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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
dfel OP
62 posts
 
" What is effective Load. He is taking 47,000/(turnsratio^2). What does that tell you, what is that calculation?

A transformer transforms impedance in proportion to the square of the turns ratio. So the load impedance seen by the cartridge corresponds to the input impedance of the phono stage (usually 47K) divided by the square of the turns ratio.

What is equation (*)? Where does this come from?

See this Wikipedia writeup on the voltage divider effect. In the first figure, consider Z1 to be the cartridge's specified internal impedance, and Z2 to be the load impedance seen by the cartridge. Consider Vin to be the voltage the cartridge would output under conditions of negligible load (e.g. 47K). "

Wow! Thanks that is exactly what I was looking for. I will take detailed look at it so I understand where the way of calculating is coming from.

Assuming it holds, which at a glance of these electricity theories it does, the problem is still not solved.

HOW DOES ONE SELECT AN OPTIMAL STEP UP FOR THEIR CARTRIDGE.

That is really the point of this thread, and I hope it will be ironed out so it can be useful to others confused on this issue.

From what was mentioned so far:

1. It seems like equation (*) on the link can be used, and should be used, over the typical multiply the turns ratio to find voltage thing most guys are doing...especially when there is distance between the cartridge imped and the efective imped.

2.Matching cart to transformer impedance is just plain wrong is most cases. it steps down 50% basically and can crush the cartridge's character.

So what else is important

1. Every phono section is different, so using equation (*) to find the adjusted voltage will give you a MV output for any given turns ratio, but what is good for your phono stage 2.5MV,10MV or anything between. Running the calculations for a sample cart, I found that if I get between 2.5 MV and 5 MV using equation (*) it shows me that just about anything between 20-40 as a turns ratio is can be used. That is a HUGE range of step ups. There has got to be a better method than trial and error to get the right step up. I mean not everyone has thousands of dollars to buy 10 of them in those ratios to try them. The other thing is that simply spending more on a step may not put you closer to the ratio you actually need...I side with matching correctly over just spending 5k on a step up and hoping it works.

2. The effect of choosing an effective load <= cart is a no no, but what happens when it is chosen too high ? The "experts" recommend 8 to 10 times as a rule of thumb, what happens if you use 3 or 50? I do not know if there is a predictable result for chosing them too wide, or if it has any effect at all. When do the transformers being to ring?

3. The MV output from your cart is not stable, nor is it always at the specified rating. .30 MV...yeah at 1000HZ, and even that number may come out exactly as the manufacturer measured it.

so in the end...using this information really only puts you in the ballpark for what you need. Hopefully someone out there knows of a way to narrow this ballpark further.

Thanks again AL for being patient and contributing to this thread!
dfel OP
62 posts
 
Yes AL, I am know very little about electrical theory and the Higher (numerically lower) and Lower (numerically higher) is what confused me, you were consistent not mistake on your part it is the semantic I was confused with.
dfel OP
62 posts
 
I mapped out a curve of the various step up turns ratios for a denon 103, for those that are into seeing the visual of what we are discussing here. I dont have one of those carts, but tons of people do, so I figure why not use it as an example. have a look, enjoy. I posted the image to the following link.

http://s28.postimg.org/9kno6o95p/Denon_Chart.png
lewm
6,770 posts
 
What low output moving coil cartridge do you own or want to own to use in your system? Perhaps the easiest approach would be to make that information known and then just get advice on what step-up ratio will work best for that cartridge. It is helpful/necessary also to know the internal resistance (= output impedance) of the cartridge itself.

What Al is telling you is that the ratio of the signal voltage seen at the input to the phono stage to the signal voltage emitted by the cartridge will be equal to the turns ratio of the SUT. However, the ratio of the output impedance of the cartridge (= its internal resistance) to that of the input impedance "seen" by the cartridge will be equal to the SQUARE of the turns ratio. (I realize you've got most of this already; sorry to sound pedantic.) So, say your MC has an output of 0.5mv and an internal resistance of 10 ohms. Say your SUT has a 1:10 turns ratio and that your MM phono stage presents a 47K ohm impedance. The phono stage will receive ~5mV of signal voltage (10 times 0.5), if the impedance matching permits, and the phono cartridge will see an input impedance of 470 ohms (47K divided by 100 or 10-squared). That is a typical OK match-up. You want the input impedance to be a multiple of the cartridge impedance. Classically, a 10:1 of impedances is ideal, but it is not a problem to go down to 5:1 and even lower. However, in this case we have 10 ohms looking at 470 ohms. We are way into the safety zone with a ratio of 47:1. What Al and one of your references was saying is that if the input impedance gets down close to the same value as the output impedance of the cartridge, then the total 5mV of signal is no longer deliverable to the phono input stage; some of it goes to ground and is wasted.

To add to the mix, some might decide that the cartridge sounds a little tizzy with this 47:1 ratio of input to output impedance, so to dampen the high end response of the cartridge they find ways to reduce the impedance ratio, such as to introduce a resistor that is seen by the SUT to be in parallel with the 47K ohms afforded by the naked phono stage input. In the end, you might decide that this cartridge sounds best when it sees a 100 ohm load. In that case, you could add a resistor in the range of 11K or 12K ohms, in parallel with the 47K to produce a net resistance of 10K ohms on the secondary side of the SUT. Now the cartridge will "see" 100 ohms across the SUT/ (10K divided by the square of the turns ratio).
lewm
6,770 posts
 
Sorry. Once or twice where I referred to the ratio of "input to output impedance" above, I should more clearly have written "output to input". That is the internal resistance of the cartridge itself (~output impedance) vs the input impedance of the phono stage, as seen by the cartridge across the SUT.
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