...what I glean from the info in this thread is that 1) transformer performance can vary between different manufacturers, 2) the output of the transformer is not a straight multiplier of the turns ratio times the MC cartridge output but is less as the SUT turns ratio increases and 3) the cartridge load with a SUT is the MM preamp's load (typically 47k ohm) divided by the square of the turns ratio.
Yes, that is correct.
I'm trying to narrow down the variables to the few that are typically listed for SUTs. Final cartridge load and output seem to be, at least to me along my ongoing learning curve, the two parameters I can reasonably determine.
Those are two parameters which you can determine if you know the turns ratio. Unfortunately the turns ratio is often not specified. Even worse, a manufacturer may label a transformer as, for example, "3 ohms". You might reasonably think that that means the cartridge sees a load impedance of 3 ohms, but in reality that's probably not the case. All too often things are left very vague in the world of step-up transformers. Maybe the manufacturer is trying not to confuse the customer or maybe they're trying to maintain some mystic around their product - who knows?
So the Shelter cartridge/1:20 SUT is not delivering 10mv to the MM preamp but something less. Can the actual output to the MM be calculated or is there a guideline I can use? Say 1:10 is 90%, 1:20 is 65%, etc.? The Shelter is loaded at 118 ohms (assuming he is using a 47kohm MM preamp) and pretty close to the guideline of 10x the cartridge impedance of 140ohms.
Yes, you can calculate it if you know the cartridge's source impedance. First you work out the reflected load impedance (47k divided by the square of the turns ratio). The cartridge's source impedance and the reflected load impedance form a potential divider, and you work out the voltage dropped across the load impedance. Then multiply that by the turns ratio. (see post #17) No, there is no "guideline" for any particular step-up ratio because it will depend on the cartridge's source impedance. If the cartridge had a source impedance of zero the output voltage from the transformer would always be the cartridge output times the turns ratio.
Maybe it's best not to get too hung-up on the maths because for low source impedances (< 12 ohms?) and low turns ratios (1:10?) you can simply multiply the cartridge's output voltage by the turns ratio and you'll be pretty damned close - within 1dB. Of course, the output voltage varies according to how loud the music on the record is, and a cartridge's output voltage may not be what the manufacturer's specs say anyway, and bear in mind that the output is sometimes specified at 3.54cm/sec and sometimes specified at 5cm/sec.
The Denon/SUT is probably not yielding 6.0mv but something less but probably around 5mv? The Denon load would be 208 ohms, not quite the 330 (10x source impedance of 33ohms) but high enough to sound better than using a 1:20 SUT. Of course it could be that 208ohms in
@mkane's system is just right. Am I correct that a 1:10 SUT might be a better choice to yield a load above 330 ohms? A 1:12 transformer might be optimal and would have a 326 ohm load plus enough output to the MM preamp.
Yes, the Denon/SUT is yielding 5.2mV, not 6mV. Bear in mind that 5.2mV is only 1.2dB lower than 6mV, so the difference isn't very much at all.
Is the load impedance critical to getting a good sound? I think it's a lot less important than many people assume it is.
Lastly, in cases where the cartridge impedance is high but the output is low a head amp might be a better choice since it may not be possible to get a high enough output along with a high enough load using a SUT.
Yes, I agree. A headamp with a variable load impedance (aka input impedance) would also allow you to test the theory that the load impedance is critical because it's possible to alter the load impedance and maintain the same gain, unlike a transformer. The Headspace lets you do that:
http://www.rothwellaudioproducts.co.uk/html/headspace_mc_headamp.html
I said I agree that a headamp might be best when the cartridge's source impedance is high and its output voltage low but there is a caveat - the headamp has to be a low noise design. When the cartridge's output voltage is low you need a lot of gain and it's difficult to maintain a good signal-to-noise ratio, ie hiss might be audible. Hiss is unlikely to be a problem with a step-up transformer. On the other hand, hum is more likely to be a problem with a transformer.
Personally, I would avoid cartridges with high source impedances and low output voltages - why bother with the hassle when there are so many other cartridges to choose from?