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MC and Step-Up Cartridge Loading

Typical beginner's question, when you start to learn more than you wanted to know: I seem to have a fairly bad loading mismatch between my cartridge (Benz Micro MC20E2L) and transformer (MA Cotter Mk2-P). I had an Accuphase AC-2 which wore out, and I replaced it with the Benz about a month ago, before I had learned enough about matching and specs. To make a long story short, the Cotter has a gain of 16.8:1 or 24.5db. Working through the transformer math on Vinylengine, this translates to a "natural impedance" of 167, while the Benz wants a load of 400-720 (i.e. >400, and using the Vinylengine formula, an upper of 720). At this point my options seem to be either get a different step-up transformer (Denon has a couple of 10:1 transformers that translate to 470 ohm loading) or get a different cartridge. So my questions are: first, if I get a transformer with the correct loading, what change should I hear? And would you go the transformer route, which is actually less expensive than a new cartridge, or would you go for one of the less expensive high output MCs and eliminate the step-up problem completely? Please keep cost effectiveness in mind, since in the current economy I'm really trying to buy more LPs and fewer bits of hardware! Thanks in advance for your advice.
palewin
almarg's avatar
almarg
9,670 posts
 
Changing the internal 47K resistor in the preamp to 120K will not affect the gain significantly, but has the two potential disadvantages that I mentioned (internal preamp modification; possible subtle electrical effects).

Putting an external 73K or so resistor in series avoids those disadvantages, but will reduce the signal level into the preamp by 8.1 db, corresponding to 20 x log (47/120). That would cause you to turn up the volume control by 8.1 db to get the volume you would have gotten without the series resistor, which in turn would raise the background hiss level by 8.1 db.

That may or may not be significant, depending on the signal-to-noise ratio of the preamp, which you can roughly judge by seeing what the hiss levels are that you presently have, and what they become if you turn up the volume moderately. Keep in mind the rough rule of thumb that a 10 db increase in volume corresponds to a doubling of subjectively perceived loudness.

For a "beginner," you ask remarkably intelligent questions!

Regards,
-- Al
almarg's avatar
almarg
9,670 posts
 
According to the specs shown here:

http://www.arcdb.ws/SP8/SP8.html

the phono input overload point is 900 mv, not 9 mv. (Of course, it's safe to assume that output clipping would occur well before that large an input were reached, given the 60db gain from phono input to main output, which is a voltage gain of 1000!).

The reference also indicates that "Magnetic phono may have any value from 10 ohms to 100K ohms substituted." Not sure how that is done physically, but see if you can determine that via the manual or by opening the top cover. 100K would cause the cartridge to see 354 ohms through your present transformer, which I'd imagine is close enough to the recommended range to be satisfactory.

Regards,
-- Al
herman
2,285 posts
 
I agree with Al's math but my gut (+ experience) tells me that the 2nd approach of a series R is the more evil of the 2 since by forcing this tiny signal to flow through a series R you are dividing the incoming voltage by 2:3 after stepping it up 17:1 = about 11:1.

At that point I think an xformer with the 10:1 ratio is a better option.

If your gain structure is good with the 17:1 then I would definitely try changing the internal R to a larger value. It says 100K max but I doubt 120K would affect anything other than the input impedance and (as Al pointed out) the bigger the input R the more potential for noise.

A good MM cartridge is also a good option or high output MC designed to work into 47K. Did you buy this preamp new? All of this assumes that somebody along the line didn't change the value of the input R to something other than 47K.

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almarg's avatar
almarg
9,670 posts
 
If your gain structure is good with the 17:1 then I would definitely try changing the internal R to a larger value.


I agree. I made the suggestion about an external resistor before finding (if I correctly interpret the statement in the specs I linked to) that the resistor value can be changed without doing soldering work on an internal pc board, and before seeing the statement that 100K is electrically acceptable.

One minor correction to your post: Inserting 73K in series with a 47K input impedance would divide the 16.8:1 ratio down to 6.6:1, not 11:1.

Regards,
-- Al
herman
2,285 posts
 
sorry, did it in my head and flipped the ratio over

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