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

Showing 6 responses by edwyun

Larry, thats what I intend to try. It is easy enough.

Lew thanks for describing the other options on the primary side. I will look into it further.

As for the need for a SUT, my pre has sufficuent gain for a MC but my experience is that using a SUT with a MM pre has benefits.

I'm familiar with the Rothwell site and links.  Thanks.

In a prior setup, I did notice a difference between 100 versus 200 or 470 ohms.

Cart is Dyna 20x2L.
Al, you are correct. I need to take into account the SUT with a 100 ohms setting on my phono preamp. But if you run the calculations as if loading on the secondary side, doesnt look like I will be able to do it - Id have to increase load, and that is a lot harder.
I agree less is more (and thus don’t like using more connections, resistors, etc, than necessary), but this is about trying different loading with a cart/SUT/phono setup to see what sounds best. If the resistors make a difference, then I would most likely change to a different SUT with different loading characteristics.

But I disagree that adding a SUT alone to a phono stage would lead to sound degradation. To me, the benefit of a SUT is not just gain.  YMMV.