# MC LOADING & STEP UP DEVICE

Hi;

After a great deal of research in this forum, vinyl engine's library of articles, and 6moons I have only utterly confused myself. I remain baffled as to how to properly match a step up device to a phono cart and MM stage. This is realy killing me as I am very eager to start experimenting with some moving coil designs. As a matter of fact I have 3 of them in my possession at present with no way to listen to them and I have been hesitant to purchase a step up without learning how to match it properly.

Is there a simple formula or rule of thumb I can follow?

What are the inputs that need to go into the equation?

What is primary and secondary impedence?

Please help me and I will forever be in your debt. Thanks guys, I really appreciate it.
dfelkai
15 responses
 07-21-2009 7:41amStep 1: Find out the winding ratio of the trannie. e.g. 20dB = 1:10; 26dB = 1:20; 30db = 1:31.6; etc.(there are more on the web, no need to calculate...)Step 2: Find out the 'natural impedance' of the trannie.Ratio^2 * phono-pre input impedancee.g. 31.6 * 31.6 * 47k ohm = 47 ohm for a 30dB trannie(this will be what the cart sees if you do not use secondary or primary loading of the trannie)Step 3: Check for phono-pre over loade.g. of cart V = 0.25mV; cart DCR = 6 ohm[cart output V * ratio]* [load impedance/(load imp + cart DCR)] = e.g. [0.25 * 31.6] * [47 / (47+6)] = 7.9mV * 0.887 = 7.0mVtry for max. input value not to exceed ~ 7.5mVWith a 26dB, you will have a higher 'natural impedance' i.e. 117.5 ohm (and not 47 ohm as with a 30 dB item) which in turn might require secondary or primary loading to lower the impedance that the cart will see.Secondary trannie loading is reflected 'back' to primary, by dividing the secondary R by the square of the winding ratio.But first you have to get the correct value e.g. if 18k ohm is loaded on the secondary then:47k*18k / 47k+18k = 13.01k ---- then reflected = 13.01k / 31.6*31.6 = 13ohm that the cart will now see.But secondary loading will ALSO have the effect of damping the trannie, as more current is pulled on the secondary side, this may be just what is needed -- but it may be also not a good thing --- only testing & listening will tell.If primary loading is indicated (if e.g. secondary gets too dull) then:e.g. 18ohm load R with 47ohm 'natural impedance' =47*18 / 47+18 = 13 ohm again what the cart sees.I hope this will be enough to give you a starting point for checking the various option.Cheers, 07-22-2009 2:46pmAxel, thanks very much for the primer. I now understand these SUT impedance calculations. Using your formulae, I was able to recalculate the 3 Ohm input impedance for the Auditorium A23 Hommage T1 MC transformer that is referenced in Art Dudley's recent column based on the winding ratio (1:30) and the secondary coil impedance (2.8k ohms) specified in the column.Very educational. 07-22-2009 4:19pmCincy Bob -- Art Dudley's writeup was perhaps worded a bit misleadingly, and I want to make sure it's clear that the load impedance seen by a cartridge working into that transformer is NOT 3 ohms. It is, to a close approximation, the input impedance of the phono stage or preamp which is connected to the secondary side of the transformer, divided by the square of the turns ratio. In other words, typically 47,000 ohms divided by 933 in this case, or 50 ohms.The voltage gain would correspond to the turns ratio, which is just over 30 times (actually 30.54, the square root of 933). The voltage gain expressed in db is 20 times the log of 30.54, or 29.7db.I'm not certain, but I suspect that the 3 ohm and 2800 ohm figures he mentions are dc resistances, which will result in a slight but insignificant attenuation of the signal.Axel -- I second the thank you which Bob offered for your excellent post.Regards,-- Al 07-22-2009 4:28pm In other words, typically 47,000 ohms divided by 933 in this case, or 50 ohms.I should have added, of course, that the 50 ohms would be less if load resistors are added.Regards,-- Al 07-22-2009 4:58pmHere is the excerpt from Art Dudley's write-up:Aschenbrenner's Hommage T1 measures somewhat differently, with a primary coil impedance of about 3 ohms and a secondary coil impedance of 2.8k ohmsI assumed that the secondary coil impedance was equivalent to a secondary trannie loading that is reflected back to primary as discussed in Axel's post above. The math seemed to confirm this:47k*2.8k / 47k+2.8k = 2,643 / 933 = 2.83 ohmsIs it coincidence that the mathematical result agrees with the "primary coil impedance of about 3 ohms" that is cited in the write-up? 07-22-2009 5:32pmArt Dudley was using "impedance ratio," in this case 933 to 1, as a round-about way of indicating the turns ratio (which is the square root of 933, since impedance is reflected between the two sides of the transformer in proportion to the square of the turns ratio).Hence the primary and secondary coil impedances he refers to (3 ohms and 2800 ohms) differ by a factor of 933. But I'm not sure how either of those numbers are defined -- as dc resistance, or as impedance at some frequency with the other side of the transformer connected to some load, or what?The reason for the 2.83 ohm vs. 3 ohm "coincidence" is that the 47K load impedance is an insignificant load in comparison to 2.8K, which in turn reflects back to the primary as 3 ohms, while 47K in parallel with 2.8K reflects back to the primary as 2.83 ohms. However, it is incorrect to consider the 2.8K as being in parallel with the 47K. Within the range of its intended operating conditions (frequency and voltage), a transformer has (to a very close approximation) no impedance of its own. In Axel's example, the 18K that he assumed was in parallel with the 47K preamp impedance represented an external 18K resistor, not the transformer's secondary coil impedance.Regards,-- Al 07-22-2009 6:10pmAl, thank you for clarifying this:>> In Axel's example, the 18K that he assumed was in parallel with the 47K preamp impedance represented an external 18K resistor, not the transformer's secondary coil impedance.<<0.26mVLoad Impedance: 30 to 50K ohms So this is the data, now the calculations: For the first cart: winding ratio 1:10 of step up,then Step #1 says-> since 1:10 = 20dB then step #2 says-> 20*20*47kohm = ? then step #3 says-> Well I need DCR, Don;t know what it is now I am lost from here on. Please help me fill in the blanks. Thanks again for helping me understand. It is greatly appreciated. 07-23-2009 2:40am31.6 * 31.6 * 47k ohm = 47 ohm for a 30dB trannie Please explain also, how you get this equality ? =998.56*47kohms=46932.32 kohms how is this = 47 ohm ? 07-23-2009 5:17amSorry for the silly typo!!!!!It must read of course: 47k ohm / 31.6 * 31.6 = 47 ohm, for a 30dB trannie 07-23-2009 1:27pmDfelkai,"DCR" = "dc resistance," which is usually just a few ohms for low output moving coils, and can be neglected without introducing significant error unless the cartridge is to be loaded to very low values (say below 30 or 40 ohms, or even less if the dcr is particularly low).Your xfmr has a ten to one turns ratio, so the 0.2mv and 0.26mv cartridge outputs will be stepped up to 2.0mv and 2.6mv respectively, neglecting the dc resistance effect.That is a little bit less than ideal (ideally I would aim for around 5mv, which a 26db xfmr would provide), but it should be ok if the gain and/or noise performance of the phono stage is not marginal to begin with. You would have to turn the volume control up by 6db, relative to the setting you would use with a 26db xfmr, which is not a huge increase, and typically would not raise noise (background hiss) levels objectionably.The square of the turns ratio is 100, so the 47K phono stage or preamp input impedance will be seen by the cartridge as 47K/100 = 470 ohms.The Carnegie One's very wide load impedance specification (30 ohms to 50,000 ohms) is probably intended to indicate that it would be within reason to drive it straight into a 47K phono stage (that provides adequate gain and adequately low noise) with no external resistor, as well as to run it into a high gain step-up xfmr. The 470 ohm reflected load obviously falls within that range, but you may want to try adding resistors of various values to fine-tune the sound to your liking. Keep in mind that adding a resistor in parallel can only lower the 470 ohms; it cannot increase that value. (And adding a resistor in series, which would increase that value, should not be done because it would attenuate the signal seen by the preamp). According to the data at cartridgedb.com, the SD-900 Super is specified for a load impedance of 2 to 20 ohms, so adding a 20 ohm load resistor on the primary side, or a 2000 ohm load resistor on the secondary side, would appear to be a reasonable way to go. The 47K load impedance of the phono stage or preamp (or 470 ohms as reflected at the primary side of the xfmr) would cause only a negligible lowering of the 2K (or the 20 ohms), so there is no point in bothering to calculate its effects. (OK, I did it anyway: The 20 ohms would be lowered to 19.18 ohms, and the 2000 ohms would be lowered to 1918 ohms).The Supex's "impedance" of 3.5 ohms probably represents dc resistance, and is small in relation to the 20 ohms so it will not affect the signal level and overload calculations to a significant extent.Hope that helps,-- Al 07-23-2009 2:37pmHi Al,I think you do make the better teacher then I feel myself, as you have it so well explained. Let's see if this gets Dfelkai sufficient clarity to proceed.Thanks again, and let's see what comes up next. 07-31-2009 8:40pmI still can't complete the calculation. This is problematic as I am looking to get myself a decent matched step up in the very near future. Seems like there are some errors in the above calculations. 07-31-2009 8:50pmWere you able to follow all of my previous post? If so, why can't you complete the calculation (which I think my post essentially does), and what are the "errors" you are referring to?Regards,-- Al