Calculating load on MC with SUT

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I should know this, but I am putting it here for help.

I have a Shelter II on which the manufacturer recommends a 20 ohm load.

My phone preamp is a World Audio Design Phono II clone with 100 ohm grid stoppers on the input tube pin and 47K in parallel from a Lundahl 10:1 step up.

So I go to wire up a 100k pot to adjust the load from what I expected to be the traditional 47,000 MM load / 10 squared = 470, but how does the 100 ohm grid stopper change the load value?

What load is reflected to the cartridge now? Does the 100 ohm resister impact the calculation?

Thanks,
Dan
 

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...Does the 100 ohm resister impact the calculation?...

The 100r resistor is in series with the internal grid resistance, which is typically hundreds of MegOhms. Say it was 100Meg or 100,000,000 Ohms. Now it is 100,000,100 Ohms. No Difference.

This 100Meg is parallel to the 47K. The true value of the input impedance is 46,978 Ohms. No Difference from 47K.

The 1:10 step-up of voltage is 1:100 of impedance. 47K/100 is 470 Ohms.

There are *several* Ohms of uncertainty here. The windings must have resistance. How much? "Small", but often not specified. Then is the "1:10" really a voltage ratio or a spec on the impedances, with winding resistance included? Who knows? And at some point, does it matter?

You want 20 Ohms? Frankly, I would just throw a 22r on the cartridge side. It can't be highly critical (it must be controlling a broad-band resonance which by nature is not too critical). Check: 470r||22r is 21.0r.

The exact-arithmetic answer is near 20.889 shunt resistor.

wire up a 100k pot

I would not do it that way. Pots are crappy resistors. Loading the secondary increases transformer loss (S/N gets worse). The transformer may not be optimized for a lower Z than 47K. Put a 20-some resistor on the needle side.
 
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