I have a pair of Cinemags that are working into a tube Phono stage. The phono stage input impedance is set with a resistor of 47K ohm. There are no loading resistors in the primary or the secondary. Does that mean that the impedance that my cartridge sees would be 47000 X square of the step up ratio? So 100squared=10000. 47000/10000=4.7ohm at the primary? The step up ratio is just for example only.
Yes, 4.7 Ohms.
> The turns (step up) ratio should be 10x for 20dB so the input impedance would be
20dB is 1:10 of voltage, yes.
In a transformer, it is also 10:1 of current.
So 10*10 or 100 is the correct impedance transformation.
> The turns (step up) ratio should be 10x for 20dB so the input impedance would be
20dB is 1:10 of voltage, yes.
In a transformer, it is also 10:1 of current.
So 10*10 or 100 is the correct impedance transformation.
I'm sorry, it doesn't work that way. (From your example it would be 10*1/10=1).
Let's look at it another way:
5mV on the input (470 Ohms)=53.2nW (.005*.005/470)
20dB voltage step up =50mV on the output (10x).
50mV into 47K=53.2nW (.05*.05/47,000)
Correct answer: power out=power in.
5mV into 4.7 Ohms=5.32uW (.005*.005/4.7)
50mV into 47K=53.2nW (.05*.05/47,000)
Incorrect: power loss of 20dB
E=I*R. Obey it (it's the law...)
Wow, radikal different answers to a simple question.
The OP wants to know what res load his cart 'sees' when he has a 10:1 SUT between the cart and the tube MM phono stage - whose Zin is 47K.
The answer is ... 47,000 / (10x10) = 470 ohms, as Pyramid stated.
If the SUT was 20:1 then the cart would see 47,000 / (20x20) = 117 ohms.
This resistance relationship is very limiting - I suggest - in terms of providing the optimum load to the cartridge when using a SUT. 10:1 is a typical ratio for a SUT to use with a 0.3-0.5mV LOMC ... but some carts (like my Benz LP) sound best at a much higher load. I have been using 3K3 and, based on some recent listening tests, am going to play around with 4K7 & 6K8.
SUT users can't do this ... which is why I use a (ss) phono pre amp (aka 'head amp').
Andy
The OP wants to know what res load his cart 'sees' when he has a 10:1 SUT between the cart and the tube MM phono stage - whose Zin is 47K.
The answer is ... 47,000 / (10x10) = 470 ohms, as Pyramid stated.
If the SUT was 20:1 then the cart would see 47,000 / (20x20) = 117 ohms.
This resistance relationship is very limiting - I suggest - in terms of providing the optimum load to the cartridge when using a SUT. 10:1 is a typical ratio for a SUT to use with a 0.3-0.5mV LOMC ... but some carts (like my Benz LP) sound best at a much higher load. I have been using 3K3 and, based on some recent listening tests, am going to play around with 4K7 & 6K8.
SUT users can't do this ... which is why I use a (ss) phono pre amp (aka 'head amp').
Andy
Not quite true.
47k input impedance is some "standard", but by raising input impedance you can also raise the load impedance.
And don't forget the DC resistances of the SUT; I don't see them mentioned.
47k input impedance is some "standard", but by raising input impedance you can also raise the load impedance.
And don't forget the DC resistances of the SUT; I don't see them mentioned.
Well, absolutely! 🙂 If you up the 47K load resistor in the MM phono stage to 100K then the maths works out as follows:
* with a 10:1 SUT, the load seen by the LOMC is 100,000 / 100 = 1K ohms.
* with a 20:1 SUT, the load is 100,000 / 400 = 250 ohms.
So still no good for a Benz LP. 😀
Andy
PS: the DC resistance of the SUT is only a negative (compared to a 'head amp').
The advantage of SUT's is that they don't introduce noise; the best MC based systems prefer SUT's 🙂.
I can't argue that, since I use SUTs for MCs; I would add that the other great advantage, and maybe the principal one, is the common mode rejection (assuming one wires the system up sensibly).
The other warning here is that input impedances of preamps almost invariably have a significant capacitive component, which is also transformed by the square of the turns ratio. So the cartridge also sees this- is that significant? I think for a low DCR MC, likely not, but I haven't tried every product out there!😀 The main effect here is in how the transformer's secondary is loaded, and generally some sort of impedance compensation (typically a series RC) is needed across the secondary to minimize ringing.
if your turns ratio is 100:1 (+40dB), then indeed the impedance seen by the cartridge is 47000/(100)² = 4.7ohms
Then make the allowance that Sy mentions.
If the step up is 10:1 (+20dB), then your cart sees 470ohms.
But do look up the Cinemag datasheet and find out what snubbing Zobel is required for your transformer.
1:100??
Never seen SUT's with that ratio.
When you would need it there is something wrong elsewhere in your chain.
Never seen SUT's with that ratio.
When you would need it there is something wrong elsewhere in your chain.
That's your opinion, pieter - not a statement of fact. My own opinion is that the best LOMC systems:
* allow loading flexibility for the cart, and
* use a high-gain phono stage ... not a stetup device + an MM phono stage.
The SLA-driven, JFET-based headamp I use has a measured noise figure of -104dB. So no noise from that! 😀
Andy
> From your example it would be 10*1/10=1.
No, it would be 10/(1/10)=100.
Is the turns-ratio 10 or 100?? OP seems to say "100"?
Apply 1V 1A to the low side of a transformer with 1:100 turns ratio. The secondary must be 100V 0.01A. 100V/0.01A is 10,000 Ohms.
So a 1:100 turns ratio is 1:10,000 impedance ratio.
Freecrowder> input impedance is set with a resistor of 47K ohm. ...the impedance that my cartridge sees would be 47000 X square of the step up ratio? So 100squared=10000. 47000/10000=4.7ohm at the primary?
His fingers may have mixed times and divide, but 4.7 is the right answer for 47K through a hypothetical 1:100 transformer.
Agree that 1:100 seems an unlikely ratio. I wish practical questions were posed with practical data.
No, it would be 10/(1/10)=100.
Is the turns-ratio 10 or 100?? OP seems to say "100"?
Apply 1V 1A to the low side of a transformer with 1:100 turns ratio. The secondary must be 100V 0.01A. 100V/0.01A is 10,000 Ohms.
So a 1:100 turns ratio is 1:10,000 impedance ratio.
Freecrowder> input impedance is set with a resistor of 47K ohm. ...the impedance that my cartridge sees would be 47000 X square of the step up ratio? So 100squared=10000. 47000/10000=4.7ohm at the primary?
His fingers may have mixed times and divide, but 4.7 is the right answer for 47K through a hypothetical 1:100 transformer.
Agree that 1:100 seems an unlikely ratio. I wish practical questions were posed with practical data.
Off course Andy, my opinion; we're on a forum, aren't we? 😉
As for flexibility of loading: there are some "strange" carts wrt source impedance, output voltage and optimal loading (however there is not just one "optimal" value for that IME), but the very large majority of MC cartridges can be loaded perfectly in combination with step up transformers (that's a statement 🙄).
OK....so first you multiply, now you divide?
Where did you learn to do math? If the voltage gain is 10x (per both of your examples) then the impedance ratio is 100 not 10,000. You posted the input impedance was 4.7 Ohms which is 47K/10,000. It seems you now concur that the impedance ratio is 100; do you still believe the input impedance is 4.7 Ohms? It will be if the voltage ratio is 100. You can't really have it both ways. So which is it?
The OP indicated he has a Cinemax SUT. Cinemax does not make a 100:1 step up (neither does anyone else on the planet). They do make a 20dB step up, so I took that to mean the OP confused 20db voltage gain ratio (10x) was the same as a 20db power gain ratio (100x). While the SUT provides 20db of voltage gain (which affects the turns ratio and impedance transformation) it provides 0db power gain which does not affect either.
I agree the OP is a bit vague in his request; I also made certain assumptions, which I tried to clarify. In PRR's first (and second) rebuttal of my answer, he used 10x voltage step up as the same assumption, but still came up with a 10,000:1 impedance ratio, which is wrong.
Yes, it's a statement (which you made) ... and I'd probably almost agree that it's a statement of fact! 🙂 But I'd say 'many MC carts', rather than 'the very large majority'!
Case in point - someone local was using a Benz LP (same cart that I have) into an expensive, valve phono stage. As per normal, there was a 10:1 SUT on the LOMC input - which meant a loading for his Benz LP of 470 ohms.
He borrowed a head amp from me, to enable him to try higher loadings (I run my LP at 3K3) and was so rapt at the way his LP sounded with these higher loadings that he promptly bought a headamp from me, so he can just use the MM input of his phono stage ... and load his LP properly. 😀
Andy
The input impedance of the valve phono stage, I am pretty sure, could easily have been changed from 47k to 330k with some DIY effort. 47k is some standard but not a must.
Then, with the 1:10 SUT, you would have had your preferred 3k3 loading (and statement confirmed...).
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