2. analog
  3. 128100-10021

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.
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Showing 1 response by lewm

lewm
12,562 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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