What type of inductor should I use for my crossover?

Isn’t it obvious? You should be using audio grade inductors. But what makes an inductor suitable for an audio crossover? First of all, let’s try and define the inductor. Like we said in a previous article about capacitors, the inductor is one of the 3 passive electrical components : resistors, capacitors and inductors. Compared to a capacitor, an inductor is also a frequency dependant resistor. However, now it rejects the higher frequencies. An inductor is measured in mH (millihenries). Depending on how many millihenries it has, some high frequencies will pass and others will not.

Inductor quality

Inductors are also called coils, for obvious reasons. So, when you are checking out some coils, if you want them to be audio grade inductors, check the following :

  • Coil core.
  • Wire used.
  • Manufacturing quality.

Let’s debate the manufacturing quality. Many DIY-ers choose to make their own inductors. This is not the purpose of this article, but you can find formulas on the internet. Depending on your coil core, you can calculate how many windings you need to get the desired inductance. You can wind them by hand or use a machine to wind them automatically.

Some people like to make their own coils, with very good results. However, manufacturers like to show off their production process and why it’s superior. For example, every coil will exhibit the effect of microphony. This means that the entire winding will vibrate to a certain extent. This mechanical vibration is turned into an electrical signal and therefore the audio signal passing through the coil is corrupted. This effect is reduced when the core is something solid (not the case for air-core) and the winding is resting on something.

To minimize this effect, manufacturers like to add an additional layer of varnish to the insulated wire. After the coil has been wound, the coil is baked. As a result, the extra layer melts and creates a solid unit, with the windings firmly bonded together. This reduces the mechanical vibrations for all types of coils. You will find other creative manufacturing processes that try to improve quality.

Types of audio grade inductors

You can image that there are quite a few types of inductors out there. But let’s keep this article to the point and focus only on the types which are most often used.

  • Iron/ferrite core inductors.
  • Air core inductors.
  • Foil coils or ribbon coils.

The order is not random. I list them in ascending order, from the sound quality perspective. This is also proportionate to price as well. You can expect that a foil coil is way more expensive than the equivalent iron core inductor. However, that does not mean that iron core coils doesn’t have their place. You can use them successfully in numerous projects.

Iron core inductor

The iron core is one of the cheapest audio grade inductors out there. While we like to call them iron core, the core can be made by various ferrous metals (not just iron). The core can have different shapes. As you can see in following image, one has a square core and the other a cylindrical one.

iron core audio grade inductors

Since the core is metal, you don’t need that many windings to achieve the desired inductance. As a result. we get few benefits from this :

  • Less windings – cheaper price.
  • Lower DCR rating. This is a solid plus. High inductance coils can have a higher resistance rating, which is rarely a good thing.
  • A solid core provides support for the winding and minimizes the microphony effect.

It is clear that these coils are top picks for budget projects. Also, you might see them in low pass filters for woofers. The tweeter is almost always more efficient that the woofer. Therefore, you don’t want to lower the output of the woofer by using a high ohmic resistance coil. Iron core are best for these types of jobs. However, the fact that there is a piece of metal in the core of the inductor, it also affects the musical signal. For these reasons, the purists out there will avoid the iron cores, or use them when not in the direct signal path.

Air core inductors

These types of inductors are the ones which are most widely spread in the hi-fi community. While they are more expensive than the iron-core counterpart, they are affordable if the inductance value doesn’t reach high levels.

audio grade air core inductors

Unlike the iron core, which has a solid metal in the middle, the air core has … air. And this is the ideal core material. Some have a piece of plastic for added rigidity. But the point is that it’s a nonconductive material. As a result, they are superior as far as accurate pulse reproduction and distortion numbers. These audio grade inductors can be used in any part of the crossover. However, since there is no metal core, the number of windwings is larger, to reach the same inductance rating. This means (besides a higher price) that the resistance is higher.

To keep the resistance lower, you need to increase the cross section of the inductor wire. This will increase the price even further. Also, high inductance coils with thick wire gauge can reach impressive dimensions and weight. This can be a nuisance when constructing the crossover board.

My advice is to use air core inductors wherever possible. If you need a big inductor on the bass side, just use an iron core, no problem. This is unless you are working on a really high end project with a big budget. If that is the case, you might want to check out the next type of inductor.

Foil inductors

These are considered to be the best audio grade inductors out there. There is quite a bit to be said here, but the conclusion is short and straightforward. Do you have a big budget and want to go all out? Get these.

foil audio grade inductors

You can clearly see the difference. No wire here. Instead, a metal foil or ribbon is used. The foil is wider than a conventional wire. Also, because of its shape, they can be tightly packed. As a result, they have a large contact surface between the windings and create a high mechanical stability. If you compare the microphony effect of different types of inductors, the foil coil is miles ahead. It has the lowest measured vibration.

The foil coil is the closest to the physically ideal coil more than any other design. It excels when it comes to music dynamics, imaging and lowest distortions. There are people out there which will argue that there is no need to spend so much money on these. On the other end of the spectrum there are guys which can have nothing but the best.

Conclusion

Depending on you project, there is a spot for any type of inductor. When it comes to building crossovers and using audio grade inductors, I usually follow some simple guidelines. If budget is a priority use iron cores. You want to build hi-fi speakers and prefer to spend more on speakers drivers? Use air-cores, or use air-cores on the direct signal path and iron-cores for the rest. Going for an exotic project? Use film coils or a mix of air-cores with film coils. It all comes down to your budget. Because, sometimes, the differences are so subtle you find it hard to justify the extra cost.


References

  1. Mundorf components catalog.
  2. Image source : link.