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SUTs and Resistive Loading Questions

Hello, I have a few questions about SUTs and adding parallel resistors to reduce the load impedance.

I have a SUT with a 1:15 turns ratio. As we know, the load for a 1:15 SUT through a MM/47,000 phono preamp would be: 47,000/15/15 = 209 ohms.

However, I want a load impedance of 100 ohms for the MC cart I'm using. To calculate the value of the resistor I need to add in parallel to do so:

100 ohms = Z/15/15 for a SUT with a 1:15 turns ratio. Solving for Z = 22,500 ohms.

Resistor Value Needed = 1/((1/22,500 ohms) - (1/47,000 ohms)) = 43,164 ohms.

So now, my questions...

If my phono preamp has a 100 ohms load setting, can I use that (instead of the 47,000 ohms setting and the above calculations for adding parallel resistor) to essentially get the 100 ohms load impedance I want?

Also, is there any difference in damping using the first method versus the second method? I would think not, but not completely sure.

Thanks for your insight.
Ag insider logo xs@2xedwyun
almarg's avatar
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almarg
9,670 posts
 
But if you run the calculations as if loading on the secondary side, the SUT’s affect is negligible.
I’m still not following you. If a 15:1 SUT is inserted between the cartridge and a 100 ohm phono stage input the cartridge will see a load of 0.4 ohms. (Approximately, that is, because additional effects such as DC resistance will become significant under such an extreme condition). And that would amount to putting a near short across the output of the cartridge, and would pretty much kill the signal it is trying to put out.

Regards,
-- Al


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edwyun OP
244 posts
 
You are correct.  I fixed my prior post before your last response.
lewm
13,223 posts
 
If you must use a SUT, you can either place a ~200R resistor on the primary side and no added resistance on the secondary side (47K only) or a ~43K resistor on the secondary side of the SUT, in parallel with 47K, to achieve a net 100R load.  However, I am puzzled when you say your phono stage affords the built-in option of 100R; that implies to me that it must have gain sufficient for at least some low output MC cartridges, in which case you might not need a SUT. Because 100R is not a load value one associates with high output, i.e., MM/MI cartridges.

But like Almarg says, you don't want to use BOTH the 100R setting AND the 1:15 SUT.
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rauliruegas
13,103 posts
 
Dear @edwyun : "  In a prior setup, I did notice a difference between 100 versus 200 or 470 ohms ....Cart is Dyna 20x2L. "

The didifferences because loading is not much because the cartridge " reaction " tha's almost no sensitive to normal load changes. Those differences comes from the phono stage " reaction " to those loading changes.

Anyway,  you will follow making load changes till you are where you like it more.

Regards and enjoy the music,
R.
larryi
4,080 posts
 

Regardless of the theoretical issues, there is no harm at all in just trying different loading. 

I experimented with primary side loading by making RCA loading plugs and an RCA T splitter.  I used cheap resistors for the experiment, with the intention of buying the good stuff of the right value when I was done experimenting.  I planned to solder the right resistors on to the back of the input jack of my phono stage (it has a built in SUT), between the hot pin and the ground lug.  As it turned out, I just preferred no additional loading, but, it was sort of fun just making a few plugs and trying them out.

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