Moving Coil Stepup Transformer
Cartridge Loading
These sections outline how to optimize cartridge loading. First we cover the procedure for setting and adjusting load resistors and then we cover the theory behind calculating the load resistor values. Once you have played with and optimized the cartridge loading you'll likely not have to worry about all the electrical theory but it may be interesting to review it anyway.
For MM cartridges there is an industry standard loading of 47K. Any MM phono stage will almost certainly have a 47K input impedance so that the MM cartridge hooked to it will 'see' this 47K impedance. Usually this is simply a 47Kohm resistor between the 'hot' and 'ground' of the input circuit.
For MC cartridges there is no standard loading. They typically have a particular load they are happiest at. This load will often be recomended by the cartridge manufacturer with a figure os 100 ohms being quite common and these recomended loads are always less than the 47K MM standard. The lower loading is typically used to tame resonant peaks in the MC cartridges. This recommended load will be a good starting point and you are not likely to end up wildly different than that point. I suggest that you experiment with cartridge loading because the typical loading recomended may not be the very best match for the ballance of your system or your listening preferences.
Load resistor posts are included with the TX103 Kit. Placing a resistor with one lead in each post puts that resistor accross the transformer secondary which is also wired to the bullet plug. Once that plug is fitted to the input of your phono stage this resistor is in 'parralel' with the (very likely) 47K input resistor inside the phono stage. We'll talk more about 'parrallel resistors' later in the theory section but for now I'll just mention that all the load resitor values listed in this text and tables take this resistor into account and assume it to be 47K.
Once you have your TX103 kit installed in your system and working it's time to setup preliminary cartridge loading. Loading is simple - just place a resistor between the posts and tighten the posts to hold it in place. One resistor is needed for each channel. Here is a picture for reference:
The only question you need to answer is how do you choose the value of this resistor? There is a way to directly calculate the values needed but it's a pain to do the math so at this point just use the table below to lookup a value close to the recomended load for your cartridge and then note the value of the load resistor for the step-up ratio you are using. The values of the load resistor do depend on the step-up ratio and the theory behind this will be explained later. Here is the table:
| Net Resistance (Rtarg) | |||
| Rload | 1:5 Ratio | 1:10 Ratio | 1:20 Ratio |
| 2K Ohms | 76 Ohms | 19 Ohms | 4.8 Ohms |
| 2.7K Ohms | 102 Ohms | 25 Ohms | 6.4 Ohms |
| 3K Ohms | 112 Ohms | 28 Ohms | 7.0 Ohms |
| 3.3K Ohms | 123 Ohms | 31 Ohms | 7.7 Ohms |
| 3.9K Ohms | 144 Ohms | 36 Ohms | 9.0 Ohms |
| 4.7K Ohms | 170 Ohms | 42 Ohms | 10 Ohms |
| 5.6K Ohms | 200 Ohms | 50 Ohms | 12 Ohms |
| 6.8K Ohms | 237 Ohms | 59 Ohms | 15 Ohms |
| 8.2K Ohms | 279 Ohms | 70 Ohms | 17 Ohms |
| 10K Ohms | 330 Ohms | 82 Ohms | 20 Ohms |
| 12.7K Ohms | 400 Ohms | 100 Ohms | 25 Ohms |
| 15K Ohms | 455 Ohms | 113 Ohms | 28 Ohms |
| 20K Ohms | 561 Ohms | 140 Ohms | 35 Ohms |
| 27K Ohms | 686 Ohms | 171 Ohms | 42 Ohms |
| 30K Ohms | 733 Ohms | 183 Ohms | 45 Ohms |
| 33K Ohms | 775 Ohms | 193 Ohms | 48 Ohms |
| 47K Ohms | 940 Ohms | 235 Ohms | 58 Ohms |
| 56K Ohms | 1022 Ohms | 255 Ohms | 63 Ohms |
| 68K Ohms | 1112 Ohms | 277 Ohms | 69 Ohms |
| 75K Ohms | 1156 Ohms | 288 Ohms | 72 Ohms |
| 82K Ohms | 1195 Ohms | 298 Ohms | 74 Ohms |
| 100K Ohms | 1279 Ohms | 319 Ohms | 80 Ohms |
| None | 1880 Ohms | 470 Ohms | 117 Ohms |
We'll use an example of a cartridge with a recomended load (Rtarg) of 100 ohms and a stepup of 1:10. These are typical settings. From the table you can see we will need a 12.7K ohm load resistor (Rload) to get the net load to the cartridge (Rtarg) at 100 ohms. Now take a 12.7K ohm resistor and put it between the small posts of each channel's TX103 enclosure. If needed trim the leads to just longer than the distance between the posts so that it will fit easily in place. That's it - at least to get started.
Now listen to the system. I recomend that you spend some time at this first setting and listen to a variety of music to get a good handle on the sound.
Once you have spent some time at the recomended setting (and have the urge to tweak your system) choose a new value resistor and play with that. If you raise the value of this resistor it tends to open up the system and possibly bring the top end up a bit. If this value gets too high the system could get a bit zippy. Lowering the value of the resistor, as you likely guessed, can bring down the top end and possibly add a bit of body to the sound. Going down too low will result in a closed in and kinda lifeless sound. I suggest a quite significant change in the resistor values when first swapping load resistors - double the value or half it at least. As you home in on the ideal load you can make smaller changes.
How the sound changes will depend entirely on how sensitive your cartridge is to load changes. Some MC cartrdiges are happy into a wide range of loads and some really want to hit a 'loading sweet spot' to perform best. If you don't hear a change in the sound (or don't like the change) then just return to the value recomended by your cartridge manufacturer and enjoy your system!
Now Continue To The Theory Behind Calculating Load Resistor Values
Handy TX103 Kit Downloads