Cartridge Loading for a phono pre amp

@judsauce The industry spec is 47K for all cartridges. When you load the cartridge at a lower resistance you are asking it to do more work (produce more power to drive that load). Even though its a tiny amount of power, it will make the cantilever stiffer and less able to trace high frequencies. It can and does affect the interaction between the arm and cartridge (effective mass and mechanical resonance).

The loading is not for the benefit of the cartridge! The cartridge generates Radio Frequency Interference (RFI) because its inductance is in parallel with the capacitance of the tonearm interconnect cable. This forms a resonant peak which can be activated by the energy of the cartridge itself. That activation is called 'excitation' in radio parlance.

That RFI can be 1 or 2MHz and is injected directly into the input of the preamp via the interconnect cable. If the preamp isn't OK with that, it won't sound right.

If the designer was aware of this fact, 47K will sound fine because the preamp will be unperturbed. Otherwise you will have to use the loading and deal with all that comes with that.

So start with 47K. If the designer knows what he's doing, that will be the position that sounds best.

@judsauce The industry spec is 47K for all cartridges. When you load the cartridge at a lower resistance you are asking it to do more work (produce more power to drive that load). Even though its a tiny amount of power, it will make the cantilever stiffer and less able to trace high frequencies. It can and does affect the interaction between the arm and cartridge (effective mass and mechanical resonance).

Wouldn’t loading it down to 250 ohms from say 47k make it softer as the back EMF os gone that “would have been” stalling the motor?

Or do I have it backwards?

@holmz 

Back EMF?? No. Any cartridge is a generator of power. When the load on any generator is reduced in resistance, more current can flow. This energy has to come from somewhere (otherwise a new branch of physics would be created...) and it comes in this case from the motion of the stylus in the groove- the cantilever becomes harder to move.

You can easily demonstrate this principle for yourself. A loudspeaker operates on the same principle of a moving coil in a magnetic field. If you have a woofer with nothing connected to it you'll find it easy to move the cone with your hand. If you then place a short across the terminals of the speaker you'll find that the cone is a lot harder to move.

What the load resistor does is three things: it detunes the resonant peak I mentioned earlier, thus eliminating the RFI and it causes the cantilever to be stiffer, possibly reducing the ability to trace high frequencies and certainly affecting the mechanical resonance of the cartridge and arm combination.

So you solve a problem for a phono section with a design bug, in exchange for introducing possible tracking issues. Not a good set of options IMO.

Thanks @atmasphere .

I was under the impression that the current from the motor was the same with and without the load, so 47k would be a higher voltage across the resistance, and lower resistance would be less voltage.

Looks like I got it backwards.

The options I have are as follows: 47K, 1K, 560R, 470R, 100R, and 47R.

There are three more options worth experiments:

257R (560R+470R)

322R (1K+470R)

362R (1K+560R)