Read: Audio Note UK IO Ltd field-coil cartridge system Review
I group audio engineering kung-fu into 3 main categories:
- Concepts which have a solid theoretical basis.
- Concepts which have a plausible, but tenuous or inadequately researched theoretical basis.
- Pure Magic, based on intuition and experimentation!
What we are talking about here, the sound of magnets, I would class in the second group.
I am sure you have heard and read many times something along the lines of: ‘The flux is 1 Tesla, it doesn’t matter whether it’s from a ceramic, alnico, neo or field coil’.
If you ask a guitarist (like me) whether ceramic or alnico magnets make a difference to a pickup or speaker’s tone, they will tell you they do. Which you chose is down to taste and the type of music you play. For example, for metal you might choose a ceramic bridge pickup and alnico neck, I have one guitar like that, the others are all alnico.
Permanent magnets are materials with a broad hysteresis loop – once magnetised energy is stored and not released. That energy is stored within the material’s crystal lattice.
In use, a magnet is placed in a magnetic circuit, and the magnet itself is a kind of battery driving the flux in the circuit. The magnetic conductor is usually a soft magnetic material, such as pure iron, and in the case of a cartridge or speaker, part of the circuit is an air gap where the coil is placed.
So, in a cartridge or speaker, the magnetic circuit is made up from Magnet, Iron, Air.
In a field coil system, some kind of coil, with a current passing through it, is used to drive the flux through the magnetic circuit.
So, the magnetic circuit here is Iron, Air.
These are two quite different scenarios, even though the DC gap flux can be arranged to be the same. One difference is what happens when an AC flux is superimposed onto the existing CD flux. – this is what happens in a speaker, and, to a certain extent in a cartridge due to the self-inducted current in the pickup coil.
In both cases, the existing field will be modulated to a certain extent, and how the magnetic circuit reacts to that, especially the permanent magnet system, goes a long way to describing what we hear.
Taking the field coil first, the superimposed field will generate an EMF in the field coil, similar to a transformer. If the power supply is low impedance, that current will disappear into it, therefore damping the modulation. Secondly, the circuit is made of a material like pure iron, which has a low loss hysteresis loop, and is quite smooth and linear in its characteristics.
The permanent magnet is quite different. The magnet does react to the superimposed field, but the system is a lot more elastic, so the field is modulated more. Secondly, permanent magnet materials have a big hysteresis loop, which is intrinsically nonlinear.
What I think we hear is at least partly due to Barkhausen Noise, which is the effect whereby Ferromagnetic materials magnetise and demagnetise in discrete steps. By their very nature permanent magnet materials generate much, much more of this noise compared to soft materials like iron. And, in fact, the fact that permanent magnets are usually hard and brittle and pure annealed iron is like soft cheese is a clue – the former has a lot of defects in its crystal lattice, which makes the material physically hard, the iron has few, which makes it very soft.
The lattice defects temporarily obstruct the field as it’s growing or diminishing, until it suddenly snaps past in a discrete jump. This is what causes Barkhausen Noise.
The effect is that different magnet systems generate different levels and types of background hash. The field coil system, due to the circuit being composed solely of soft iron, generates the least, and that is apparent in the lack of grain and mush – you hear more detail without it being harsh. It’s an improvement in dynamic range.
I guess some may read this and continue with their 1 Tesla is 1 Tesla ‘”Electronics 101″ worldview, but that comes about due to an incomplete worldview, and it flies directly in the face of millions of musicians worldwide.
Regarding analogue tape vs IO Ltd., if we consider master tapes, meaning those which are used to create vinyl, then one would expect the transcription and replay processes to introduce certain ‘modifications’ – even if we might prefer the result.
I believe, though, you are referring more to replay of what might be commercially available tapes, on stereo playback equipment. In that case, the tapes may be several generations away from the master and will also undergo ‘modifications’ by the duplications process, and by the machine you are replaying on.
In the latter case, I think, yes, it’s possible for vinyl to outdo tape, especially when considering the IO Ltd. (and related Audio Note equipment), which is designed to be excellent at audio, compared to tapes deck which are most likely designed to be good on a test bench.
Overall, though, the devil is in the details…
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