I used some of these recently, with a crossover frequency of 400Hz. I only did measurements at low power, but the measured THD was about the same with the crossover as the drivers alone, so the inductors were distorting less than the drivers themselves (compared to inductors wound on small ferrite cores that I used for prototyping, which brought THD up to a very unpleasant 10%).
I bought them for the low external magnetic field of toroids, plus they have reasonably low DCR.
I bought them for the low external magnetic field of toroids, plus they have reasonably low DCR.
That worried me too, but I measured them just fine with a DE-5000, and they were very close to their nominal values. I wonder if it's not some sort of translation error.
Here's an interesting article about hysteresis distortion from Purifi. It's more about amps and speakers, but would certainly apply to inductors in crossovers.
Hysteresis Distortion
Hysteresis Distortion
Steel laminate => transformer-style stampings of thin steel usually <1mm.
Iron powder => Powdered iron mixed with glue (or thereabouts) to reduce effective permeability through internal (distributed) gapping.
There are certain iron powder core grades with good linearity and little hysteresis like the -2 (red colour code) from Micrometals/Arnold used in Class-D output chokes. However, such cores would still require a lot of turns to make crossover (large value) chokes, that they may or may not be able to accommodate, according to the size (window area).
All cored inductors can saturate at some point. However, if the core is an open geometric structure (e.g. I-core, drum core etc.), the resulting saturation currents can be very high (almost unreachable) due to the large air-gap between the ends of the core. The price paid is electromagnetic interference due to fringing flux. For example, in an I-core choke, 50% of the magnetic path length is made of air, which means that the flux is all over the place, just as it is in an air-core choke. Thus, there doesn't appear to be any advantage to using such cores, as opposed to an air-core inductor, at least not at usual audio power levels.
Any perceived change in bass etc. due to steel / ferrite / powdered iron cores is possibly due to harmonic distortion effects of the non-linear B-H curve.
Iron powder => Powdered iron mixed with glue (or thereabouts) to reduce effective permeability through internal (distributed) gapping.
There are certain iron powder core grades with good linearity and little hysteresis like the -2 (red colour code) from Micrometals/Arnold used in Class-D output chokes. However, such cores would still require a lot of turns to make crossover (large value) chokes, that they may or may not be able to accommodate, according to the size (window area).
All cored inductors can saturate at some point. However, if the core is an open geometric structure (e.g. I-core, drum core etc.), the resulting saturation currents can be very high (almost unreachable) due to the large air-gap between the ends of the core. The price paid is electromagnetic interference due to fringing flux. For example, in an I-core choke, 50% of the magnetic path length is made of air, which means that the flux is all over the place, just as it is in an air-core choke. Thus, there doesn't appear to be any advantage to using such cores, as opposed to an air-core inductor, at least not at usual audio power levels.
Any perceived change in bass etc. due to steel / ferrite / powdered iron cores is possibly due to harmonic distortion effects of the non-linear B-H curve.
Saturation will occur in any ferromagnetic or ferromagnetic core material inductor so there is a power limit, there will also be hysteresis effects, where energy is lost in reversing the polarisation, this causes distortion of the waveform, more so a high frequencies. The best option unfortunately is to use air core inductors, as the air is in effect paramagnetic saturation does not occur, and there is no hysteresis effect. The downside is that the air has a very low susceptibility to external magnetic fields caused by current flow through the inductor, so a much longer wire is required for the same inductance. So to reduces changes to the Q of the system you need to use a thick guage inductor, or compensate for the change in Q due to the resistance with a larger box volume or more box stuffing.
I'm surprised there aren't metallic glass cored inductors for audio as they have no long range crystal order they have very low magnetocrystaline anisotropy constants allowing high susceptibility with next to no hysteresis effects.
I'm surprised there aren't metallic glass cored inductors for audio as they have no long range crystal order they have very low magnetocrystaline anisotropy constants allowing high susceptibility with next to no hysteresis effects.
If you are concerned about component crosstalk the toroidal core inductor will contain the produced magnetic fields better as it has a closed loop domain.
The iron or steel laminate core inductor may make a woofer sound better for simply it has lower DCR, and hence bass is a bit more powerful vs air core which will have a longer length of wire. If one controlled the DCR to be constant between both air and iron core that would be an interesting test.
if I absolutely positively need to use a cored inductor, it would be a laminated steel "I"core type with the largest core i can find and only use it as a LP series inductor on a large woofer with a low xover frequency. Everything else gets an air core inductor and a litz wire type if used in a midrange LP circuit. Every other non-air cored inductor I've tried imparts distortion, which is audible at higher level transients. It can sometimes sound like the amp is clipping or the recording level is overdriven. Alot of people will go through the hassle of changing out all the electrolytics in a crossover and leave those nasty little powdered iron cored inductors in place. That's a foolish and uneducated thing to do, yet I see it done so many times.
Foil inductors in a series LP circuit can make a woofer sound tighter and even more so with a litz wire inductor. I personally wind my own inductors after having bad luck with consistency and the higher price of ready made inductors. Its not as hard as people think, but you have to soak the inductor in some potting agent like polyurethane to keep it from buzzing under higher drive levels.
Foil inductors in a series LP circuit can make a woofer sound tighter and even more so with a litz wire inductor. I personally wind my own inductors after having bad luck with consistency and the higher price of ready made inductors. Its not as hard as people think, but you have to soak the inductor in some potting agent like polyurethane to keep it from buzzing under higher drive levels.
Ben, have you played with the Jantzen C-cores yet? If so, what are your thoughts?? Not so concerning anymore now that steel laminates are pretty readily available again, but I (think I) noticed a slight improvement in bass on a larger woofer using that C-core over a steel laminate. Not sure why and maybe that was just in my head.
I used a pair of 3.0mH C-Coils on the EMP TMM towers to use in a notch at 200Hz. I don't notice the imparted 'slam' of the typical I-cores on this pair of speakers, so the C-Coils must not influence toward exhibiting the same characteristic near the in box resonance of the drivers used.
In this case, the Lowpass was much higher at 1.8kHz, and the C-coil was in parallel with a CR circuit to take out the broad peak, kind of like first order 6dB filtering in parallel. It worked really well, but needed 650uF of capacitance. Being the C-Coil was influencing the signal below the notch, that was my point of focus and I like what I hear.
A few years ago, I used the 1.5mH C-Coils in the Scandivifias project as the BSC coil on a series xover. I did not hear anything detrimental there either.
I have not used C-coils in the coil comparison jig that I contrived for the Cecropia project. The values are far enough off that it might not be a valid test.
I think they are worth it in the bass range as explained by the manufacturer. I am not certain it would be better than air-cores in the upper passband of a woofer for a 2-way.
In this case, the Lowpass was much higher at 1.8kHz, and the C-coil was in parallel with a CR circuit to take out the broad peak, kind of like first order 6dB filtering in parallel. It worked really well, but needed 650uF of capacitance. Being the C-Coil was influencing the signal below the notch, that was my point of focus and I like what I hear.
A few years ago, I used the 1.5mH C-Coils in the Scandivifias project as the BSC coil on a series xover. I did not hear anything detrimental there either.
I have not used C-coils in the coil comparison jig that I contrived for the Cecropia project. The values are far enough off that it might not be a valid test.
I think they are worth it in the bass range as explained by the manufacturer. I am not certain it would be better than air-cores in the upper passband of a woofer for a 2-way.
I just recently built a new test load specifically to test the power handling and saturation currents of various inductors and the effect of the inductance value and load impedance on said current rating.
I've posted some preliminary harmonic distortion measurements that compares baseline hysteresis distortion between the inductors types. I'm working on the high power level saturation tests now.
That is posted here:
https://www.mtg-designs.com/tips-tricks-tests/inductor-testing
But when it comes to actually hearing a difference between inductor types in listening I've ever only noticed one when swapping out two inductors that have a sizeable difference in DCR (such as an 18 gauge air core and 18 gauge laminated I-Core) where said difference in DCR causes a measurable change in the frequency response as well.
I've posted some preliminary harmonic distortion measurements that compares baseline hysteresis distortion between the inductors types. I'm working on the high power level saturation tests now.
That is posted here:
https://www.mtg-designs.com/tips-tricks-tests/inductor-testing
But when it comes to actually hearing a difference between inductor types in listening I've ever only noticed one when swapping out two inductors that have a sizeable difference in DCR (such as an 18 gauge air core and 18 gauge laminated I-Core) where said difference in DCR causes a measurable change in the frequency response as well.
Very cool project! One thing to note is that the wirewound / cement filled resistors generate a lot of self distortion when used as dummy loads for amp power testing. I have measured this myself and they can add several percent of THD. The best low distortion high power loads are either series parallel banks of metal thin film resistors or a large flat film low inductance power resistor such as the EBG UXP 300/350/600 series. They handle 300w and have a large flat surface for active cooling with a heatsink.
https://www.ebg-resistors.com/en/products/ultra-high-power-resistors
I haven't noticed this or at least my amps aren't clean enough to notice this issue. When I measured the Aiyima A07 into that 4 ohm resistor bank at 1w/10w levels THD was below 0.01% (0.003% and 0.005% at 1kHz). With my bridged Behringer A500 when at the 200w level it was still near 0.01%. I needed to use my inuke 3000 for the high power testing, it's a bit more noisy but i'm still measuring below 0.5% at 800w which goes down to ~0.1% or less below 1khz. This is lower then the distortion levels the magnetic core inductors generate, so it's not really issue while testing them.
Everything is a compromise.
Cored / No gap => Hysteresis / Non-linearity / Early saturation / Few turns / Excellent flux containment
Cored / Small gap => Hysteresis / Non-linearity / Controlled saturation / More turns / Fringing flux from gap (maybe shielded if desired)
Cored / Large gap / Open geometry => Hysteresis / late saturation / Even more turns / Poor flux containment
Air-core => Lot of turns / Poor flux containment
Among the cored ones, Steel => LF only, anyone interested in details, please check the following:
https://en.wikipedia.org/wiki/Permeability_(electromagnetism)#Complex_permeability
Air-core appears to be the wisest with minimal issues. Orthogonal placement could solve the flux problem.
Cored / No gap => Hysteresis / Non-linearity / Early saturation / Few turns / Excellent flux containment
Cored / Small gap => Hysteresis / Non-linearity / Controlled saturation / More turns / Fringing flux from gap (maybe shielded if desired)
Cored / Large gap / Open geometry => Hysteresis / late saturation / Even more turns / Poor flux containment
Air-core => Lot of turns / Poor flux containment
Among the cored ones, Steel => LF only, anyone interested in details, please check the following:
https://en.wikipedia.org/wiki/Permeability_(electromagnetism)#Complex_permeability
Air-core appears to be the wisest with minimal issues. Orthogonal placement could solve the flux problem.
For lowest distortion, use air cored coils. If this results in too small an inductance for a required specification, such as physical size or wire resistance (because more turns will be needed compared to an iron or ferrite core) then it may be necessary to consider using a magnetic core of some description. When designing cores which are intended to provide an inductance as opposed to transformers, an air gap is usually essential to minimise saturation, since a non-gapped core will have a high inductance but not be able to pass much current, A.C. or D.C. without saturation at some point. It is similar to designing a Class A output transformer with DC current flow. You have to decide what the maximum possible AC current to be conducted is, and aim to achieve the inductance with a low flux away from saturation. Typically for high saturation iron like 0.35 mm GOSS (1.7T) I would not push the core flux any further than 0.6T. at peak current. Stray fields can be controlled better using EI cores where the gap is internal to the E, but may mean you have to deliberately cut some cores to achieve that. Normally the air gap is achieved with a simple spacer (thickness being half the required gap as this makes two gaps in the magnetic circuit) between the E and I lams, and that can work with a small stray field.
If you consider buying ready made inductors, you should check, carefully, the specifications. Open ended cores (such as a straight I core with open ends) should have almost unlimited saturation, but for closed cores, when specifying X henries at Y amperes you need to know the magnetic levels of field compared to the core maximum to have an idea of how close to saturation, hence distortion, this may be.
If you consider buying ready made inductors, you should check, carefully, the specifications. Open ended cores (such as a straight I core with open ends) should have almost unlimited saturation, but for closed cores, when specifying X henries at Y amperes you need to know the magnetic levels of field compared to the core maximum to have an idea of how close to saturation, hence distortion, this may be.
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Sure it woud. Any volunteers? Ok, I can help a bit.
For one air force core 8.2mH:
awg 8;
magnet wire mass =10.3kg;
bobbin diameter 70mm;
inductor diameter 286mm;
inductor thickness 1";
No. of turns=246;
No. of layers=33;
Wire length=137.3m
Re=0.283;
This testing verifies my suspicions and the fact I don't like ferrite core inductors at all. 7th order harmonics appear to be the most objectionable sounding of all odd order HD components IMO. The higher quality steel laminate heavy gauge models are my favorite for use in lower xover points. Otherwise I'll always use air core if it fits in the.given location available. I can share with you a severe case of distortion with a ferrite core inductor used in a LP midrange circuit, making sax and vocals sound almost like it had autotune mixed in with it. It was the inductors fault for sure being it was gone after changing it out with air core. Problem was it needed to go into a tight space, so it was wound very shallow but wide. Not an ideal solution but was a night and day improvement nonetheless.
I used 18awg icore coils and 14awg air cores and foils when i did my testing to minimize dcr differences. The steel laminate definitely imparted an upper bass emphasis where the other 2 did not. The people present did prefer the air cores over the other 2, and even though the Pcores also were an option present and at lower dcr, they were the worst of the bunch audibly.
I've finished generating graphs for all the the additional inductor testing and updated the pages to include the new data for those who find it useful:
https://www.mtg-designs.com/tips-tricks-tests/inductor-testing
https://www.mtg-designs.com/tips-tricks-tests/inductor-testing