2. PS Audio GHPG phono stage, loading jumps from 100- to 400 ohms, and the Zu likes around 300ohms??, Ive tried 100ohms, but seem's a bit muted and boring, when I go to 400ohms, the bass comes alive, but with harshness,!!The loading is there for the preamp, not the cartridge.
This is how it works: The cartridge is an inductance (the 'coil' part of 'moving coil'). The tone arm cable is a capacitance.
Together they form a resonant circuit. Quite often the resonance is several MHz, but it could be in the high 100KHz range too, depending on the coil inductance and the cable capacitance.
Usually loading is thought to reduce ringing in the inductor, but with LOMC cartridge, this does not happen until well outside the audio band. The reason that an unloaded cartridge can sound aggressive on some preamps is that the resonant peak is injecting RFI (Radio Frequency Interference) into the preamp. If the preamp isn't happy with that, it can sound quite aggressive! The loading resistor detunes the resonant peak, thus getting rid of the brightness by removing the RFI.
The problem is, now you are forcing the cartridge to do more work because the loading resistor is a much lower resistance. This can cause the cantilever to get stiffer, much like how shorting a loudspeaker can stiffen its cone.
Now if the preamp is designed such that it does not care about the RFI, then no loading is needed. A side benefit of this is that quite often there are less ticks and pops in the playback as well. We can discuss why later.
So the loading is there for the preamp, not the cartridge.
My previous post was deleted for some reason- it put all this in a nutshell. I've not been able to sort out why it was deleted.
BTW there is a great thread on the What's Best Forum regarding this topic. I don't recall the thread title, but it was about cartridge loading. Jonathan Carr of Lyra fame participated and essentially stated what I've stated above. Additionally, take a look a Jim Hagerman's site: http://www.hagtech.com/loading.html
In this site you can see that the peak I mentioned is a good 30db higher than that of the cartridge. This is a significant amount of RFI and can play havoc with sensitive phono input circuitry unless the issue is anticipated by the designer! In far too many cases, it is not.
BTW, it is far easier to design a tube phono section that is RFI resistant, due to the fact that semiconductors have diodes integral to their operation. This diode can rectify (convert from RF to audio) the RFI, so more precautions have to be taken by the solid state designer to get around this problem. IOW, a good phono circuit is not just enough gain and proper equalization, it must also have good overload margin (to deal with that 30 db peak) and RFI resistance.