Ralph, how does cartridge impedance affect all of this? I assume on a
good phono amp not at all. If a higher load increases the motor's
stiffness why wouldn't a cartridge track better at lower loads?
When one **increases** the load, the actual load resistance is lower. If there is less load, the resistance is higher. That should answer your second question as well.
Is there a measurement on Stereophile or whatever that will show if a
phono stage has resistance to RFI and/or good internal stability?
Otherwise the only way to know is to compare the sound of a LOMC at say 300 ohms vs 47k, right?
I've not seen Stereophile do any such measurements- which IMO suggests that whomever is doing the testing isn't paying attention to this parameter. So yes, trying the two different loading values is about the only way you can do it.
BTW, here is an excellent tutorial on this topic:
http://www.hagtech.com/loading.htmlThis tutorial is seeing everything from the cartridge point of view and ignores how the phono section responds. But the resonant peak I've mentioned is obvious. Cartridge manufacturers often suggest a value for loading, but that is usually a general value and this calculator reveals why: the tone arm interconnect cable is playing a role (as well as the input capacitance of the phono section) and this value is an unknown. So the loading value often seen in the literature for a cartridge is really only a suggestion.
What I am maintaining is that if the preamp is properly designed, the resonant peak will be of no consequence to the phono section. Its pretty obvious that a peak like that if driven into excitation (which it often is by the cartridge itself) has the ability to overload the input of a phono section unless that phono section takes this issue into account.
Again, the presence of RFI or ultrasonic energy at the input of an active circuit can affect the way it sounds. As I mentioned earlier it can also affect the compliance of the cantilever of the cartridge. Jonathan Carr (of Lyra fame) and I had a conversation on this topic at Munich a few years ago- it was after a thread on this topic on which we were both active as on the What's Best Forum. He pointed out to me this issue of compliance- prior to that I'd not really thought it thru. Its a really good point because into 47k the cartridge might be doing X amount of work, but into 100 ohms it will be doing 470X **more** work, and that energy has to come from somewhere otherwise a new branch of physics is created :)
That of course will limit the ability of the cartridge to trace higher frequencies. But cartridge compliance is not measured with such low resistance loads, so when looking at the specs to see if a particular cartridge is going to work in a particular tone arm this issue can throw off the setup!
