3.2 More practical considerations!

Lets now have a first (and last...) look at... simplicity itself: A high output cartridge connected directly into a standard MM phono stage.
And, after changing our cartridge for a low output one, a SUT is introduced into the picture.
Notice the R=x resistor? Well, time for you to know that when a SUT is introduced between the cartridge and the MM phono stage's load resistors, it changes the load that the cartridge will see in a quite dramatic way. This is because the SUT acts like an 'impedance divider', dividing the impedance (47k) by the square of the turns ratio between the primary and the secondary.

Too long and (seemingly...) complicated? Let's then take an example:
Just suppose that you have the primary of your SUT wired for 1:10 gain. This means that the impedance presented by the phono stage (47k, remember?) will be divided by 10 square. As a result, the real cartridge load will be 47000/100=470 Ohm (also called "natural load"). If your cartridge is happy with this kind of load, then you are also a happy man. If not (or if you would like to experiment...) you should be able to calculate the resistance value which, soldered in parallel with 47k, will give the desired loading:

Let's stay with the same example, supposing that the desired load (Rtarget) is 100 Ohm (a very common load for MC cartridges, by the way...). We have already said that, because we are using the 1:10 ratio, the cartridge sees a load 100 times lower than the actual resistance on the secondary. This means that the 100 Ohm we want on the secondary are – in fact – 100x100=10k Ohm. As a result, we must calculate the value x of a resistor which, in parallel with 47k, will give us a 10k Ohm net resistance.

We can now use the well known formula for parallel resistors:

Rnet = 1/(1/R1 +1/Rx)

(where: R1=47k, Rnet=10k)

And, solving the formula for Rx:

Rx = 1/ (1/Rnet - 1/R1) or Rx = 1/ (1/10.000 - 1/47.000)=12.7k

Simply install a resistor close in value to the calculated 12.7k and you are in the ballpark i.e. you will have a +-100 Ohm load seen by your cartridge.

Please, finally, note that resistors can only be found in typical values (E12 and E24 ranges -- Jensen Stepup Transformer). I have used a good number of these values (Rload) to make a table indicating what kind of net load your cartridge will 'see', once you solder the Rload resistors in parallel, with the input resistors of your phono stage (47k). And, this, taking into consideration most (if not all...) gain ratios, currently used by SUT manufacturers.
You can find table in Jensen Transformer web