Transimpedance phono sections do not present the cartridge with a low impedance in the same way that a resistor might. This is why 'virtual ground' is used to describe the junction of the input resistance (the cartridge in this case) and the feedback resistor in an opamp circuit. The word 'virtual' means 'almost or nearly as described, but not completely or according to strict definition'. Were it actually ground of course the opamp wouldn't amplify as the input signal would be shorted.
To emphasis added, this statement is false. It suggests that somehow phono cartridges are exempt from physical law. Any time you load a magnetic transducer you will affect how easily it is moved. This is why an alternator in a car takes considerably more energy to spin when its loaded.
The statement is true in the same sense that I might say that
the idea that a man flapping his arms up and down gives him the ability to create lift and fly is a myth.
Of course, it stands to reason that if you flap your arms you will create lift, but the effect is so negligible that it is meaningless in much the same way as the electrical power dissipated in a cartridge's coils/resistance is capable of changing the compliance by any real degree. The short-circuit/trans-impedance condition leads to the highest level of electrical power dissipation (and therefore mechanical resistance) since the system's resistance to current is minimised, and even then it is a tiny fraction of a percent compared to the mechanical power in the system.
Cartridge coils are most certainly not efficient transducers designed for maximum power extraction such as alternators. They are sensors optimised for tracking/bandwidth and linearity - much like acoustic loudspeaker drivers with eye-wateringly low power efficiencies.
This is the classic Audiophile intuition fallacy. It
sounds plausible on the face of it, but mathematically it is negligible to the point of meaninglessness. I would highly recommend you do a little calculation at 5cm/s and compare this via mechanical compliance vs. the electrical dissipation of 500uV through a 10 ohm coil...
Take the Audio Technica AT33SA cartridge. It has an output of 0.4mV at 5cm/s cantilever velocity. This means that if we shorted the output with the lowest impedance possible (maximum mechanical resistance from the coils due to maximum electrical power absorption) we would get 16nW of power removed from the system.
If an object moving at 5cm/s is dissipating 16nW of power against an unknown force, we can easily work out the force to be 320nN (nanoNewtons).
Take a frequency of interest, say 100Hz, to find the excursion at 5cm/s (dynamic compliance is specified for the AT33SA at 100Hz), which gives us an RMS value of 76micrometres for a sinusoidal function.
So we now have 320nanoNewtowns and 76micrometres - translating to a resistive/dynamic compliance figure of 238 metres per Newton. The AT33SA give a figure for microcentimetres/dyne so let's convert, giving a figure of
238,000×10-6cm/dyne - you can see where this is going already with all the zeros.
Now, the AT33SA specs mechanical compliance as
10×10-6cm/dyne (100Hz)
So our electrical compliance is 23,800 times greater than the mechanical compliance... If we sum them together, we see that shorting the cartridge degrades reduces the compliance from 10×10-6cm/dyne to 9.99958×10-6cm/dyne - or
something like a 0.004% change.
So yes, my sincere apologies to you old chap. You're quite right - a most substantial difference of
one 250th of a percentage point...
This is why audio is in the miserable place it is right now. All of the above, perhaps bar the RMS excursion calculation, is A-level/high school mathematics (yours truly did mechanics at A-level and we did all of the above).
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and it's also not easy to predict the minute level change and compensate without the kind of competence you represent .
