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.

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.

Gorilla Wrangling

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.

dave
intactaudio
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