Magnetic Myths

As an engineer I tend to sigh when I come across myths presented as ‘fact’. I’m quite happy with the idea that people prefer what suits them. But it’s a problem when people present myths as if they were a ‘scientific’ basis for their preferences. It can mislead others or make them feel their own preferences must show they are ignorant or an idiot.

One particular myth I’ve seen a few times recently is that Moving Coil (MC) cartridges are better than Moving Magnet (MM) designs because their lower output resistance means they have “lower thermal noise”. The idea being that they can deliver a wider dynamic range. hence MC designs are inherently ‘better’. Alas, ask almost any learned academic physicist or engineer about this idea and you’ll be told the politest technical term they’d use for this proposition is “twaddle”!

The most basic point they’d then go on to make is that the amount of thermal noise power is the same for all values of resistance that are the same temperature. That’s a fundamental part of reality. What does vary is the noise voltage it will deliver into a loading which ‘matches’ that resistance. However the available noise current it can provide falls by a corresponding amount.

The amount of signal voltage you can get from a cartridge varies from example to another - but so does the cartridge's source resistance (and inductance). Look in a textbook and you can see that the thermal noise voltage scales in proportion with the resistance. And in practice what really matters here is the signal/noise ratio you can get. i.e. how the signal levels compare with the amount of noise. Experience shows that, yes, in general MC designs have a lower output resistance, and so lower output noise voltage. But they also output a lower signal voltage.

To clarify that I went though dozens of old reviews. The results are summarised in the diagram. The graph on the left plots each cartridge’s properties. It shows how large the cartridge's wanted signal output will be compared with the thermal noise it generates, assuming you use a amplifier which is ideal for that particular cartridge.The key bit of basic physics here is that the thermal noise voltage rises with the source resistance. So the real signal/noise ratio depends on the ratio of the signal to this resistance.

Since it is common to use an MC design with a step-up transformer I’ve also plotted - above on the right - the results when using a ‘perfect’ transformer that gives a step up ratio of x 10 in voltage. This increases the signal voltage from the MC. But it also increases the output resistance presented to a following amplifier! That means it also steps up the noise voltage. The powers haven’t changed, but the voltages have. And this is what we’d get using an ideal transformer. Real ones tend to lose some signal and add some noise! Looking at the result we can see that the MM and MC designs generally end up on the same part of the graph. Although in practice the MC designs have a far larger variation from one model to another, in essence, the fundamentals of physics allow both types to reach much the same sort of signal/noise behaviour when made and used with care.

Assuming the same audio bandwidths I worked out the optimum signal/noise of each cartridge, and these results are shown on the right. This shows how many examples give a result at various levels. If noise is your concern, the higher the signal/noise ratio value, the better. Looking at these results its quite clear that typical MM designs tend to perform just as well as the good MC ones. But poor MC designs tend to lag behind and give worse signal/noise performance.

In practice, a further set of complications arise because the ‘loading’ presented to the cartridge (and any transformer in-between) can alter their frequency response and/or distortion levels. This effect is most well-known because the effects can be quite dramatic with many MM designs and amplifiers where even the choice of connecting cable can have easily measurable and audible effects. A more subtle problem is that the noise performance of an amplifier also varies with the source impedance connected to its input. Given all these variable (and quite often a lack of reliable measurements) it is understandable that the results can vary from system to system in a way that audiophiles can find hard to predict!

(N.B. If you're an engineer and want to know more about some of the above, the basic physics involved is explained by this free book. In particular, chapters 3, 13, and 14.)

So the plain truth is that the end result isn’t determined by a simplistic any “MC good, MM bad” belief. Its a matter of choosing a good example that suits your preferences. Don’t be taken in by the magnetic attraction of myths. Just choose what you prefer.

850 Words

Jim Lesurf