In all the years I have been reading and talking to engineers who are
involved with step up trans and carts this is the first time I ever
heard this.
Apparently they don't know the physics of phono sections.
In the case of an SUT you have a different set of variables! The SUT is the major player in terms of loading and you have to satisfy its requirements for the best sound. That will vary according to the cartridge and interconnect used, as transformers transform impedance, so if your cartridge x is lower impedance than cartridge y, the loading required to prevent the transformer from ringing will be lower too. Go too low and the transformer will start to roll off the highs.
My comments precluded the use of an SUT, which will usually block RF interference due to a lack of bandwidth. If you are using an SUT you most definitely have to pay attention to loading! You also have to be careful about the interconnect cable, as the inductance of the SUT is part of the resonance equation. Keeping the interconnect cable capacitance down is key.
If we are simply talking about loading of the cartridge (no SUT), sometimes it is helpful to use test equipment and ears at the same time! Turns out that when the cartridge is loaded properly (called critical damping) that is also the value at which it sounds best as it is not ringing and not rolled off in the highs. The only thing is, **that really does not apply to low output moving coil cartridges** (instead applies to higher output moving magnet types). You can load LOMCs or not, and when you 'ring' them with a square wave, all you get in response is a square wave. In essence, the loading is only affecting them at RF frequencies and any engineer worth his salt can tell you why: the inductance of the cartridge is very low so the interaction with the loading resistor is occurring at ultrasonic or radio frequencies.
If you search this and other forums you will find this has been discussed quite a lot.
To summarize:
1) Loading of a LOMC cartridge generally does not affect the cartridge. There will be audible effects if the phono section is unstable with RFI at its input.
a) for unstable phono sections, the 'loading' resistor detunes the resonance caused by the inductance of the cartridge in parallel with the capacitance of the interconnect cable. Therefore the 'loading' value will change with the cable. We can see from this that the 'correct' value is not likely found in manufacturer's recommendations.
2) In the case of a LOMC cartridge and an SUT, loading of the output of the transformer is paramount for proper performance. Often this will include a capacitive value as well as a resistive component.
3) In all cases the results can be measured and readily correlate with what is heard, meaning that if you know what you are doing you can often get pretty close to what is correct in your system by ear. examples:
a) if the phono section is stable and no SUT used, the loading will have no electrical effect on a LOMC cartridge. The extra energy asked of the cartridge by loading it may cause the tracking characteristics of the cartridge to change- IOW it may track better without loading. This will be found to be an interaction with the effective mass of the arm/cartridge combination as the loading essentially affects compliance. If a value of 10x or more of the cartridge impedance is used, the effect on tracking will be minimal. If the phono section is not stable then the loading resistor will be needed to detune the tank circuit at the input of the phono section (cartridge and cable). The best loading value will the highest one that halts the stability problems in the phono section.
b) if an SUT is used with a LOMC cartridge, the best sound will occur when the transformer is critically damped.
c) If a high output moving magnet cartridge is used, the best sound will be when the cartridge/cable combination is critically damped.
d) in all cases it will be found that a low capacitance cable makes things go easier.
