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Need to be educated re: MC Step-up transformers

I recently purchased the EAR MC-4 Step-up transformer. I was having some noise problems with my analog rig. I spoke with Tim de Paravicini of EAR directly in the UK. He answered all the system matching questions and pronounced the MC-4 a fit with my system. He is very direct, extraordinarily knowledgeable, and seemed very sincere and honest.

I still don't understand the "How" though. Here's what I do understand and relayed to Tim:

1. I use the ClearAudio Stradivari, a MC at 0.7mV output.

2. Currently, my phono stage provides a total of 57 dB of gain: 20 in the MC mode using a JFET and 37 in the MM mode using all tubes. My tube line stage provides 12 dB of gain SE, which is how I run it. So, currently without the MC-4, I have a total of 69 dB gain when running my phono stage in MC mode (20 + 37 + 12).

3. The MC-4 has four taps – I was told by Tim to use the 40 ohm tap which provides a 10x gain in voltage and is compatible with the 32 ohm impedance of my cartridge. This would change the output voltage my phono stage “sees” from 0.7mV to 7.0mV.

4. The phono stage must be run in MM mode, bypassing the JFET in the MC mode.

5. Given all this, then, my phono stage will receive as input a 7.0mV cartridge output from the MC-4. This signal will run through the MM mode and receive 37 dB gain and then another 12 dB gain from the line stage SE for a total of 49 dB gain, down from 69 dB, as we are no longer “gaining” the additional 20 dB from the JFET MC mode.

6. I need to remove the 600 ohm Vishay resistors and get the phono stage back to the stock 47 kohm setting, as the MC-4 will reduce the resistance by the square of the voltage gain or 10^2 or 100: 47,000/100 = 470. That is within the range recommended by Clearaudio of 320 – 900, preferably toward the lower end, though let your ears be your guide.

It was based on this information (which I provided) that the MC-4 was pronounced a fit - I certainly don't doubt that.

Here's my confusion: Am I to understand that 7.0mV of cartridge output from the MC-4 is so much more voltage that all I need is the 49 dB the system provides in its new configuration for low noise and analog bliss?

I just don’t understand the science, I guess. I appreciate the education.

Brent
flyfish2002

Showing 4 responses by axelwahl

axelwahl
788 posts
 
Brent,
are still talking SUT? Please note that the values you had discussed related to non-SUT, straight into your phono-pre gain stage, the way I understand it, and the values look to me. I do not know your specific cart but ~ 500ohm could be fine to give you some "air" without getting the sound too "fluffed out", loose, chirpy, uncontrolled, what ever you want to label it.

There are not no many carts that go OK with 47k, as it seems to depend on the internal damping of the typical MC rise in the treble.
Only some MC designs are OK with 47k, and some would argue NONE at all.
Again, if you talk SUT, and specifically truly low output ones then the loading values for SUT use change drastically in my experience, but we have already gone into all of that, no?
Cheers,
axelwahl
788 posts
 
Hi Flyfish2002,
seems you are NOT the only one that uses a somewhat 'wacky' SUT setup e.i. going over the top with the input voltage, etc.

I finally got my NEW Ortofon Windfeld back and was trying it, something just didn't gel when I used the SUT as I had it with the old one, which had an 'VTA error' according to Ortofon --- it sat very low, etc.

Following a hunch, I removed the 13ohm primary loading resistor from the SUT --- just to see what would happen. I be darned, it works VERY WELL without. In theory I'd be overloading the MC input but it looks like I don't. Also I recall that with the old cart, voices seemed too recessed in the stage, none of that either now.

My current thinking is, that since an SUT is an 'impedance-converter', it would AMPLIFY not only the voltage but also any problem with a cart.

Go figure - and enjoy the music.
Axel
axelwahl
788 posts
 
Hi Salavat,
your Phoenix has 0.4mV output, yes?
A 30 dB SUT has a 31.6:1 voltage ratio. Therefore 0.4mV*31.6 = 12.64mV output of SUT secondary to your phono-pre ---- and that will pretty surly OVERLOAD your phono-pre input stage, usually starting to overload already if the above calculation gets to ~ 8.5mV

So, before you want to work on the impedance side of things, the first thing is to make sure you will not get into OVERLOADING the phono-pre input --- all else comes next.
Mid spec for phono-pre input is actually 4.7mV. Check any MM/MI cart output and it will give you a pretty good idea what seems an acceptable range: 3mV - 6mV, so 12.6mV is too much by far.
Only if you consider SUT primary or secondary resistive loading will this value it come down somewhat, but not enough by far for the above example.

Greetings,
Axel
axelwahl
788 posts
 
Hi Al,
you quote Gundam91: "The first rule of thumb is that most (some exceptions exist) MC cartridges like to see a load impedance of 3X to 6X their output impedance value."

This is entirely dependant on the cart DCR in the first place.
Note: The lower the cart DCR, the closer the actual cart impedance to the cart's DCR value.
What you actually try to do is to get an impedance match 1:1 between cart and primary. This way you double the current from the cart in lieu dropping it’s output voltage. The cart is working in: ~ 'current mode'~, which in turn provides for the best and highest voltage output on the secondary.

Very often this coincides very closely (also in experience) with the lowest quoted "recommended" loading.
If e.g. a cart it quoted as >10ohm (at a DCR of ~ 4ohm) or say >30ohm (of DCR ??) etc., then this often turns out to be your point of departure.
Caveat! If a ~ 6ohm cart is e.g. quoted >100ohm then it is VERY probably targeted only at a non-SUT load.
You could see what I mean when visiting some of the Lyra specs (on web) and will see that they actually quote 2 values - one for normal (non-SUT) and one loading for SUT.
Usually some ~ 10:1 difference
Greetings,
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