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 3 responses by larryi

larryi
3,757 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.

larryi
3,757 posts
 

This can be a particularly complex subject because there are some experts who advocate loading on the secondary side of the transformer (selecting the 100 ohm setting on the phonostage does that), while others advocate loading on the primary side of the transformer (at the input side of the step up transformer).  I have generally found that no additional loading on either side usually works well and it really does not matter that much what is the output impedance of the cartridge; the only matching that really matters is the right amount of gain.  This is pretty much the same conclusion that is found in this white paper from Rothwell, a manufacturer of step up transformers:


larryi
3,757 posts
 

Oops, don't know why the link does not appear:


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