Hi Myles: Most likely, all low-impedance MC cartridges will cause ultrasonic resonances when played through the capacitance of a phono interconnect, and burning off the resonant energy is the primary benefit of the "cartridge loading resistor" at the phono stage input. But since the Air Tight and MySonicLabs cartridges are designed to have very low internal impedance, there is even less self-resistance available than normal that could be useful in damping out reactive resonances. Therefore, differences in capacitive loading are more likely to be easily audible with cartridges that have very low internal impedance.
His contention (and I don't mean to put any words in Nick's mouth) that higher loading leads to issues with Johnson noise.
I don't buy this explanation, because the higher-value loading resistors will be in parallel with the cartridge's self-impedance, therefore the net impedance, and therefore the net Johnson noise, will be dominated by the low impedance of the cartridge's coils.
Plus Nick also pointed out that the added "detail" at the say higher loadings actually obscures "real" information.
If the phono stage does not have high overload margin at ultrasonic frequencies, or not-so-favorable linearity at ultrasonic frequencies, the ultrasonic spike resulting from high-value resistive loads (the spike can be in excess of 30dB at 5-7MHz if the phono stage input termination is 10kohm or higher) can easily result in ringing and intermodulation distortion which will obscure real information, which is undoubtedly what Nick is referring to.
But if the designer does give his phono stage high overload margin and good linearity at ultrasonic frequencies, the phono stage will treat the ultrasonic spike as simply another signal, and no intermodulation distortion products will be generated that could otherwise be low enough in frequency for the ear to hear. IOW, there will no longer be any distortion products to conceal real information with.
The reactive ultrasonic spike between cartridge inductance and cable capacitance (or phono stage input capacitance) is a real problem that the phono designer should take into consideration during the design process.
One countermeasure would be to design a phono stage input network that efficiently damps out the ultrasonic spike without needing to resort to heavy-handed resistive damping. Way back in 1936, Alan Blumlein published a patent that discusses how to make reactive loads behave like purely resistive ones. The US patent number is 2035457, for anyone who is interested in reading it.
A second countermeasure would be to design the phono stage for high overload margin and good linearity at ultrasonic frequencies, so that even when ultrasonic spikes do occur, the phono stage isn't flummoxed by this.
A third countermeasure would be to reduce the capacitance between cartridge and phono stage to the smallest value possible, since the resulting ultrasonic spike can be damped out with higher-value resistive loads than if greater capacitances were involved.
I hear what he's talking about with the Atlas and other cartridges - but I attribute that to his phono section being very neutral and resolving without the need for extra kick.
As most phono stages are designed are right now, the input resistor needs to serve two functions. The sonically dominating function is to make sure that the resonant ultrasonic spike isn't so large that it affects the behaviour of the phono stage. The sonically lesser function is to load the cartridge. I would like to see these two functions separated so that we can start discussing cartridge loading in terms of what it does for the cartridge, rather than as a phono stage band-aid in disguise.
kind regards, jonathan