Coils for loudspeakers
Copyright 2007-15 Troels Gravesen

Advice for beginners in building loudspeaker crossovers

This page to try answering some of the most often asked questions about which coils to use in crossover networks. Basically I don't think you get much better coils than those perfectly wound from round copper wire. Foil coils may be marginally better when wound very hard and soaked in wax.
Wire must be baked to reduce microphony. "Baked" means the coils have been heated after winding to soften the lacquer covering the wire so the windings will stick together. Heating can be done in an oven or applying heavy low-voltage current for a short period of time.
If you wind your coils yourself, you made submerge the coils in lacquer - preferably under vacuum to make the lacquer penetrate all the windings, making a rock-solid coil.

Bass section:
Usually you should use coils with as low DCR as possible*. DCR = the resistance of the coil in ohms. If you only have one single coil in series with the bass driver, you may get good results from DCR = 0.3 - 0.4 ohms. The lower the better. Usually I use coils wound from 1.2-1.8 mm wire.
The cored coils should be made from non-ferrit material and I like them a lot. They have high power handling; they are small and they have very low DCR resistance, thus do not ruin the damping factor of the bass driver and provides very low loss in sound level. The higher resistance in series with the bass driver, the more power is converted to heat rather than sound.
*: If you want to up-grade an old crossover with new coils, measure the DCR (Ohmic resistance) of the coils you intend to replace. Sometimes, even for bass, relatively high DCR of the coil is intentional in tuning the system. The lower the better is not always true.

Coils in series with the middriver can well have a DCR of 0.2-0.5 ohms. Quite often the middriver has series resistors for attenuation and sometimes a series coil can have a very high DCR to provide attenuation. However, unless you have proper measuring equipment, keep coil DCR below 0.5 ohm and use resistors for attenuation. I suggest air-cored coils wound from 0.8-1.0 mm wire.

Most often we have a single coil in parallel with the tweeter, e.g. 2nd and 3rd order filters, and anything from 0.2-0.5 ohms usually goes goes well here. However, these coils are usually small and getting 0.15-0.30 ohms coils is no problem. I suggest coils wound from 0.5-0.8 mm wire.

Notch-filters = LCR circuits
These filters consist of a coil, a resistor and a capacitor and are usually placed in parallel to the driver. As a resistor is usually included, the DCR of the coil can be quite high. If the resistor is e.g. 5-8 ohms or above, the coil DCR can well be 0.5-1.0 ohm. No need to buy humongous air cored coils for LCR circuits. It's a serious waste of money. I suggest cored coils wound from 0.8-1.0 mm wire.

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