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.

edwyun

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:


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


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.
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?
Edwyun, your calculations look correct, but this question isn’t clear to me. A 100 ohm load within the phono stage would present a 100 ohm load to the cartridge only if the SUT (and the gain it provides) is not in the path. If the SUT is present the cartridge would see a load of 100/15/15 = 0.4 ohms, which of course would not work too well :-)

As Larry alluded to a resistor could be placed on the primary side of the SUT, that would provide a 100 ohm load in combination with the 209 ohms, but I would not want to speculate as to how the resulting sonics would compare with the paralleled 43K/47K secondary side loading approach.

Regards,
-- Al

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.
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


You are correct.  I fixed my prior post before your last response.
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.
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.

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.

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.

Dear @edwyun : "  my pre has sufficuent gain for a MC but my experience is that using a SUT with a MM pre has benefits. "

That statement makes no sense to me other that the MC stage in your preamp be a bad design, that could be but I can't know because I don't know wich unit you own.

I said that makes no sense because: how or why can you think that adding an external SUT and additional IC cables and connectors can improve or gives benefits to the quality level performance of the same cartridge signal?

Look, if I assume that your preamp has an active gain MC stage and is a " decent " design then what you posted can't be true because those additional external devices you are talking about and in a " perfect audio world " ( that does not exist. ) the best it can do is no cartridge signal degradation and NEVER an improvement in any way.

Those additional devices you want to use only can degrade the cartridge signal and NEVER can improve it: that different levels of degradation are name it DISTORTIONS, additional distortions.

If you like those additional distortions is not the main audio subject here.


Regards and enjoy the music,
R.
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.




Dear @edwyun : It's your privilege to disagree.

R.
Free opinions and free speech.