VAS MC One Transformer

Thanks, all. I’m experimenting with the two inputs, and whilst both sound remarkably nice, the low/47Ω input sounds as if it has a richer, fuller bass than the high/100Ω input (maybe I’m being fooled by an increase in volume: 17.3dB vs. 10.9dB). In neither case am I aware of any excessive top end, but then again, I probably can’t hear it if present!

I bought this transformer many years ago as the pre-amp I used at that time (Copland CTA-301) only had an MM phono section. I don’t think I want to dive into fine-tuning via a choice of SUTs. This arose again now as the MC input of my phono stage has some hum, and this cures it. I think it sounds better, but we all know how easily we can think something new or different can sound better, and the proof of the pudding will be in whether the benefits last.

I read that phono stage impedance/turns ratio squared = Impedance apparent on primary coil.

Phono stage impedance =47kΩ

turns ratio = x (not given on VAS website)

primary coil impedance 47Ω

Which should make x = 21.7, probably 1:20. Similarly the high/100Ω input should have a turns ration of ~1:30 by that calculation. But I’m doing something wrong, as the high impedance input has a 10dB gain vs. 17dB for the low, and surely a higher turns ratio should result in the higher gain, not the other way round?

I was also reading that getting the output voltage of the SUT to match the input of the phono stage is what matters. I can select input sensitivity of the phono stage as 1.3mV, 2.6mV or 5.2mV. That complicates it further!

Don’t worry if you can’t drum sense into my thick skull—I’m only going to use what sounds best anyway!

Chris

 

lewm is right, my Fidelity Research FRT-4 settings are not what you might think:

 

3Ω:31.1dB(x factor: 35.84 times);

10Ω:26.3dB(x factor: 20.68x);

30Ω:25.2dB(x factor: 18.27 times)

100Ω:20.0dB(x factor: 10.55 times)

it gets a bit complicated, you need to balance:

desired impedance (guide: cartridge’s coil resistance x 10, get close to that is all it means)

with enough/too high resultant signal strength.

......................................

My AT33PTGII: coil 10 ohms; signal strength 0.3mv

FRT-4 setting 10 ohms = x factor 20.68.

signal strength 0.3mv x 20.68 = 6.2mV. allow for some loss, guess 5.8mv signal strength. (not too high)

RESULTANT impedance is 47,000 divided by x factor squared. thus setting 10 ohms 20.68 x factor squared is 428.

47,000 divided by 428 is 110 ohms RESULTANT impedance. Guide for my cartridge: 10 ohm coil x 10 is 100 ohm guidance.

....................................................

As mentioned: listen, find what seems right.

Avoiding too high signal strength, thus overload of phono stage’s input is important.

 

 

Long article, interesting chart simplifies recommendations

http://www.rothwellaudioproducts.co.uk/html/mc_step-up_transformers_explai.html

I found this which, if you scroll down has a chart for many cartridges by model 3, and a calculator where you plug your #s in

http://www.mh-audio.nl/Calculators/StepUpTransformer.html

It's very confusing in my opinion, but generally if the inputs of a SUT are labeled with resistance values, in the case of vintage Japanese SUTs, this usually meant that one should plug the phono leads into the pair of inputs with the label (in ohms) that most closely approximates the internal resistance of the cartridge.  In the case of the SUT you are using, and because it is not a vintage Japanese creation, all bets are off, since no LOMC cartridge has an internal R in the 47 to 100 ohm range. Luckily for you, you can just contact the manufacturer, as Elliot already suggested.  Why they don't just label the inputs according to the turns ratio obtained with that particular pair of inputs is beyond me, except perhaps they think audiophiles are too stupid to figure out what turns ratio does to impedance or what turns ratio would result in an appropriate voltage gain for the cartridge.

Possibly the 47 ohm inputs give an input impedance of 47 ohms with a 47K standard load at the input of the phono stage. But that would imply a turns ratio of 1:32, which is a lot of gain.  For the 100 ohm inputs, you would have a 1:22 turns ratio. (In other words, for the cartridge to see a 100 ohm load using a 47K resistor on the secondaries, the turns ratio would be 1:22-ish, which is the square root of 470.) The load seen by the cartridge is equal to the load on the secondaries (47,000 ohms) divided by the square of the turns ratio (~22-squared = ~470).

Meantime, just use what sounds best.

Those are the numbers from VAS:

Steve has replied that I should simply go with what sounds best.

The load numbers look wrong, mine start at 3ohms and go up to 40ohms.

I think a 100ohm load would have little gain.

I'm of the 'I don't care' school and use the setting that sounds best.

Yes, I have a question in to Steve, with whom I correspond fairly often. But he will be biased in favour of his transformer (quite rightly) so I thought to ask you well-informed gentlemen as well.

VAS is alive and well

Steve and son Ray Leung are terrific people, certainly can answer your questions

cayinus@yahoo.com