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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

Showing 3 responses by herman

herman
2,280 posts
 
You have 2 issues, gain and impedance. If you lower the gain to 10:1 to raise the impedance you may not end up with enough gain.

The 167 ohm assumes that the MM input has an impedance 47,000 ohms, which would be typical. This is usually controlled by a single resistor on the input of the stage so the simplest thing would be to change the value of the resistor. A 120K ohm resistor would give you a reflected (natural) value of about 425.

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It is proportional to the square of the turns ratio so 16.8 times 16.8 is about 283.

167 times 283 = 47,000
425 times 283 = 120,000
herman
2,280 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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herman
2,280 posts
 
sorry, did it in my head and flipped the ratio over

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