I can understand the benefits of having very low resistance inductors in crossovers: less losses and more accurate behaviour. In that sense, foil is a form factor which is an improvement over conventional round copper wire, allows for more compact coils, fully agree on that.
But I don´t understand why worry about coil potting material *at audio frequencies*, surely any difference must be infinitesimal, hard to measure if at all, and definitely below any threshold of audibility.
Do you guys have plots of , say, 3 coils, same inductance, same DCR, 3 different mechanical constructions?
As in: conventional round copper, foil potted in "plastic" , foil potted in "wax" ?
Thanks.
But I don´t understand why worry about coil potting material *at audio frequencies*, surely any difference must be infinitesimal, hard to measure if at all, and definitely below any threshold of audibility.
Do you guys have plots of , say, 3 coils, same inductance, same DCR, 3 different mechanical constructions?
As in: conventional round copper, foil potted in "plastic" , foil potted in "wax" ?
Thanks.
All that is needed is that the turns stay in place, which is mainly down to former shape and winding tension. Potting would need to use something a bit more substantial than wax.
I suspect nostalgia is playing a role here. 1940s and 50s electronics used wax for potting coils, but the aim was simply to keep moisture out.
I suspect nostalgia is playing a role here. 1940s and 50s electronics used wax for potting coils, but the aim was simply to keep moisture out.
Can cause inductive coupling: when using more than one inductor in a crossover, the electro-magnetic fields of the inductors can interfere with each other causing an unpleasant result. That is why it is best to keep the inductors as far apart as possible. Also, keep the fields out of phase with each other by rotating the inductors 90 degrees.
Has anyone characterized copper foil inductors? As frequency increases, proximity effect will tend to push the current to the edges, essentially starving the center of each foil. This will cause a non linear, second order resistance component, the inductor will have higher resistance during high slew rate of current. The only way to prevent that is with litz.
Edit: btw, this is exactly what happens with jelly roll capacitors if the foils are connected at opposite ends of the foils. They are best constructed with both leads at the same location. Otherwise, this proximity effect not only starves the foil cross section, it causes the dialectic along the center to not be involved in the current. It is one reason large electrolytic a can have it's capacitance fall off with frequency.
Also, another caution... Do not place any inductor against copper foil of a PC board, it lowers inductance vs frequency and increases resistance via eddy losses. As to wax vs plastic, it should only be affected by thickness and permittivity of the insulator, as it will create a distributed shunt capacitance through the inductor.
Jn
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Thanks Merlin and JNeutron for taking the time to provide info. I started my little cap upgrade on the crossover pcb board at the back of the speakers about 8 months ago but somehow it kept growing till I had to make an external board. I have 4 coils on my board and 3 of them are in the same plane but I made the board big and they are at 150mm centres so I'm hoping that shoud be sufficient?
Ah, Rod Elliot. He's actually not bad, most of his errors are not that significant, some are.
The inductor vibration thing.... Not an issue with a solid inductor unless the board it is mounted to can vibrate mechanically within the magnetic field of the woofer. In my experience, at high drive levels the external magfield of the woofer will modulate enough to affect CRT type tv's and monitors. That is usually due to keeping the magnetic structure small for cost, the trade off can be saturation of some of the iron and resultant stray fields. Another factor is, the saturation of the iron is second order to the music because it happens mainly when the vc is towards the back plate, not towards the front.
As to his statement on low resistance cables, he's concentration only on power loss and damping of lows. Soundstage creation and stability may be more relevant to the midrange system.
BTW, it would be nice to credit an author for their work, you did in your last post but not in your previous ones. I for one appreciate the link, I am always looking to learn stuff.
Jn
The inductor vibration thing.... Not an issue with a solid inductor unless the board it is mounted to can vibrate mechanically within the magnetic field of the woofer. In my experience, at high drive levels the external magfield of the woofer will modulate enough to affect CRT type tv's and monitors. That is usually due to keeping the magnetic structure small for cost, the trade off can be saturation of some of the iron and resultant stray fields. Another factor is, the saturation of the iron is second order to the music because it happens mainly when the vc is towards the back plate, not towards the front.
As to his statement on low resistance cables, he's concentration only on power loss and damping of lows. Soundstage creation and stability may be more relevant to the midrange system.
BTW, it would be nice to credit an author for their work, you did in your last post but not in your previous ones. I for one appreciate the link, I am always looking to learn stuff.
Jn
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If you have an inductance meter, you can easily test coupling. Measure one, and short the others. If they couple you will see the inductance go down on the measured coil. Measure each one in their frequency use range, although it would be stronger to measure all 20 to 20k.
On some crossover boards they fill unused areas with copper. Be aware that if you mount the coil flat against the copper it will alter the inductance and resistance vs frequency.
I have plots of this effect in my gallery somewhere. They are in the inductance test area. Sorry the text is a bit blurry, center of the R and L graphs are 1 kHz.
Jn
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Attached Troels Gravessen placement coils.
Attachments
Potting in varnish would be better, far more monolithic. Wax would deaden vibration, but might not keep the turns from moving relative to each other.
In all cases, litz will support better parameters vs frequency, independent of potting material. It's just a pita to solder to.
Oh, and if you make or buy potted litz inductors, be very very careful of the exit wire. If you bend it too much where it exits, individual strands can break, which will increase resistance based on fraction broken. Inductance will remai the same, but it is easy to break them.
A stranded wire inductor does not suffer that fragility, but does have a L and R vs freq dependence, not to mention packing factor.
Jn
Merlin, in future please present quoted material as a quote with an introduction as to the source (I know you posted a link). Rod is quite particular about copyright and he may ask for post #12 to be taken down.- Status
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