SUT - electrical theory and practical experience

@drbond 

from rothwell...

In order to make this transformer match the cartridge with a load impedance at the primary of 5 ohms, a load on the secondary of 6480 ohms could be employed instead of the 47k normally found on an MM phonostage.

Lets say for example the goal is to load a cartridge @ its internal impedance.  I agree with Rothwell that heavily loading down the secondary of a SUT to reflect the desired load to the cartridge is a mistake.  However....  One could simply accept the 47kΩ termination on a 1:36 will reflect back 36Ω. In order to reduce the cartridge load to 5Ω, simply parallel a 5.8Ω resistor with the cartridge. (it is kinda cool how 36^3=47K)

I have done this experiment and can emphatically state that in this case 5Ω≠5Ω.  In the case with the load applied through the transformer it seems like a blanket has been thrown over the speaker.  This is the exact "thick" sound people report from this type of loading.  The error all too often made is incorrectly attributing the sound of the reflected load to the cartridge when it is actually the sonic nature of the loaded transformer.  Simply doubling the turns ratio of the SUT and then loading the cartridge at the same 5Ω nets a very different sonic result. Thick, dark and muddy are the last terms I would use as descriptors.  To complete the circle on this I have then taken a transformer loaded Jfet based head amp and compared the same cartridge above loaded at 47kΩ and at its internal impedance and I would say the overall pattern of sonic character followed that of the transformer with the additional load placed on the cartridge directly and the sound of the "traditional" heavily loaded transformer secondary is the outlier.

dave

@rauliruegas 

So, Dave hopefully you can share your know-how and experiences about.

All I can give is my subjective opinions from my experiences comparing the myriad of options out there.   In my system the SUT's are always silver wire on 80% round loop nickel cores with an emphasis on air as a dielectric.

dave

@intactaudio Your last response is what I would call an IP reveal and very generous as an offered information. 

I carry info' like this from my Bespoke Designs I own,but have always felt my liberty was best constrained when the opportunity arose for sharing the Designer/Builders IP.  

I once had an awkward conversation with a not very well known to me EE at an event where I was demonstrating a device, they were keen for me to allow them to remove a Chassis bottom plate and let them see inside. I was adamant that that would be quite a betrayal to the designer with whom I have developed a close to 30-year friendship.  

Some observations:

I always found the specs for a cartridge confusing: 0.25mV output at 5 cm/second.  Into which load?  Open circuit?  Load=Cart impedance?  Load=recommended impedance?  Why not state the output voltage and the impedance at which it is measured?  Most recommended load impedances are specified over a large range (100Ω-1K), and they are all usually 10x the cart source impedance so the voltage divider effect will be small, but in some cases, it could have a significant impact on the output.

Matching the cart source impedance into the load would only be relevant if you are trying to transfer the maximum power from source to load which is not the goal of a phono preamp.  It would be like trying to match the load a speaker presents to the output impedance of an amplifier which can have an output impedance measured in milli-Ohms.

What you do want to match between cart and load is the conjugate match of the imaginary (reactive) part of the impedance so the cart sees a purely resistive load.  The cart has a complex source impedance which may not be a simple series or parallel capacitive or inductive component but a combination of those elements.  The complex impedance will most likely change with frequency as well, complicating the matching network calculations. 

Perhaps what is needed is an S-parameter measurement file for the cart that can be used in a linear simulator to design an appropriate conjugate matching network.  Most RF devices are spec'd this way and is the only practical way to design matching networks that work;  absent this data, it's like trying to find your way out of a maze with your hands tied behind your back and wearing a blindfold.

Dear @intactaudio  : Thank's.  I understand that normally the dielectric in SUT is air but I can be wrong.

The only SUT silver wired I tested in my system vs SUT's cooper wired was the Audio Note Kondo that was compared vs the AT 1000T, Denon 340 and Denon AU-1000 and in all tests I prefered the cooper wired SUTs ( phono input 100k. ) and you the silver wired ones.

Maybe the SUT kind of wire is more or comes along with the room/system and owner targets. Different with speaker croosover inductors where silver is superior to cooper wired and ribbon shape over rounded ones.

Btw, the Kondo goes down to 2hz ! ! and Audio Note builded around the end of the 70's a SUT with an exceptional FR: 2hz to 200khz at -o.5db and 0.001% distortion. I never seen in the market and I don't know neither if was silver wired but probably not because was at the midle of Audio Note SUT line.

R.