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

this fidelity research FRT-4 is quite versatile
don’t anybody buy it, I might buy my 75th birthday gift 3 years early.

I certainly won’t be buying it :-)

Even if I needed it, which I don’t, personally I would not purchase from a seller who ...

-- has more than 3300 items listed for sale at eBay, very few of which have anything to do with audio.  (His "store" states that "we mainly sell Japanese figures, apparel and daily goods");

-- lists this 40+ year old item without providing any indication of whether it has been tested and found to work;

-- has more than 2% negatives among his nearly 1000 feedback ratings in the past 12 months;

-- and is overseas, which might make returning it and obtaining a refund problematical should that be necessary. At the very least you would have to pay return shipping costs, per the stated terms.

Just my $0.02. Regards,

-- Al

impedance is what effects the sound
too low = dull. too high = too bright

As a general rule of thumb that is often true in the case of low output moving coil cartridges. But the effect is also dependent on the phono stage, and for that matter on the capacitance of the cable connecting the output of the SUT to the preamp. (The shorter that cable is and the lower its capacitance per unit length is the better, assuming accuracy is the goal).

.3mv cartridge x 21.5 = 6.45mv = TOO HIGH SIGNAL STRENGTH

Nope. See below.

.3mv x 10 is 3mv, TOO LOW? (mx100z phono sensitivity is 3mv)
.3mv x 20 is 6mv" TOO HIGH?

I PICKED THE WRONG DAMNNN CARTRIDGE!!!

While sensitivity specs for power amplifiers can be useful, I suggest that you completely ignore such numbers in the case of preamps. All it means in the case of a preamp is what I described in my initial post, and pretty much all it is useful for is calculating the gains of the phono and line sections of the preamp (if they are not explicitly specified), as I did in that post.

A 20x (26 db) SUT will boost the rated output of the 0.3 mv cartridge to 6 mv, as you indicated. 6 mv is only about 1.6 db greater than the 5 mv ratings of two of your present MM cartridges. For a single-turn rotary volume control (as is provided on your MX110Z) a 1.6 db change in volume typically corresponds to around 10 degrees of rotation, and perhaps even less. So with the new cartridge and a 20x SUT you would simply be setting the volume control a tiny bit lower than the settings you presently use with those two cartridges, to achieve the same volume.  (This assumes that the specs are reasonably accurate, of course).

Regards,
-- Al

As far as I can recall my Lionel transformer didn’t hum :-)

Also, while I’m not certain I believe that in at least most of the models made in the 1950s or thereabouts those "transformers" were actually autoformers. Meaning that they just had one winding, rather than separate primary and secondary windings, with a sliding tap on that winding to control the output voltage. The maximum setting being mechanically limited to result in a safe voltage.

Regards,
-- Al

Again, keep in mind that the higher the boost provided by a SUT the lower the number of ohms that will load the cartridge.

I don't know of any SUTs which provide a bypass function, although perhaps there are some.

Regards,
-- Al

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

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