MC LOADING & STEP UP DEVICE

Cincy 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

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

Art 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

The secondary coil DCR is ALSO reflected back into primary, again by dividing it by the square of the winding ratio. B U T, it would be negligible in the case of my example and would be about 0.06ohm added to the calculated 13ohm of the example.

Yes, with the slight further clarification that the dc resistance of the secondary coil is reflected back into the primary by dividing it by the square of the winding ratio, in terms of its effects on the range of signal frequencies and voltages over which the transformer acts like a transformer. Which is why the 1.6 ohm and 60 ohm dc resistance numbers (dc being zero frequency) do not differ by a factor of 900 or so, in case anyone wonders.

Best regards,
-- Al

Dfelkai,

"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

Were 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