There are many different ways to create audio filters, but arguably the most favored by audio aficionados are those that incorporate inductors. Even legendary designers like Rupert Neve, Bob Moog, and the engineers at Abbey Road Studios knew that this is where the sonic magic is. They’ve incorporated inductors in their EQ filter designs from the beginning, and made some of the most coveted and sought after equalizers. There is an unearthly and organic quality to the sound of these filters due to their unique properties and behavior. One of which is core saturation, which you don’t get from a standard resistor-capacitor filter. The other comes from inductive reactance. Products like the EVE Equalizer, use our own precision multi-tap inductors that we make in-house for use in our own equipment. Not only because we need something that is custom, but also because it gives us our own sonic imprint. And it’s totally badass.
What is an inductor? How does it work?
This can be complicated, but I’ll try to make this is easy as possible to understand. Conductive materials like copper wire, have 3 inherent properties. Resistance, capacitance, and inductance. Sometimes more of one property than the other, but they all have these 3 inherent properties.
Another basic concept to understand, is that audio signals are simply alternating currents. They “wiggle” back and forth in a wire. The faster it wiggles, the higher the frequency. So when we pass an audio signal through a wire, the inherent 3 properties of a conductor, will affect that signal wiggling back and forth.
Inductance in particular, is the ability of a conductor to oppose, or resist an electrical “flow” (current) through it, based on its electromagnetic properties. When electrical current flows through any conductor, an electromagnetic field is created around it. When a magnetic field moves, this field is called magnetic flux. With inductance, its this magnetic flux that creates the opposition to electrical current.
Interesting things happen when we change the current through something that has an inductive property. The opposing magnetic field is stronger at certain frequencies than others, depending on its inductance. This varying opposition is called inductive reactance. Something with inductance might not be sensitive to opposing low frequencies, but could be very sensitive to opposing high frequencies. More on this later.
Another interesting thing is that we can modify a conductive material like wire, to purposefully exploit its inductive properties. This is in essence what inductors are. Inductors, are purposefully fashioned in a way where inductance is the prominent attribute. They are usually nothing more than a coil of wire around a magnetic conductor. The magnetic conductor could be air, or some type of ferrous material like iron, or different alloys. Its much more complicated than this, but by changing the diameter of the coil, the length of the wire, the diameter of the wire, or changing material in the “core” of the coil, we’re able to change its inductive reactance. We do this to increase inductance. This extra inductance we can use to “stop” or “choke” ac signals such as audio signals, at different frequencies.
Ferrous materials like iron are fantastic conductors of magnetic flux. After all, its what magnets are made of. However, in real life, the ferrous materials we use are usually designed so that we can somewhat control this flux. Every core is different. A core’s ability to carry magnetic flux is called magnetic permeability. This is important in the design of inductors and transformers, because we exploit it’s magnetic properties, to be able to control the inductance.
If you’ve followed along so far, great! If not, that’s ok. its complicated. In essence we can use an inductors ability to “resist” current at different frequencies in the audio spectrum. Using additional components along with inductors, we can create a beautiful tone circuit.
Elektrophonik Inductors
In the EVE Equalizer, we need multiple inductance values. Instead of using multiple inductors, we use one large inductor called a multi-tap inductor. We just “tap” off from different parts of the coil to get different inductance values. Its much more efficient and space saving to do it this way, but just as effective.
At the heart of the inductor is the core. We start with two ferrite cores. It’s what is carrying our magnetic flux. It is just ceramic that is impregnated with iron powder. Using a calibrated coil of wire, we can measure these cores to find their exact magnetic permeability. Since every core is different, and can be different up to 20%, we need to precisely measure them first before we start our design.
Now that we’ve figured out the cores permeability, we can figure out the specifics to make our coil of wire around it. Every inductor is born from it’s core, and every one is unique. This is where math starts getting very complicated. Because we need to make a ton of these things, we’ve created our own software to make the math process go faster. It’s no fun doing calculations all day.
Using our software, we can enter all of the values we need to make any inductor, based around the selected core’s permeability. Then it figures out exactly what we need to make a custom inductor based on the core. Next we need to know the wire diameter, length, number of coils, bobbin width etc… luckily we’ve also told our software how to factor all of that in as well.
Then our software performs double duty. It takes all of our initial requirements, compares it with the permeability of the selected cores, then calculates everything we need to make the perfect sounding inductor. It takes this information and turns it into machine code.
To build the inductor, we need to coil enameled wire (aka magnet wire) around a “bobbin”. It’s just a hollow spool, but will eventually have the core inserted.
Now for that machine code… we’ve also built our own machine to wind our inductors. It’s faster than manually doing it, and it isn’t prone to human error. Using precision motors, we can do everything like set the tension of the wire, and turn the wire around the bobbin to within 0.2 degrees of accuracy. If and when we need to make a “tap”, the machine pauses to allow us to make the tap out to a pin, then continues it’s journey where it left off. It’s not self-aware yet, but we’re working on that 😉
After the machine has completed its work, we insert the cores into the bobbin and perform an initial measurement of the different inductance taps.
After the machine has completed its work, we insert the cores into the bobbin and perform an initial measurement of the different inductance taps.
Once we’re satisfied, we permanently fuse the cores together so they are mated for life. Even the slightest movement of the cores can drastically impact the inductance values. Which will then impacts how well it can impede frequencies. So every core permanently belongs to that specific coil that was created for them. It’s a symbolic relationship that can be very prolific if you think about it. Each one is unique.
Every Elektrophonik inductor is specifically created for their set of chosen cores, and as such need to be made from scratch. After which they are tested to ensure less than a 0.1% error margin, kissed by angels, and put into service.