Thoughts on cartridge loading with a SUT.
Hi, My name is dave slagle and I am an audio geek.
This is my first post here, Larry and Steve (Vetterone & Celloist) encouraged me to start participating in the forums here during the Audiogon meet and greet during RMAF 2K8. What follows is payback for the 2 for 1 margaritas they forced down my throat.
There seems to be a lot of confusion on how to select a transformer for a particular MC cartridge. Many people consider this a black art and I have to admit I have been shocked by the sonic differences from devices with similar electrical characteristics. I'll try to be brief and cover some of the relationships at hand in a few short paragraphs.
It’s about the turns ratio.
The first thing you need to know about the SUT in question is the turns ratio. This is an easy number to find out and is the most important. The idea here is the turns ratio tells you the gain but it also plays a major part in the load the cartridge sees. The rule is impedance is the square of the turns ratio. If you have a 1:10 turns ratio, you get a 1:100 impedance ratio. If you assume that the 1:10 will feed a phono stage with a 47K input resistor the 47K will reflect back 47K divided by the impedance ratio (100) or 470 ohms. It is really that simple. Many transformers (particularly the vintage ones) are speced with impedance numbers like 150:50K which simply translates to a 1:18 step up ratio. (sqrt(50000/150)). The impedance numbers are somewhat important since they suggest the ballpark of where the unit was designed to operate, but it is easy enough to toss that info out the door and measure things for yourself. One other important thing to realize about the turns ratio is that "More" isn't always better. It is surprisingly easy to have too much of a step up resulting in a situation where you overdrive (clip) your phono stage.
Relating the turns ratio to load.
Sticking with the above example of a 1:10 driving the 47K input, our cartridge in a perfect world sees a load of 470 ohms. Now what if we have a 103R with a 15 ohm internal impedance and we want to play with larger loads. (A larger load is one that is smaller in value) The traditional way to do this is to add additional resistance in parallel with the 47K to get the desired value. Again this is a simple application of the turns ratio. If we desire a 150 ohm load we would need to parallel a 22K resistor with the existing 47K, which nets us 15K across the secondary. (47000||22000=15000) Dividing the 15K by our impedance ratio of 100 nets us our desired 150 ohm load.
Houston, We have a problem
Everything here is nice and clean upon firs glace, but we have actually hit our first speed bump. When we terminate the transformer with a different value, we not only change the load seen by the cartridge we change the behavior of the transformer itself! This means we are changing two parameters which creates a very unpredictable situation which goes a long way to explain why results of playing with secondary loading on SUT's has lead to such varied results since you cannot be sure what you are fixing.
Why the need for a load anyways.
The simple answer is all cartridges have a peak (resonance) at some high frequency and by increasing the load the peak is damped smoothing out the measured response. The general effect is when the load it too low (say unloaded) the cartridge sounds bright and when you take it to the other extreme and increase the load the highs start to sound rolled of. At some point in the middle the "sweet spot" is found and the only way I know how to this is in system by ear. The 800 pound gorilla sitting in the corner is the fact that the SUT will show the exact same behavior as the cartridge and loading will have a similar effect. Typically the resonance in the SUT will be an octave to a decade higher than that of the cartridge. Unfortunately there is no way to know if your choice of secondary load has damped the resonance of the cartridge taming the highs or simply rolled of the SUT masking the brightness of the cartridge resonance.
How do we tame that gorilla in the corner? Menno van der Veen
presented the best approach I have seen in his white paper on his SUT’s. Essentially his approach is to determine the load needed to make the transformer behave as desired and then add any additional load required by the cartridge to the primary of the transformer. He even goes as far as to measure his SUT’s under a number of conditions and provides the needed loading info for various impedance cartridges. This is the little brother of the 800 pound gorilla, lets call him the 400 pound gorilla on the couch. Just as increasing the load on the secondary damps transformer resonances (ringing) decreasing the source (cartridge) impedance tends to increase ringing. This makes the once simple loading of a transformer become a far more complex relationship. A few other gorillas strolling about the room involve the belief by some that the loading of transformers can cause more sonic harm than the problem it fixes and the idea that loading a transformer secondary causes phase shift at high frequencies.
Jane Goodall to the rescue.
Rather than ignore all of these primates strolling around our listening rooms the best approach is to attempt to understand them and learn how to live with them. The best way I know how to do this is to converse about experiences and understand that ultimately it comes down to first understanding ones musical preferences so their experiences can be related to the info available to us.
A final anecdote.
For some time now I have been a fan of primary loading of the SUT. Over the past few years I have encouraged a number of people to play with primary vs. secondary loading and the results have been mixed. The one interesting thing was that consistently the people who preferred secondary loading said primary loading was “harsh” sounding and the people who preferred primary loading stated it brought more of the music out of their system. In looking at the measured response of the transformers in question with the known source and load impedances it quickly became apparent that the people needing the secondary loads were not only damping the ringing in their cartridges, but also taming a peak in response in the transformer. By loading the primary they actually made the transformer ringing worse, which is a very plausible explanation of what they heard. When looking at the measured response of the transformers used by those who preferred primary loading, the resonant peak was reasonably controlled and substantially beyond the audio band.