Is AlNiCo the superior guitar pickup magnet material?
Alnico magnet is a term that gets thrown around quite a bit when it comes to guitar pickups. Especially among guitar and bass players, of course. Many people consider a pickup built with alnico magnets superior. At least compared to ones made with ceramic (ferrite) magnets. Most of these people don’t really know the technical aspects, nor the differences between all these things. They either just heard that they are supposed to prefer the alnico sound. Or they simply prefer the sound of these things. Why is that? Is there a real difference sound wise between these magnet materials? Let’s check into this a little bit.
Is alnico the real deal?
So why does even the average AlNiCo magnetic pickup sound better? And “crisper”? What does this iron alloy containing Aluminum, Nickel, Cobalt, Copper and some Titanium offer? Well, my friend, the keywords are magnetism and inductance. The stronger the magnetic field the strings vibrate in, the more treble we get. But the higher the inductance of a pickup, the less treble will be present. What’s interesting in a pickup made with alnico is that the core(s) – the alnico magnets – add to the inductance of the coils. It results in a bright yet more or less balanced output. In a passive circuit and with the regular cables and amplifier, the resonance sits right in the frequency range our ears are the most sensitive to. The 2-5kHz range. It also results in plain simplicity that’s easy to manufacture. Think Fender. Such a simple alnico equipped pickup design uses the magnets for both bobbins and pole pieces at once. It would be difficult to replace those certain shaped alnico rods with the softer and more brittle ferrite magnets directly. It would also change the sound significantly, because ceramic magnets don’t add to the inductance near as much as alnico does. So the resonant peak would get into the ear piercing territory. Some pickups that use separate bobbins and blade shaped magnets for pole piece can get away with the single ferrite core. They do sound rather trebly though.
Ceramic, ferrite, neodymium – the other kind of magnets
The workaround is to use strong and easy to manufacture steel rods. Then put the rectangular bar magnets under those. It will affect the sound though, because we still haven’t changed the pickup design itself. Using steel will add enough inductance to get the resonance lower. The problem is, it might now be lower than in the case of the initial pickup designed for the alnico rods. Placing the magnets under the steel poles will usually result in a somewhat weaker magnetic field around the strings as well. Unless we use neodymium magnets. Or completely redesign and fine tune the pickup for the ferrite magnets and steel poles. Just to get the same sound. Most manufacturers won’t do so. A good pickup was designed to sound the exact way it does. If it was meant to be an alnico pickup, it will sound the best with alnico magnets.
You make this statement – The stronger the magnetic field the strings vibrate in, the more treble we get – which I have seen elsewhere, but never any explanation of why you get more treble. Can you explain?
when I wrote the blog post, I simply took that information for granted; after all, that’s what I observed many times while setting pickup height, so it was clearly the “nature of the beast”. Your question made me look into this deeper. I’ve found this Princeton University article: http://puhep1.princeton.edu/~mcdonald/examples/guitar.pdf
Apparently, the fundamental frequency of the vibrating string is carried only in the string vibration component perpendicular to the guitar’s top (y); the vibration parallel to the top (x) induces the 2nd harmonic and higher frequencies only, no fundamental. The strength of the magnetic field decreases as the distance between the pickup and the string increases, according to the inverse square law. A string vibrating at a greater distance from the pickup, where the magnetic flux is weaker will still vibrate into a lower layer of relatively stronger flux with its x vibrating component, but the y vibrating component parallel to the face of the guitar will remain in the plane at the initial distance, so the higher frequencies will be induced in a relatively weaker flux layer, resulting in a lower voltage of that frequency component.