SUT Problem - Help needed

[martinswimmer]

Yes your right. The actual voltage into the phonostage is lower than I stated, as I failed to take into account the modified source impedance as seen by the phonostage.

So to recap, the load as seen by the cartridge is (100 + 12470)/900 + 42.5 = 56.47Ohm. A potential divider is formed with the 3.5Ohm coil impedance of the cartridge, so the input to the primary of the SUT is 0.2 x 56.47/(56.47 + 3.5) = 0.188mV.

The source impedance as seen by the phonostage is (3.5 + 42.5) x 900 + 12,470 = 53,870 Ohm, and this forms a potential divider with the 100Ohm input impedance of the phonostage. So the voltage at the input of the phonostage is 0.188mV x 30 x 100/(100 + 53,870) = 0.01mV.

I can find no information for any small signal transformer marked as "T/5346/P" (either as an EMI manufactured device or as one made by Partridge (who use a similar designation system)), so cannot verify the values of the winding resistances; but they do seem high. However Martin reports that the SUT worked with his Denon 103 (0.4mV output with 40Ohm coil resistance) and now works with the correct input impedance setting on his phonostage, so the SUT would appear to be adequate for the job, if less than ideal.

Thanks for this Barry - really helpful. Now another question. Is there any benefit in using a SUT and reducing the gain in the phono stage as opposed to using just the phono stage which has enough gain in max configuration settings? Is there a potential for a further impedence match/mismatch? More calculations to be done?

[Barry]

Thanks for this Barry - really helpful. Now another question. Is there any benefit in using a SUT and reducing the gain in the phono stage as opposed to using just the phono stage which has enough gain in max configuration settings? Is there a potential for a further impedence match/mismatch? More calculations to be done?

Now you're opening a can of worms there Martin. This topic has been discussed many times and caused much heat and division of opinion.

Without wishing to side-step the issue, I would say it come down to individual preference as much does in audio (analogue v. digital sources; valve v. solid state; passive v. active volume control etc.). Why not listen to your Entré both via the SUT and the phonostage set to 47K impedance and something like 3mV sensitivity, and then through the phonostage set to 100Ohm and 0.3mV sensitivity and report back with your findings.

At the end of the day it's your ears.

[martinswimmer]

Fair enough ;-)

[RothwellAudio]

Is there any benefit in using a SUT and reducing the gain in the phono stage as opposed to using just the phono stage which has enough gain in max configuration settings? Is there a potential for a further impedence match/mismatch?

Now you're opening a can of worms there Martin. This topic has been discussed many times and caused much heat and division of opinion.
Without wishing to side-step the issue, I would say it come down to individual preference as much does in audio (analogue v. digital sources; valve v. solid state; passive v. active volume control etc.). Why not listen to your Entré both via the SUT and the phonostage set to 47K impedance and something like 3mV sensitivity, and then through the phonostage set to 100Ohm and 0.3mV sensitivity and report back with your findings.
At the end of the day it's your ears.

Yes, in my experience the difference between a good step-up transformer and a good headamp or solid state mc phonostage is fairly subtle, though I'm sure there are some people who will claim the difference is night and day.
However, in this instance I'm not optimistic that the transformers used are well-suited enough for LOMC cartridges to give a truly hi-fi performance. The resistance of the primary and secondary windings are effectively parasitic elements and in an ideal transformer would be zero, ie the impedance would be all inductance and no resistance. In the real world that isn't possible but the resistance of the primary winding is usually only one or two ohms, not the 42.5 ohms of these transformers. Similarly the resistance of the secondary windings is usually a few hundred ohms whereas these transformers have 12470 ohms. As Barry's calculations show, those resistances merely waste voltage that should be going to the phonostage and in this case they're wasting a lot, even when terminated with the correct 47k rather than 100 ohms. Furthermore, the high resistances and high turns ratio suggest there are a lot of turns on the windings which will have a lot of capacitance, and that isn't good for a broad bandwidth free from ringing.
In short, although these transformers will work I doubt very much that they will be anything like approaching state-of-the-art. It would be interesting to compare the SUT+phonostage set to mm (with 47k input impedance, not 100 ohms) and the phonostage for mc operation. Let us know your findings if you do.

[martinswimmer]

What stops me from experimenting with these further is the ground noise - from memory I was able to reduce that to completely manageable with a careful placement and grounding. I don't have time at the moment to do it properly, so what I did instead is wired up a 1:10 SUT (Tamura TF21019A - Primary DC Resistance = 17 ohms, Secondary D.C. Resistance = 1,700 ohms) - but they are clearly not a good match and I don't have enough gain. I'm running them into a 4W single ended, so back to using MC option on the phono stage. I generally have good experience with SUTs and I'm using them in 2 of my setups, but this is the lowest output MC that I've got and it needs something else.

[RothwellAudio]

A turns ratio of 1:10 would be ok for that cartridge but something a bit higher, up to 1:20, would be better. Above 1:20 you're getting into other problems, so I'd avoid higher turns ratios if possible. Unfortunately, the DC resistances of the windings won't tell you what the turns ratio is.
The can't-go-wrong way to wire up the transformers is the have the L input earth and L output earths connected together, the R input earth and R output earth connected together, and the surrounding metalwork/casework/shield connected to a separate earth post (not connect to either the L or R earths) which goes to the earth point on the phonostage. Ok, maybe it's not quite can't-go-wrong because there will always be unusual setups and exceptions to the rule, but I think that gives you the best chance of trouble-free performance.

[martinswimmer]

Thanks Andrew, much appreciated!