It's a means of 'nulling' almost all cartridge loading capacitance/cartridge inductance interaction in the audio band, and allows near-zero capacitance performance for more or less any normal phono preamp/cable arrangement. It works without modification to cables, phono preamps or cartridges, and can be implemented in an RCA 'loading plug', the same way as R loading is commonly varied, using RCA Y adapters.

I've modelled it for cartridge inductance 400mH to 700mH, and for total loading capacitance 150pf to 600pf, where the standard internal preamp load is 47k. That covers most likely cartridge/cable/preamp combinations. The results are pretty good, as illustrated below.

All one has to do is add this network across to the existing RCA connection :

Where L is the same inductance as that of the cartridge coil, and R depends on specific C (total capacitance loading) but is typically between 68k and 100k. Default values of R=94k for L=700mH, and R=68k for L=400mH are good for normal cables.

Here's are plots of predicted frequency response for 2M like cartridge inductance, versus ‘theoretical 0pf response’, and versus ‘normal’ loaded response. Firstly loaded at 200pf total capacitance :

Predicted response of the modified case fairly closely resembles that of the 0pf loading response. And is quite distinct from the 200pf ‘normal’ case.

Here's a plot with the same 2M like parameters, except for 450pf total capacitance loading:

In this case, resemblance to the 0pf case is still good, and quite distinct from the ‘normal’ 450pf case.

This is a plot for AT95 like inductance, firstly at 200pf capacitive loading :

Again, predicted response of the modified case fairly closely resembles that of the 0pf loading response. And is quite distinct from the 200pf ‘normal’ case.

Here’s the AT95 like case at 450pf total capacitive loading :

Again, resemblance to the 0pf case is still good, and quite distinct from the ‘normal’ 450pf case.

Note that to achieve these predicted matches, it was necessary to optimise the value of R, using values as marked up on each plot, but not very closely. Default values of 94k for 700mH and 68k for 400mH still give pretty good all round results, particularly for C loading around 300pf total.

So there you go, the near-ultimate attainment for zero-pf seekers of sensation !

This is fresh off the simulator, and I confess I’ve not yet

*actually*measured or listened to results. That's physicists for you, caveats apply as usual, but I'm confident :wink: If anyone feels inclined, please beat me to the listening/measuring test !

One last point, it seems very convenient to use a spare cartridge body of the same type (minus stylus assembly) to provide the inductance necessary for the modification. Advantages are that inductance/resistance should match near exactly, and hum cancellation intrinsic to most MM/MI types would avoid the need to shield the inductor, should that prove necessary.

All of the response plots above are for the electrical part of the cartridge, not the overall cartridge response, of course, which is a composite of electrical and mechanical responses.

Interesting ? Comments/questions ?