Thom, Palasr:
Yes, 28dB is a lot! (25 times greater energy than the flat-response amplitude)
My experience has been that when a low-impedance MC cartridges changes in sound and energy balance due to different electrical loading, what we are hearing are mainly the effects of altered distortion characteristics (particularly intermodulation distortion) in the phono stage, and to some extent, dynamic compression of the cartridge (which is what too-heavy loading will do). The oft-repeated mantra that loading-induced alterations in the sound and energy balance are due to the frequency response in the audible frequency range changing is a myth, because there is no measurable sign of this happening. OTOH, factors that I don't see normally mentioned will cause significant changes in the measured frequency response, such as when the ambient temperature changes by a few degrees, or where on the LP the cartridge happens to be tracking (groove radius).
I should now debunk another myth regarding loading, which is that low-impedance MC cartridges are insensitive to capacitive loading. OK, the MC cartridges themselves aren't particularly sensitive to capacitance, but the inductance of the cartridge coils will resonate with the distributed capacitance of the coils and the capacitance of the tonearm cable to create a high-frequency spike, and this spike certainly is sensitive to capacitance. In general, the less the capacitance the better. Having more capacitance (across the plus and minus cartridge outputs) will increase the magnitude of the high-frequency spike and lower its frequency, neither of which is good news for phono stage stability or phase response.
Generally speaking, the greater the capacitance across the plus and minus cartridge outputs, the heavier the resistive loading needs to be to control the resulting high-frequency spike. Conversely, less capacitance allows the resistive load on the cartridge to be reduced, which will benefit dynamic range, resolution and transient impact.
From the above we can deduce that tonearm cables for low-impedance MCs should have low capacitance. As a test, some time ago I built some 5-pin low-capacitance tonearm cables of 1.2m length (configured for use with a Graham Phantom). Including 5-pin and RCA connectors, the smallest-capacitance versions got down to 32pF, there was an intermediate version at 42pF, and the highest-capacitance version had 50pF. I felt that these low-capacitance tonearm cables resulted in greater flexibility in loading, a more natural tonal balance with better dynamics and resolution, and were a worthwhile upgrade.
hth, jonathan carr (preamp and cartridge designer)
PS. The possible frequency range occupied by the high-frequency resonant spike also includes the frequency range encompassed by LP pops and ticks, and these can likewise be of quite large magnitude (larger than any music signal inside the groove). Just as with the high-frequency resonant spike, controlling pop and tick energy is the task of the phono stage (although it is a big help if the cartridge has a low-mass moving assembly). The phono stage and cartridge can have an immense influence on how "noisy" your LPs appear to be.
