Lewm, I don’t see any incompatibility. As you say, loading less than 10x the internal impedance of a cartridge will have an audible effect on its performance, due to voltage drop. This is usually experienced as a loss of output/dynamics and a rolled off treble. If a person likes it that way there’s probably no harm done, but the cartridge will not perform according to specifications.
Loading with values above that 10x number will normally let the cartridge perform as intended. Which explains the often used ’>100 ohms’ recommendation from manufacturers of low impedance MC’s. Based on my own experience I’d say that the loading range between the 10x minimum value and 1000 ohms is most critical in optimizing the sound of the cartridge in relation to the system it’s used in. Higher values seem to be fairly irrelevant to the cartridge performance itself, but the design of the phono amplifier could interfere with electronic resonances, as experts like atmasphere have explained.
All of which still doesn’t explain why certain designers demand a very precise loading value for their cartridges. Obviously they have no idea what phono amp a customer will use with their product?
Take for example the Jan Allaerts MC2, generally considered to be one of the top MC’s with unbelievable specs (how about 70dB channel separation?). It has a coil impedance of 32 ohms and according to the specs the load impedance needs to be set at exactly 845 ohms. Not 800 or even 850, but 845. Why this precision when the result is system dependent and that system is unknown?
The spec sheet of Lyra Atlas is even more strange: recommended loading is between 104 and 887 ohms. A wide range as usual, but why these very specific numbers at its limits? Perhaps jcarr can explain, as I assume there must be a logical explanation for this.
