Rerurn to Romy the Cat's Site

rickmcinnis wrote:

Having tried many types of coils nothing was as transparent as the NORTH CREEK 8 gauge air core coils. I do not think you will be happy with the JANTZEN or any other using a metal core. They are much cheaper than the NORTH CREEK and look better. I, too, have used this combo approach to get a crossover curve and found it works well when one wants to avoid more amplification stages to get a sharper slope.

Rick, thanks for the opinion.  I generally would agree: nothing beats air core coils. There are a few “howevers” of to look at the subject a bit deeper.

If we are use a very low crossover frequency then the air core become huge, really, a true barrel of copper. The inductance of could and driver, combined with capacitance of the cables and HUGE air coil create a perfect resonating detector. Combined with a driver high sensitivity one might pick radio stations and all imaginable noises right in this detector. So, to do the things properly as 10mH air core 8ga coil will be 15” in diameter and near 60 points in mass. Now, to shield this coil you would need ½ inch steel. The problem is that if you locate such a sarcophagus too close to the coil then the core will talk with it and you have no air core anymore. So you need to locate the shield no closer than let say 5-7” from the end of the coil. So, we are taking about a table size enclosure for a single coil. Do not forget that that I gave an example of 10mH air core 8ga coil but in my case I would need 30mH – I will not be able to lift up such a coil. Alternatively the shield might be done with co-netic like alloys that have 800 times more shielding capacity. Prepare to pay for the shield much more then you paid for the coil. That all combined with the fact that such a coil can’t effectively filter and the inductance of driver’s VC will crew up file in the driver’s peaks.

Do not forget that even 8ga coil the DCR of the coil is large as it has a LOT of wire, 10H of the air core will have .4R. The toroid coil with metal core will have literally a few turns of wire and will have DCR of .02R. You get purity or air core but you lose dumping as you have too high DCR. Also, I am pretty confident that all air-core advantage exist only at HF, at lower frequency it is very questionable.

How I know all it? I when over it, not once and it made me to feel OK with metal core for LF. In my case the main roll off will be by line-level filter but I can wire more curves in my amp. I need one very soft first order and I think line level will do fine for now. It possible to add a filer in plate voltage but it would require to change my amp that it a bit complex without it.Rgs, Romy the caT

Rerurn to Romy the Cat's Site