Need to be educated re: MC Step-up transformers

You underestimate yourself! Sounds to me like you understand it perfectly. Basically, you are substituting the 20db of gain provided by the step-up transformer (20db = a factor of 10 gain in voltage) for the 20db which had been provided by the jfet section of the phono stage.

So the overall gain will remain the same. And by removing the resistors you are keeping the loading on the cartridge in the same ballpark, and within specification.

The benefit of the change will be elimination of the noise problems you mentioned, IF those noise problems were due to the jfet section of the phono stage. That is probably a good bet, because since noise generated by the front end of a phono stage is amplified by all of the other electronics that follows in the system, that is typically the most significant contributor to overall system noise.

The only possible concerns that occur to me, which seem unlikely, would be the possibility that the 7mv might overdrive the mm section of the phono stage, or that overall system gain (including your power amp) might result in having to operate the volume control down towards the bottom of its range. But if you haven't had either of those symptoms running the cartridge through the 20db gain provided by the jfet front end, I don't think you will have them with the step-up xfmr that provides similar gain.

Regards,
-- Al

Brent -- Regarding using a 47K load, iirc correctly even the estimable Harry Pearson (founder of "The Absolute Sound") wrote many years ago of preferring the sound of some particular mc cartridge when loaded (or perhaps I should say "unloaded") with 47K! But as Axel indicates, that would be optimal only for a small minority of mc's.

Basically, what the typical lower value load does is to dampen what would otherwise be a large high frequency (usually ultrasonic) resonant peak. You can find an explanation of that here:

http://www.hagtech.com/loading.html

The degree to which that peak would produce adverse audible consequences, such as brittleness or edginess, would depend on its amplitude, its frequency, the bandwidth of the rest of the system, and the intermodulation and other effects that may be produced in the rest of the system by ultrasonic or very high frequency spectral components, as well as on the mechanical damping provided in the cartridge itself, as Axel points out.

Axel & Saudio -- Thank you for your comments. I don't question your experienced-based comments, and I think we all agree that mc cartridge matching necessitates that listening be the final arbiter even more so than with the rest of an audio system.

I haven't seen any good explanations of why use of a step-up transformer would typically call for heavier loading than would be the case if a high gain phono stage or head amp were used. After giving it some thought, I've developed the following theory:

A real-world transformer can be modeled as an ideal transformer in parallel with an inductor on the primary side, representing "magnetizing inductance," as well as other resistive and inductive elements representing loss mechanisms. See the diagram around the middle of this page:

http://en.wikipedia.org/wiki/Transformer

Referring to that diagram, if we assume that the series leakage inductance Xp is relatively insignificant compared to the cartridge's inductance, then by Thevenin's Theorem the magnetizing inductance Xm can be considered as combining in parallel with the cartridge inductance. That will result in a lower total inductance, and therefore an ultrasonic resonant peak which is higher in amplitude if a heavier load is not applied to damp it.

Although the peak will be higher in frequency as well, and it will also be affected by the bandwidth limitations of the transformer, all of which sufficiently muddles the situation such that, again, listening would seem to be the only way reach any meaningful conclusions.

Regards,
-- Al

Outstanding, Brent! Thanks for the well written update.

Regards,
-- Al

Saudio -- I would respectfully disagree. In general there is no reason to expect that providing a load to the cartridge which numerically equals its output impedance will provide best results, or even a good starting point. Quoting from the excellent reference provided by Gundam91:

The first rule of thumb is that most (some exceptions exist) MC cartridges like to see a load impedance of 3X to 6X their output impedance value.

And it seems to me, at least as a starting point, that the manufacturer's recommendation for the specific cartridge should supersede any general guidelines.

Also, I'm not sure why placing a load resistor on the primary side should necessarily be given preference to placing a resistor on the secondary side, with the value chosen to reflect an equal value to the primary side when divided by the square of the turns ratio.

Regards,
-- Al