MC and Step-Up Cartridge Loading

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

I agree with Herman's comments, but I'll add a couple of things:

1)The main effect of resistive loading of a moving coil cartridge is to smooth out what would otherwise be an ultrasonic peak in the frequency response:

http://www.hagtech.com/loading.html

With excessive loading (167 ohms instead of the recommended 400+), the frequency response may roll off within the audio range (perhaps somewhere between 10 and 20 kHz), instead of above the audio range. It's not possible to say in a more quantitative manner without knowing the cartridge inductance, which doesn't appear to be specified. Perhaps you could contact the manufacturer and ask them what the inductance is, and plug that value into the calculator at the link above.

2)If your preamp input impedance is fixed at 47K ohms (some preamps have selectable input impedance, sometimes including 100K), another approach might be to have someone prepare a small shielded enclosure with rca female connectors at each end, and containing a roughly 73K good quality resistor, in series with the hot lead of each channel. 73K in series with the 47K input impedance of the preamp = 120K.

That would avoid any internal modification of your preamp, as well as the possibility of any possible subtle sonic effects that might result from changing its internal input resistor (e.g., on leakage or bias currents into its input stage).

As Herman mentioned, however, you might possibly still face the problem of inadequate gain (resulting in excess hiss), depending on how good the signal-to-noise ratio of your preamp is.

Regards,
-- Al

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.

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

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