Moving Coil Stepup Transformer

Cartridge Loading Theory

This next section explains some of the mechanics and math behind cartridge loading. First a quick overview of resistors and then an example and some formula for calculating load resistor values.

#1 - Resistors:

Since this section is all about load resistors we'll first talk a bit about resistors. Resistors are electronic components that 'resist' current flow. There is a commonly used analogy to water and pipes that will be a useful aid to picturing resistance. If electricity (or current flow) is like the water then the wires are like the pipes. A length of plain wire has very little resistance to the flow of electricity and so is like a big diameter pipe that would have very little resistance to the flow of water. A resistor is like a smaller diameter section of pipe. It has some resistance to the 'flow' which depends on it's value. This value is measured in 'ohms'. A low value resistor (like only a few ohms) is not too different to a length of wire (big pipe) A high value resistor is like a very small pipe - which blocks the 'flow' to a large extent. Here is a resistor represented in both a electrical and a 'plumbing' point of view:

#2 - Resistors in Parallel:

Once we introduce the load resistor on the TX103 secondary we then have 2 resistors connected in 'parallel' - both connected accross the input of the phono stage. These 2 resistors function just like one resistor with a value of less than either of the 2 resistors.

If the 2 resistors values are 'R1' and 'R2' we can calculate the 'net' resistance value 'Rnet' value as follows:

1/Rnet = 1/R1 + 1/R2 or Rnet = 1/ (1/R1 + 1/R2)

A special case that's easy to visualize is when we have equal values of R1 and R2 - Rnet works out to be ½ the value of the 2 resistor's resistance. If we have 2 pipes the same size connected in parallel then the net resistance to flow would be ½ the resistamce of one pipe. If R1 and R2 were each 100 ohms:

Rnet = 1/ (1/100 +1/100) = 50 ohms

Two resistors in Parallel always have aa lower net resistance than with of the resistors alone. To return the the plumbing analogy - Two pipes in parallel have a lower resistance to flow (overall) than a single pipe would. Here are parallel resistors represented in both a electrical and a 'plumbing' point of view:

#3 - Your phono stage input:

Your MM phono stage input very likely has a 47K input impedance. This is usually a 47K ohm resistor accross the input. This drawing shows this resistor.

Note that the 47K ohm resistor is connected across the input and not in 'series' with it.

#4 - Transformers - They 'Transform Things...

Here is a drawing with the stepup transformer introduced to the phono system:

When we introduce a step-up transformer (any step-up transformer) between the cartridge and the load resistors it changes the load that the cartridge see's in a quite dramatic way. The stepup transformer acts like an 'impedance divider' and the amount that the transformer divdes the impedance by depends on the square of the turns ratio between the primary and the secondary. Here are the 'divisors' for the 3 stepup ratio's that the model TX103 can be wired for:

1:5 = 5 * 5 = 'divided by 25'

1:10 = 10 * 10 = 'divided by 100'

1:20 = 20 * 20 = 'divided by 400'

Since the Moving Coil cartridges tend to like lower loads than the 47K ohm standard Moving Magnet phono stage's input impedance this effect tends to be a benifit. So if we only have the MM input stage's 47K input resistance as a cartridge load then the load that the cartridge would see in each case would be:

1:5 = 47000 / 25 = 1880 ohms

1:10 = 47000 / 100 = 470 ohms

1:20 = 47000 / 400 = 118 ohms

I like to call these values the 'natural resistance' since they are the values the cartridge will naturally see if we do not add a load resistor to the system. Now recall that when we connect a load resistor at the TX102 stepup that it is in 'parallel' with the MM input's 47K load. The value of the 2 resistors in parallel is always less than either resistor's value and so when we add a load resistor at the TX103 the net resistance the cartridge sees can only be lower than the 'natural resistances' listed above.

The affect of the stepup ratio changing the impedance can be a factor in choosing a step-up ratio. You should check that the recomended load from your cartridge manufacturer is less than the 'natural load' in the table above for the ratio you would like to use. It very unlikey that your cartridge would want a higher load than the 470 ohms that the 1:10 ratio naturally provides. There may be cases when the recomended load would be higher than the 118 ohms that the 1:20 ratio naturally provides. This also tends to ballance out pretty much as it seems that the lowest output MC cartridges (that are likely to need the 1:20 ratio) tend to have recomended loads that are close to or lower than the 118 ohms this stepup ratio naturally provides.

Lets take an example of a 1:10 stepup ratio and a target load of 200 ohms. At the 1:10 ratio we have a natural load of 470 ohms. This is higher than the 200 ohm target resistance so we'll add a load resistor at the TX103 load posts that gives us the net 200 ohm load we want. We can use the table in the above section to choose this value or it can be calculated as follows:

Simply install a resistor close in value to the calculated 34.8K in the load posts and you will have a 200 ohm load to your cartridge. The value does not have to be exact so don't worry if you are off by 5 or 10%. The ideal load for you and your system will be found by swapping resistors and listenning so there is no need to get hung up on exact values.

Here are the general formula's to use to calc a target cartridge load value:

Rnet = Rtarg * (ratio * ratio)

Where

'Rnet' is the load resistor 'temp' value for the formula below

'Rtarg' is the target load for your cartridge in ohms

'Ratio' is the stepup ratio (5, 10 or 20)

Rload = 1/ (1/Rnet - 1/47000)

Where:

'Rload' is the value in ohms of the load resistor to install

'Rnet' is the temp value calc'd above

I hope the above description was understandable and helpful! Please send along any suggestion on how it could be improved or any questions you might have and I'll try and incorporate those ideas into the write-up.

Now Continue To The Theory Behind Calculating Load Resistor Values

Handy TX103 Kit Downloads

• TX103 Wiring Schematic

• TX103 Plate wiring DWG - 1:5 stepup

• TX103 Plate wiring DWG - 1:10 stepup

• TX103 Plate wiring DWG - 1:20 stepup

• TX103 Kit Manual - Microsoft Word

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