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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.
Mid-drivers
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.
Tweeters
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.
Visit my DIY loudspeaker page.
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