MC and Step-Up Cartridge Loading

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

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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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

sorry, did it in my head and flipped the ratio over

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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."