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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
128x128flyfish2002
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saudio
16 posts
 
Brent, there is much more to do to optimize the sound of your Stradivari when using the MC-4 step up. You want to load the cartridge on the primary side of the transformer in order to get an appropriate impedance match. Right now you are letting your cartridge with a DCR of 32 ohms see a reflected impedance of 470 ohms. This is not optimal. Using Axel's 2.5 times the DCR to give an estimation of the cartidge's output impedance gives you the starting point. 32 times 2.5 is 80 ohms. If you put a 100 ohm resistor across the primary the cartridge now sees a reflected impedance of about 80 ohms. You will be amazed at how better things will sound. You have to listen and see what the correct multiple of the DCR to use is. Small changes in primary side resistors have large effects on the sound. There is no easy way to know any cartridge's true output impedance so you must listen and decide what you like the best. The point I am trying to make is that the set up you have now with no primary side resistors is not correctly matched . I have done this with Tim's EAR MC-3 step up which is basically the same as what you are using.

As an aside, this seems to be a great deal of trouble. And it is. IMHO, MM cartridges are more musical and overall better than MC and you bypass all of the set up complexities. If you can get better sound for less trouble ( and much less money! ) why bother?
almarg's avatar
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almarg
8,673 posts
 
Saudio -- I would respectfully disagree. In general there is no reason to expect that providing a load to the cartridge which numerically equals its output impedance will provide best results, or even a good starting point. Quoting from the excellent reference provided by 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.

And it seems to me, at least as a starting point, that the manufacturer's recommendation for the specific cartridge should supersede any general guidelines.

Also, I'm not sure why placing a load resistor on the primary side should necessarily be given preference to placing a resistor on the secondary side, with the value chosen to reflect an equal value to the primary side when divided by the square of the turns ratio.

Regards,
-- Al
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saudio
16 posts
 
Al, I have placed resistors on both the secondary and primary sides of step up transformers and prefer using them on the primary side. I am not an engineer and have no technical background but I try to listen very carefully. I'm guessing that the transformers are designed so that they work optimally when the secondary sees 47K. If you are changing this perhaps the transformer is subject to ringing or other anomalies.

I believe the manufacturer's recommended loading in most cases is for direct into a high gain phono stage. Some companies like Lyra, for example, quote a loading for high gain phono stages and a different loading for step up transformers. In Clearaudio's case the top of the range 900 ohms is close to the 25 times starting point recommended by many when not using a step up.

I'm still learning about all of this as is everyone else. I'm just suggesting that everyone have an open mind and just experiment and listen. Maybe everything that we've assumed in the past is not correct.
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saudio
16 posts
 
On their website, Clearaudio recommends 200 ohms as the load that gives the best sound when using their own phono stage. If that is going to give you the best sound then 470 ohms into a step up is way off and not even close to being optimum!! It has to be much lower because the correct value into a step up will always be lower than that when going direct into a high gain phono stage.
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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