Air cored single wire, multi-strand (litz) and foil-based induction coils:
Our Air Core Wire Coils, Litz Wire Wax Coils, Cross Coils and Wax Coils are all either made without a core or with a neutral wooden core.
These coil types can generally be used across the board for all positions on the crossover, however from certain technical or cost related perspectives, it may not make sense to use air core coils for certain crossover positions.
Ferromagnetic cored coils:
Inductors with ferromagnetic cores tend to become saturated as the current increases.
Any induction coil made with a ferromagnetic core will always have a certain level of audible distortion / noise.
The cores material and construction will dictate how profound the distortion / noise will be and under what circumstances the core will saturate.
I will also depend on the driver used, the crossover design, and filter order.
Ferro-magnetic cored inductors can largely be used for bass and lower mid-range sections of the crossover without any audible distortion or interference, or as rule of thumb for positions below 1.2 kHz.
Air cored coils:
Air core induction coils have no core that will saturate, and are therefore not affected in the same way when it comes to electrical currents.
Important notices about coil selection (inductance/DCR in relation wire/foil gauges):
It is important to note that even coils made from the same wire/foil gauge and with the same inductance, may have quite different DCR (ohmic resistance) figures, beyond the industry accepted +/-5% tolerance.
Therefore the formfactor and dimensions of a coil (which winding tool it was wound on) also plays an important role.
For ferromagnetic cored coils, the above also applies, with the addition that the type and mass of the ferromagnetic core will also influence the inductance and DCR of coils.
For this reason it is important that when looking for a coil to use on a new crossover design, the DCR target value must be known and ma need to be adjusted when tweaking the crossover design via listening tests.
When replacing an existing coil, knowing the inductance and wire/foil gauge is not enough to determine the DCR of the coil and it must be measured with a high quality LCR meter with a milliohm function.
Wire/Foil Gauges in general:
The gauge/thickness of wire or foil will influence both the resistance of the coil (DCR) and the power handling of the coil.
Using a thicker gauge of wire or foil can also add more dynamic headroom.
The wider the surface is of the conductive material for a coil, the more freely the electrons can travel, which makes Litz wire and foil-based coils superior compared to single strand wire-based coils.
Coil power handling:
The reason why it is difficult to exactly determine the exact power handling data for a specific coil (gauge/core type and size) has to do with the complexity of doing such measurements, below are listed some of these complications:
- Are we talking 4Ω or 8Ω impedance when measuring?
- At what frequency are we measuring?
- At what amplifier output voltage / amperage are we measuring?
Music consists of a very wide range of signals across the frequency response range.
How a specific coil will perform in a crossover will therefore depend on the music type played and volume levels over extended play time, etc.
To measure the exact wattage load where a coil would start losing inductance, a core saturates or when audible distortion would occur is therefor complicated.
It will often be possible to get advice or share experiences with audio experts and DIY audio enthusiasts alike, who have a lot of hands-on experience with coil selection. Such advice or experience sharing can usually be found in online articles or via online audio forums.
In many cases it is also a matter of personal preference, budget, and philosophy which coils could or should be used and if any theoretical or measured potential “issue” is in fact audible to the human ear.
If you want to go down the route of getting a better idea of what a coils overall power handling would be, a setup will have to be made, where you use a white noise test signal.,
It would simulate/represent the full power bandwidth energy area in the audible band (20-20kHz) run through the coil during such a test.
Another thing to note is that even if you make a test using a white noise signal, the results may not translate into something that you can physically hear for your specific speaker design
Which coil type will sound the best or give the best performance:
Here we get in to a complex topic where several factors should be considered.
As a very generalized rule of thumb, we could say that it is always best to use an air cored coil to avoid any distortion/noise or to worry about saturation of a ferromagnetic core.
Another very general observation could be that using better and more expensive coil types follow the same pattern as where you would use more expensive and better capacitors, e.g. for crossover positions that are in direct line with the tweeters and upper midrange.
However, it is not that simple, as ferromagnetic cored coils have many good use cases and will make sense to use both in terms of cost saving, power handling and for reaching a desired inductance and DCR for certain positions on a crossover.
As an example if a high inductance and low DCR coil is needed for the bass section of a crossover, it is unlikely that any distortion would be audible, if crossed below around 1.2 kHz (can vary from listener to listener).
Replacing a ferromagnetic cored coil with an air core coil for such a crossover position would require a coil using a lot of copper in a thick wire or foil gauge, which makes the coil both very large in size and expensive from a cost perspective.
When it comes to air cored coils we can make a general hierarchy as follows:
1. Foil based inductors
2. Litz wire based inductors
3. Single wire based inductors
In general the larger the surface is of the wire or foil used, it allows for more free travel of electrons.
It also gives the potential for added dynamic headroom and can also further eliminate any unwanted distortion.
When it comes to choosing between the 3 main air core coil types mentioned above, the most important factors are:
1.) Is there room on the crossovers to use a larger foil or Litz wire based coil, versus using a single wire based coil?
2. Can an actual improvement be heard in a listening test when using a better and more expensive coil type?
3.) Does the cost versus gain ratio make sense, when using a better and more expensive coil, when factoring in the cost of the drivers and other crossover components used?
The conclusion is that when choosing any crossover components, there can be some physical (size) limitations and some cost limitations that will dictate if we must make compromises for a crossover design.
Even if size or cost is not a factor, then we get into to something as illusive as personal preferences and what the designer’s ears are able to pick up when experimenting with using different component types for a crossover.
There is no substitution for experience and when it comes to crossover design and choosing which components works to the designer’s taste there are unfortunately no easy shortcut to gain an overall understanding of which components makes the most sense to use for various speaker designs.
We can offer some general advice and share feedback from other speaker designers, but that should be treated as inspiration and cannot replace personal listening tests.