The correct internal-inductance of Windfeld cart.?

I learned something. To paraphrase Gertrude Stein, a SUT is not a SUT is not a SUT.

Hi all,
there is something that seems to 'fox' Lewm with the different loading values, and Intactaudio got some of it --but not all.
A cart working in 'voltage mode' MUST work into some 'highish' level of impedance, in order to work at all.
Most MCs need somewhere around 100 ohm loading but hardly ever less (with no SUT!).
Actually more ideal would be the 47k of the pre's input-impedance --- but there is this resonance issue with (still) most MC carts, starting at ~ 10kHz and producing just too much HF energy up to 16 - 20 kHz before roll-off.
To handle this the cart must be damped as we know.
This can be done either electrically or 'electro-mechanically' i.e. you are either loading it down electrically (pulling more current through that load resistor and thereby the cart-coil, which creates the damping effect) or, like more of late, built into the cart is some proprietary 'electro-mechanical' means of damping. Ortofon by example uses a mixed bag of 'tricks' to achieve that, I won't go into it here. Just check their Website and see what they do e.g. with the Windfeld.
Dynavectors has their own bag of 'tricks' for their top carts, etc.
Now, more current (I) also means even less voltage (V)(Ohms law) for the signal => pre-amp.
Try to load the cart with 10 ohm (no SUT) and there is a big drop in output added to the treble-roll off.
Newer MC carts are now often run into 47k (the pre's impedance only) and can sound great, it is the damping system in the cart that makes this possible.

The better the cart the less loading is required! (Because it is more 'well behaved', less HF resonance).

Now take the SUT situation (this where I say Intactaudio got it 'almost' right), there is such a thing as a PERFECT impedance-match. When this occurs NO 'reflections' are produced by the signal (check out why there are BNC connectors and the like). The result is maximum power transfer from cart to primary with no reflections. In order to get there, the input impedance of the trannie first needs to be matched to the cart coil's output impedance.
Now, it also is known by ROT that this cart-output-impedance is DCR*2.5 i.e. WP RDC 4ohm*2.5 = 10ohm and quoted as such (by Ortofon) as the minimum loading impedance, yes?

So now it might make more sense WHY 13ohm loading R || 47ohm the 'natural impedance' of the SUT = 10.18ohm is a good fit.
Forget the .18ohm mismatch since there is, as always, more to it than just that, but it is a good approximation and the rest is trial and error.

Last part: Therefore "Current mode" means that the cart delivers the most current it can, into the SUT's primary and the best / most when properly impedance matched.
This 'maxed out' cart-current produces the best possible damping of the cart --- BUT most of all, the current of the primary is converted into VOLTAGE at the secondary according to its ratio. So you find the best of both worlds, given a decent SUT of good bandwidth i.e. VERY low losses.

Maybe this helps to get some more better idea. Google Impedance matching if you want the 'low-down' and the details of the concept.
Best,
Axel

Axel, No one is arguing with your logic, and I am not foxed. I will leave it to Dave (Intactaudio) to comment on the second half of your post. Dave knows more about transformers than both of us. I did learn something from your story, so thanks.

Hi Lewm,
excellent all’s as clear as can be, jolly good show.
I, incidentally, didn't imply you where arguing my logic, hm. Sorry if it sounded that way.

One fact remains that I was 'foxed' at the start with SUTs, and there are a-plenty reviewers that seem to be utterly oblivious of the possibility of loading SUTs, never mind the issues surrounding primary or secondary loading... However, they merrily do review the product (SUT) and tell you how it sounds.

If I would have followed that route, the XF-1 I'm now using would be completely and utterly unacceptable to be used with a 0.3mV cart i.e. unloaded using only the natural impedance of 47ohm (what the cart sees) sounded far to phasey and rolled of, completely unacceptable.

Yet, if you get 'un-foxed' there is more than expected, as so often, and that SUT sounds just great – if you know what to do.

I'm glad to share some of that little I learned, and in that spirit - thank you for sharing.
Axel

Axel, I am often vexed by the same phenomenon-reviewers who comment on the "sound" of a SUT without regard to the myriad of other parameters that affect the result. Particularly egregious is the "SUT shoot-out" type of review, where many different SUTs are compared using the same cartridge and the exact same loads, often without regard to the differences in turns ratio, let alone other less important factors. The result is worthless information, but it fills pages in a magazine.