-10 logarithm.

Q4) Given Q3, 40 db would be way too much gain. It would also result in an absurdly low load impedance being applied to the cartridge (see below).

Regarding ...

... 10 ohm coil x 10 = 100 ohm

... That is a good basis for choosing a SUT. See this writeup, which also addresses some of your other questions.

The voltage gain of a SUT corresponds to its turns ratio (i.e., the ratio of the number of turns in its secondary (output) winding to the number of turns in its primary (input) winding. The impedance transformation provided by a SUT corresponds to the square of that ratio. So to apply a 100 ohm load to the cartridge when the SUT is working into 47K the turns ratio (and hence the voltage gain) should be:

Square root(47000/100) = 21.7.

A voltage gain of 21.7 corresponds to 26.7 db. So a 26 db SUT (i.e., 20x, sometimes also referred to as 1:20) should be a good choice. You may want to take a look at the SUTs offered by Bob’s Devices, where the paper I linked to earlier can also be found.

Regards,
-- Al

almarg

thanks, I could handle multi-million budgets for my clients projects, but this kind of math, my mind just won't go there at my age. I appreciate simplifying it to 20x

Given that 20x will be good for this .3mv cartridge, I suppose it might be wise to have some extra boost for a possible lower signal MC, what would you suggest as max?

That’s hard to say, Elliott (see the last paragraph on the first page of the writeup I linked to), but I wouldn’t suggest going much higher than 20x, if at all. For example, if you went to 30x (which is about 29.5 db, just 3.5 db more than 20x) you would be loading your new cartridge with only 47000/(30 x 30) = 52 ohms!

Also, given the 20 db gain of the preamp’s line stage (which is significantly higher than average), coupled with its 40 db MM gain and 29.5 db of SUT gain, the 0.3 mv rated output of your new cartridge would be boosted to about 9 volts if, hypothetically speaking, the volume control were at max. And the peaks of some recordings can cause cartridges to exceed their rated outputs (which are based on certain standard test conditions) several-fold. Which means that you may risk overloading some of the circuitry on the dynamic peaks of some recordings (especially circuitry which is "ahead" of the volume control in the signal path), causing distortion, and/or having to use the volume control undesirably close to the bottom of its range.

Regards,
-- Al