A quick work around moving coil cartridge loading - Work in Progress

[Frankyc2003]

Evening everyone,

This thread is a bit of a brain dump, so please bear with me. I wanted to share a quick set of (soft) rules that helped me determining the correct loading of my cartridge. So here we go, and please let me know if I made any mistakes, if I have forgotten anything, or if you feel something is just plain confusing...

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HOW TO DETERMINE THE OPTIMAL LOADING OF YOUR MC CARTRIDGE

Rule 1. A Step-up transformer of 1:10 ratio will have an internal impedance of 100 Ohm.
It's internal impedance is the square of it's step-up ratio.

1:5 ratio / 25 Ohm
1:10 ratio / 100 Ohm
1:20 ratio / 400 Ohm

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Rule 2. The reflected impedance back to the cartridge you want to load is the impedance from the RIAA phono input divided by the internal impedance of the step-up transformer.

So for a 1:10 step-up plugged into a 47K Ohm phono, the reflected impedance would be 470 Ohm.

1:5 / 1880 Ohm
1:10 / 470 Ohm
1:20 / 117.5 Ohm

This is obviously without any form of loading on the secondary winding.

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Rule 3. The average optimal loaded impedance for a cartridge is about 10 x it's internal impedance.

Lyra Clavis 2 Ohm / 20 Ohm
Denon Dl103R 14 Ohm / 140 Ohm
etc…

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Rule 4. To obtain the correct loaded impedance, you need to put a resistor in parallel on the secondary winding (output) or if you prefer in parallel with the input of the RIAA.

So the 2 resistors in parallel equation applies here.
(Product divided by sum)

R total = R1 x R2 / R1 + R2

So let's say I am using a 1:20 SUT with a Lyra Clavis. I will need to get the RIAA input impedance (or the secondary winding impedance) in the region of 8000 Ohm. You will get to this resistance value of around 8000 Ohm by adding a 10K resistor across the output of the SUT (secondary winding).

47000 x 10000 / 47000 + 10000 = 8245 Ohm

Then we see that by adding this 10K resistor we have achieved near enough optimal loading of our cartridge.

8245 / 400 = 20.61 Ohm (following rule 2.)

This gives you an easy way to determine the best resistor to use in your system.

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[NRG]

Maybe some small corrections...

1) 1:10 SUT has a voltage ratio of 10 to 1 and and impedance ratio of 100:1 A SUT does not have an internal impedance as such but the primary and secondary has some DCR

2) The 47K of a typical MM stage is the loading...

3) Just a rule of thumb and can be adjusted to suit needs. I prefer to load a little more at around 7x and some time more as this damps the SUT, see 5)

4) Dont forget to add in the DCR of the secondary and primary into the equation. It wont affect the result greatly but should be taken into account.

5) By secondary loading you are in effect loading the SUT rather than the cartridge, if you have the test kit its probably better to load the primary for the cart and then add a RC network to the secondary to damp the SUT.

[Moko]

Ok you guys obviously know this stuff, so maybe you can help me with what I hope is a simple question where does the resistor go in a set up like this?

http://i787.photobucket.com/albums/yy154/MokoMilk/Beyerdynamic%20SUT/IMG-20120815-01063.jpg (http://s787.photobucket.com/user/MokoMilk/media/Beyerdynamic%20SUT/IMG-20120815-01063.jpg.html)

Does it have to be soldered onto the pins of the transformer, or does it go across the RCA outputs somehow?

I am using a Ortofon SPU Classic GM2 E which I have been advised to add some 82K in parallel with the 47K of my phono stage to work with the 50 Ohm setting of my Beyerdynamic SUT, but as an electrical novice I have no idea of where to solder the resistor (I can solder though and follow basic instructions but the theory stuff does tend to go over my head).

Thanks for any advice.

[NRG]

Hi Miko,

The resistor would normally be wired across the secondary winding in parallel with the phono stage load. Pins 1 and 2 according to the diagram below....so across the Phono connector is fine.

82K in parallel with the typical MM loading of 47K presents a 30K load to the SUT secondary.

I found your wiring diagram:

http://mob787.photobucket.com/albums/yy154/MokoMilk/Beyerdynamic%20SUT/attachment.jpg?t=1345032783

It seems the SUT can be configured for 50 or 200 Ohm. If 50 Ohm, your SUT is configured for a 30:1 Turns ratio IE: From the diagram: Secondary Impedance of SUT (45000) / Primary impedance (50) = 900. Square Root of 900 = 30

30K / (30*30) = 33.33 Ohm loading to the cartridge

This is good for low impedance cartridges of around 3 ohms or so. The voltage step up ratio of 30:1 is high though so the low o/p MC of the Ortofon at .2mV will output a good 6mV into your phono.

If configured 200 Ohm the turns ratio works out to 15:1 and the loading to the cartridge would be 30K / (15*15) = 133 Ohms. .2mV would step up to 3mV which is probably below the minimum required.


Hope this helps

Edit: With an output impedance of just 2 Ohm you could load the cart a bit more and use a 33K resistor in parallel with the 47K of the phono stage to give a loading of 19.4K / (30*30) = 21.5 Ohm

This would damp the SUT more and may sound better, listening test would confirm. It will lower the output a touch though...

[Reffc]

As per Neil's advice above but just to add a simplified way of selecting loading:

As Neil mentions, as long as you have the test kit, it is in some ways better to load the primary then test the result for ringing at high frequencies. Many SUTs will require some electrical damping at HF to smooth the HF response and this is why many are loaded at the secondaries.

For secondary loading, in order to work out the reflected load to the primary and hence to the cartridge, you take the step-up ratio and square it. So a 1:10 means an impedance ration of 100. Add the 47K RIAA load into the equation and you can work out the reflected load that the cartridge sees as:

1/x= (1/47000+(1/y2)/100) where Y=load at cartridge and X= additional load in parallel with the output (ie between positive and negative on the output). You wont go far wrong by simply loading the secondary in this way because many SUTs will benefit from electrical damping.

[Barry]

Why not make life simple and just use one of the primary windings?

A single primary winding has an impedance of 100 Ohm when the secondary is loaded by 45 KOhm, thus the turns ratio is 21.2.

When the secondary winding is loaded by the nominal 47 KOhm of the preamp the primary input impedance will be 104.2 Ohm.

The Ortofon SPU Classic GM Mk. II has an output voltage of 0.2mV and a coil resistance of 6 Ohm. It is designed to operate into an impedance of >10 Ohm. A transformer turns ratio of 21.2 will transform the cartridge output to 4.24mV and most moving-coil cartridges like to operate into a load that is no less than 10x the source impedance (that is > 60 Ohm). The second unused primary winding is left open circuit.

In practice the input impedance seen by the cartridge will be > 104.2 Ohm as the resistance of both the primary and secondary windings have to be taken into account.