Low Inductance MM ?

Think of a cartridge as a generator - you know, like the opposite of a motor. It has wire windings and a magnet. In a MM cartridge the magnet moves in sympathy with the stylus and the windings (the coil) are fixed (stood still). The magnet has to be light and therefore small so we can gather it isn't a strong magnet. To get a good output the coil needs to have plenty of what's called "ampere turns", in other words a "big" coil.

Inductors are coils of wire. The more turns to a coil (which means more layers usually), the more inductance.

On average the inductance of a moving magnet cartridge is 0.5H (H means Henries after the bloke who discovered inductance). The input impedance of a phono input is designed to match this average inductance. The IEC/DIN standard calls for 47k Ohm resistive load, 100pf of arm cable capacitance and a 220pf input capacitor. Often though, phono preamp manufacturers make the input capacitor 100pf in case the arm cable is more capacitive.

In reality we end up with a load of 47k Ohms in parallel with around 250pf on a 0.5H coil. Using a bit of electronics math(s) we find the 47k load gives a -3dB upper frequency roll off at around 15kHz, which incidentally is yesteryear's high fidelity. The 250pf being in parallel with the 0.5H of the coil is in fact a tuned circuit (like they use in a radio) and that gives us a frequency of 16kHz with a bit of what's called peaking - a bit of a hump - which when it subsides has carried us to around 20kHz thus fulfilling the 20kHz upper frequency requirement for "modern high-fidelity".

Now, all MM cartridge manufacturers know the above. But they may want to do it differently - they all have their own ideas on how to push the performance that little bit further. Some use a higher inductance and modify the stylus resonance to give the cartridge a bit more high frequency extension. Some instead modify the inductance downward and keep the stylus pretty well taught with similar results. And some pitch it somewhere in-between.

Swapping the stylus of a particular cartridge for a different variant may well make a difference but I'd have thought a subtle one. The "proof of the pudding" etc...

We'll be interested in your findings.

PS. There is a lower inductance sort of MM. It's moving iron (MI) in fact. It works on a different principle and often needs some parallel input resistance to damp high frequency peaks to get the best sound. There is also high output moving coil (MC) which often requires similar measures. tg and fatmangolf have written quite a bit on load-plugs elsewhere on this forum and if they could provide the links it may also be of interest.



Edited by Graham Slee - 20 Dec 2011 at 9:39am

The stylus won't alter the inductance.

The stylus tuning and the inductance of the cartridge (in the cartridge body itself) work together in determining the overall cartridge performance, for the given load. I will add that cartridge manufacturers usually quote a capacitive loading "spread" indicating that the cartridge is tolerant of a narrow span of capacitance in which it will deliver what they consider to be right.

Changing from one type of stylus to another "allowed" stylus for a specific cartridge may lead to a subtle difference. The Goldring 1000 series works in very much a similar way, each stylus offering a difference in performance.

That's a useful data point, thank you. I seem to recall the capacitance of the OL RB250 wire was different ( and dependent on the plugs fitted too ) to the Rega original, though I can't remember if it was significant. Either way, it all sounds good right now and I have my eye on a replacement stylus ( not the "Ea" type ) that I may try shortly.

Paul