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MC Step Up Math

Hi all,

after posting a thread on here years ago and becoming exceedingly confused about cartridge step up maths, I gave up, embarrassing for a math major..perhaps I should have studied electrical engineering. Recently I have been reading up on this topic and would like to once and for all figure out how to run the math/electronic theory to find the correct step up to mate with a MC cartridge.

I have looked at 2 different links.

Link (1)

http://www.theanalogdept.com/sut.htm

and

Link (2)
http://www.rothwellaudioproducts.co.uk/html/mc_step-up_transformers_explai.html

Now, everything I read in link 2 falls apart after reading what is on link 1 and I am once again confused about what to look for in a MC step up.

In the second link the author explains that you simply apply a 2 step process: A. multiply the turns ratio by the cartridge output to find the voltage and make sure that it is not overloading the MM phono stage input (i.e/ between 2.5 and 10 MV) and then B. Perform the calculation to show you how much resistance the cartridge actually sees and apply a rule of thumb at least 3 to 10 times ratio between the source impedance and the input. The rule is for the most part out of thin air, though he does explain that matching to equate the 2 is a bad idea.

In the first link however, the author takes a different approach. He explains that a turns ratio cannot just be multiplied to give you the voltage on the other end. For example the cinemag 3440 cart used with the dynavector illustrates the point. The output is .30 MV and the turns ratio is 35.4 resulting in 10.6 MV out.

Now here is the bit I need help with. He explains that in reality the with this combination the output is really 5.1387mV NOT 10.6MV. He uses this equation to adjust the 10.6 MV to 5.1387MV:

(Vout / Vcart) = (R(Load_effective) / (R(Load_effective) + (Rcart)))

he finds Vout and then Multiplies by the turns ratio.

The parameters are as follows:

Rcart: is internal resistance of the MC cartridge
R(Load_effective): resistive load seen at the MC cartridge
Vout: Voltage output at secondary side of tranny
Vcart: Voltage output at MC cartridge

Hi all,

after posting a thread on here years ago and becoming exceedingly confused about cartridge step up maths, I gave up, embarrassing for a math major..perhaps I should have studied electrical engineering. Recently I have been reading up on this topic and would like to once and for all figure out how to run the math/electronic theory to find the correct step up to mate with a MC cartridge.

I have looked at 2 different links.

Link (1)

http://www.theanalogdept.com/sut.htm

and

Link (2)
http://www.rothwellaudioproducts.co.uk/html/mc_step-up_transformers_explai.html

Now, everything I read in link 2 falls apart after reading what is on link 1 and I am once again confused about what to look for in a MC step up.

In the second link the author explains that you simply apply a 2 step process: A. multiply the turns ratio by the cartridge output to find the voltage and make sure that it is not overloading the MM phono stage input (i.e/ between 2.5 and 10 MV) and then B. Perform the calculation to show you how much resistance the cartridge actually sees and apply a rule of thumb at least 3 to 10 times ratio between the source impedance and the input. The rule is for the most part out of thin air, though he does explain that matching to equate the 2 is a bad idea.

In the first link however, the author takes a different approach. He explains that a turns ratio cannot just be multiplied to give you the voltage on the other end. For example the cinemag 3440 cart used with the dynavector illustrates the point. The output is .30 MV and the turns ratio is 35.4 resulting in 10.6 MV out.

Now here is the bit I need help with. He explains that in reality the with this combination the output is really 5.1387mV NOT 10.6MV. He uses this equation to adjust the 10.6 MV to 5.1387MV:

Equation (*)
(Vout / Vcart) = (R(Load_effective) / (R(Load_effective) + (Rcart)))

he finds Vout and then Multiplies by the turns ratio.

The parameters are as follows:
Turns ratio: The turns ratio of the step up device
Rcart: is internal resistance of the MC cartridge
R(Load_effective): resistive load seen at the MC cartridge defined as 47,000/(Turns Ratio)^2
Vout: Voltage output at secondary side of tranny
Vcart: Voltage output at MC cartridge

for this example they using a denon 103 + cinemag 3440 are:
Turns Ratio: 35.4
Rcart: 40
R(Load_effective): 47,000/(35.4^2) = 37.5 ohms
Vout: to be solved for
Vcart: .30 MV

Putting it into equation (*) and solving yields
.1452mV for Vout.

He then takes Vout and multiplies by the turns ratio.

.1452 * 35.4 = 5.1387mV

NOW: If you take the simple method (from link 2 by multiplying turns with output) you get 10.6 MV, using this adjusted method with equation (*) you get 5.1387 MV. So my question is this. What is equation (*), is there some theory here that I am missing, is this voodoo? I would like a reliable way to select components that match, though I have trouble trusting the equation (*) method without knowing where why he is using it and what it is. I certainly want to get this ironed out before I start buying different transformers to play with, and any help with this would be greatly appreciated. Thanks.
dfel
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almarg
9,670 posts
 
With the dynamic nature of the impedance characteristic, would it not make sense to pick a "flattest" segment of the curve to operate on. The X Axis turns ratio variable can be converted to effective load, then picking a impedance load such that its relative operational neighborhood is sufficiently "flat" would make sense.
I see no reason that choosing a point on the flattest part of the curve would have any relevance, since the turns ratio, once chosen, remains constant. Choosing a point on the flattest part of the curve would improve the accuracy with which the input voltage to the phono stage can be predicted, but I don't see that improvement as being necessary or beneficial.
Turn Ratios between 10-80 give 2.5-5 MV which seems perfect. BUT the i/o impedance ratios in the same range go from 0.20 to 22.0 ! A range that has extremes FAR AWAY from 10X, and the equation says they are all fine.
The equation says that they are all fine in terms of the voltage level that is sent into the phono stage. But it says nothing about how sonics may be affected as a function of the turns ratio, and hence loading, within that range. As indicated in my previous post, the turns ratio will affect sonics in ways other than by its effect on voltage.

Regards,
-- Al
dfel OP
62 posts
 
Al, what is the reason for this 10X rule ? I mean at the core of it, do you just take it on blind faith. The ideal match really hinges on this figure, is there any science behind it or is it just an intuitive thing ?

I am not trying to be rude, please do not misinterpret, I just don't get how this is used so commonly with no science or reason blindly...and MANY do this, just about every source I have read from so far.

If Equation (*) is correct, and needs to be ignored and tossed out in favor of something else like this 10X rule, should there not be an explanation for the 10X rule using some scientific/physics/math/electronic property and theory?
dfel OP
62 posts
 
As for the turns ratio flatness...

Is the issue you are talking about the "corner frequency"?
You explained that there may be issues from the change in impedance all three variables are related. I can put the voltage in terms of Impedance, effective impedance, or cart impedance. Either way it will trace out a transformation of that same curve. You are chosing a point yes, however as the cart impmedance changes you now have a neighbourhood. suppose it is +-10%, from 40 ohms..you are operating on 36-44, and it has now become a variable. looking at a the tranformed curve would it not make sense to seek the nbhd that has the flattest voltage change given the change in impedance? Because I was thinking that this can be done by changing the location you build your neighborhood around...this can be done by changing the turns ratio since the nbhood is fixed for the cartridge.

Like I have said many times, I only know enough to be dangerous, so please correct me if I am wrong.

Is the amount of inductance relevant in this setting ?

In terms of phase and frequency, well those variables are not part of equation (*), and I suppose it is the bit I am missing so that the magic number 10X makes sense in my mind.
dfel OP
62 posts
 
Al, if you don't mind my asking..where the heck did you learn all this stuff? Do you have a background in electrical engineering? I have struggled to try and really understand the science behind this hobby and the learning curve can be steep at times...depending on what you are reading.
intactaudio
300 posts
 
In addition to what Al has said above. Once you understand that there are two different things at play here that should have no fixed relationship to each other, it becomes a little easier to understand.

One is the load the cartridge wants to see. This is not a fixed value but something that needs to be selected by ear. There are all kinds of ROT's but ultimately your ears are the only thing to tell you what this value should be.

The other thing at play here is the voltage gain. This is also not a fixed number. Again ROT's abound on this but in order to make a proper choice you need to not only take into consideration the input headroom of your phono but the overall gain structure of your system and your listening habits.

My approach is to get the gain set up properly first then address the load. In the case where Rsource=Rload you will lose 6dB but that is rare.

The mistake I see most making is attempting to link the numbers together to find some sort of convoluted solution. Sure one has the effect on the other but that is only based on other assumptions like the 47K input resistance of a phono. Lets face it, that 47K has no real meaning for a MC cartridge. For MC it should be much higher in value anyway to avoid the situation where the needed turns ratio loads down the cartridge too much.

dave
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