The correct internal-inductance of Windfeld cart.?

Hi Lewm,
That a trannie sounds better (ONLY MC!) is simply due to it using its "strength" - current delivery, rather then it's "weak point" - voltage (micro Volts).
It enables the cart to 'dig up' more detail / dynamics etc. More current through the cart's coil, not to forget, gives it superior damping, a mostly ignored fact these days.
However, there are A-PLENTY phono-pres that do it any way, since they use trannies as part of their design, EAR and Manley, come to mind immediately, it's just not too obvious.
EAR not only with their tube pres, but also with their SS-pre like the EAR 324. So it's not SO out of the ordinary after all.

Now to the 500ohm loading of your MC7500. Try it by all means, BUT if I estimate it right, the "mechanical damping" of the MCxxxx carts is NOT at all the same as Ortofon's newer cousins PW, Jubilee, Kontra-Punkt, Vienna, Venice, etc.

That simply means you will have to load higher (using a SMALLER R value!) simply to damp out the typical > 10-12kHz resonance frequency, please bear that in mind! 500ohm might get already too "loosy-goosy", 1k I can't imagine.
Now recall that a PW still works jolly fine with 47k (no loading at all, other than the phono-pre input impedance). The top Dynavectors do the same too, as their damping scheme(s) take care of this typical > 10-12kHz over-shoot / resonance.

So, the SUT secondary "sees" the 47k input-impedance of the phono-pre! If you put -another- 47k R (as you implied) and it will be || with the 47k pre input-impedance and so you'd have the SUT see: 47k*47k / (47k+47k) = 23.5k! on secondary.
This 23.5k, reflected back to primary, as we saw (taken a 1:31.6 xfactor (30 dB) SUT => 23.5k/31.6^2 = 23.5 ohm) is now in || with the primary load! AND is now WAY too low, (about 5ohm) if you'd have a 13ohm R for primary load!! So, NO secondary loading, the 47k input impedance of the phono-pre is your load.

Now, it MAY not end there... since (depending on the trannie's resonance behaviour) you might have to consider an RC (on the secondary) to damp any trannie resonance. It will be THEN when you want to know your cart's L! My original enquiry, to figure out the values for this RC.

My XF-1 does not have an issue (as it seems) but if you get some of the new fangled 'high-speed' core material trannies it could well be in need to damp out somewhere around 20kHz. It's a tricky lot of calculations and completely dependant on the trannie / cart combination, there are ball-park figures but I guess it would require some trial and error. This RC would act as a Notch-filter in case that makes it more clear.

The other SUT loading method: secondary loading, might take care of such resonance, I mentioned it earlier since e.g. 18k secondary load (for a 1:31.6 xfactor!) will dive you ~14 ohm the cart will see, AND it damps the trannie also. That is why primary loading (not damping the trannie) gives you more detail / clarity -- if you can make it work for your trannie / cart / phono-pre combination.
Cheers,
Axel

Axel, Now it is YOU who have misunderstood ME. You wrote, "So, the SUT secondary "sees" the 47k input-impedance of the phono-pre! If you put -another- 47k R (as you implied)". No, I did not mean to imply nor did I think that you have two 47K resistors in parallel on the secondary, I did mean to ask whether the sole parallel resistance on the secondary side of your SUT is the 47K-ohm input Z of the phono section. That's what I wrote. Your whole paragraph on the undesirable result of paralleling two 47K resistors is beside the point. I just wanted you to summarize what resistances you have and where they are in your circuit to give you the sound you like from your PW.

And thank you for the advice on SUT installation, but there is no way I am going to be buying and installing a SUT, when my phono pre has oodles of gain and sounds great. I am just interested in your findings.

Hi Lewm,
+++ I did mean to ask whether the sole parallel resistance on the secondary side of your SUT is the 47K-ohm input Z of the phono section.
+++
YES SIR, right as you say!

+++
...when my phono pre has oodles of gain and sounds great.
+++
Well, as with all things --- "sound great" that's relative, no? :-)
And "oodles of gain" got nothing to do with it. My Levinson phono-boards (through my 326S) has 78dB gain if I open it all up (the ML has adjustable input gain +6, +12, +18dB, and the phono-board does 40dB or 60 dB. Believe me, SUTs are only to some degree, and in some more 'needy of gain' situations about gain.
The point I'm making is, that a trannie will make it sound 'greater' still --- but that is a personal thing, like buying another speaker, component, or such. Some day you may recall this little conversation about SUTs, is all :-)
Take care,
Axel

I really think it's quite interesting and unexpected (to me) that with pre-loading of a SUT, the optimal impedance for the PW seems to be about 10 ohms net, whereas, as you say, without the SUT the PW seems to like 500 to 1000 ohms net. I am not close-minded about using a SUT; this is the first report that suggests there could be an advantage notwithstanding any need for extra gain.

I think Axel is spot on with his comments and approach. I'm not quite sure I follow his distinction between "current" and "voltage mode" but maybe that is best for a different thread.

Lewm wrote:

I don't know of anyone else who uses a SUT and loads the cartridge on the primary side.

I covered this in a short thread back in november which can be found here

dave