MC and Step-Up Cartridge Loading

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

.

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."

Palewin,

Thanks for the good update and summary. Yes, cartridge output specs are usually defined for 5 cm/sec groove velocities at 1 kHz, and the RIAA equalization that is utilized on records provides a boost of around 20db at 20 kHz, relative to that 1 kHz reference frequency (and also a cut of about 20 db at 20 Hz). The preamp provides an inverse response curve during playback. 20 db is a factor of 10 in voltage terms, so a cartridge with a 5 mv output at 1 kHz would output 50 mv at 20 kHz, for the same groove velocity. Of course, real-world recordings are unlikely to have that kind of content, at least for any significant amount of time; if they did tweeter meltdowns would be a common occurrence!.

I would think, also, that worst case groove velocities can exceed the 5 cm/sec reference velocity that is the basis for these specs, although I don't know to what degree.

My other comment, which you most probably realize, is that if you make BOTH changes, restrapping the xfmr to 10:1, and also changing the preamp to a 100K input impedance, the cartridge will see a 1K load, which goes too far toward the other extreme.

Enjoy!

-- Al