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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 2 responses by palewin

palewin OP
58 posts
 
First, thanks to both Herman and Almarg. With regards to gain, the MC20E2L puts out .5mv, so a 1:10 step up would get me to 5mv, a good value. In fact, the 1:16.8 of the Cotter is giving me 8.4mv, which is close to the overload on my ARC SP-8, which specifies a max phono input of 9mv. One thing I wasn't clear about, if I make the resistor mod(s) you suggest, does that also have the effect of lowering the input voltage to the phono input circuit? ( I was a math major, not an electrical engineer, so my ignorance shows - my guess is that the resistor will turn the "extra" voltage into heat, and have the effect of lowering the voltage to the phono input, but you are teaching me, and I know that guesses will get me in trouble!) Thanks again.
palewin OP
58 posts
 
Once again, thanks to Al and Herman for your kind tutoring! To put this to bed for now, I'm going to do a few things. First, the SP-8 Rev2 pre-amp is supposed to have special "under the circuit board" soldering posts (i.e. accessible from the bottom cover) for both reducing output gain for amp matching, and for upping the phono input to 100K, so I'm going to see if my ARC dealer can do this (while I practice soldering on something less valuable...). I am the original owner, so I know nothing has been changed from factory specs to date. Secondly, it turns out that the Cotter transformer can be "restrapped" to a lower step-up ratio which will increase its loading, again a bit of soldering that I will ask my engineering friend to do (and count this as a soldering tutorial). Lastly, I confess to having been fooled by the overload specs for the ARC. The extensive tutorial on step up transformers on Vinylengine states that the usual input range for MM phono circuits in 2.5-10mv, with a more usual max around 7.5-8mv. So when I read the 900 mv overload in the ARC specs I thought there was a problem with decimal points or units, but now understanding that input voltage will vary with frequency, I see that the discrepancy was that while cartridge manufacturers specify output for a specific frequency, ARC gives a total overload number which covers all frequencies. All said, I will still be more comfortable with a "standardized" input of closer to 5mv (10:1 step up) than the higher number I'm getting from the higher step up with my current strapping. Since both of you have been so kind, I wanted to give some "closure."
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