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HIGH-LEVEL 12 dB/OCTAVE CROSSOVER DESIGN CHART
Values selected for typical loudspeakers
and their associated impedance characteristics.
FREQUENCY 4 OHMS 8 OHMS 16 OHMS
in Hz CAP. COIL CAP. COIL CAP. COIL
|---------------|-------------------|-------------------|-------------------|
| 50 | 564 18 | 283 36 | 141 72 |
| 60 | 472 15 | 236 30 | 118 60 |
| 70 | 400 12.8 | 200 25.7 | 100 51.4 |
| 80 | 354 11.3 | 177 22.6 | 88.4 45.2 |
| 90 | 313 10 | 156 20 | 78.1 40 |
| 100 | 283 9.0 | 141 18 | 70.7 36 |
| 120 | 236 7.5 | 118 15 | 59 30 |
| 150 | 188 6.0 | 94.5 12 | 42.2 24 |
| 200 | 141 4.5 | 71 9.0 | 35.4 18 |
| 250 | 113 3.6 | 56.4 7.2 | 28.3 14.4 |
| 300 | 94.5 3.0 | 47.2 6.0 | 23.6 12.0 |
| 350 | 80.5 2.57 | 40.3 5.14 | 20.1 10.3 |
| 400 | 71 2.26 | 35.4 4.52 | 17.7 9.0 |
| 450 | 62.5 2.0 | 31.3 4.0 | 15.6 8.0 |
| 500 | 56.4 1.8 | 28.3 3.6 | 14.1 7.2 |
| 550 | 51.3 1.64 | 25.6 3.28 | 12.8 6.56 |
| 600 | 47.2 1.5 | 23.6 3.0 | 11.8 6.0 |
| 700 | 40 1.28 | 20 2.57 | 10 5.14 |
| 800 | 35.4 1.13 | 17.7 2.26 | 8.8 4.52 |
| 900 | 31.3 1.0 | 15.6 2.0 | 7.81 4.0 |
| 1000 | 28.3 0.9 | 14.1 1.8 | 7.07 3.6 |
| 1200 | 23.6 0.75 | 11.8 1.5 | 5.9 3.0 |
| 1500 | 18.8 0.6 | 9.45 1.2 | 4.22 2.4 |
| 2000 | 14.1 0.45 | 7.1 0.9 | 3.55 1.8 |
| 2500 | 11.3 0.36 | 5.64 0.72 | 2.83 1.44 |
| 3000 | 9.45 0.3 | 4.72 0.6 | 2.36 1.2 |
| 3500 | 8.05 0.26 | 4.0 0.5 | 2.0 1.0 |
| 4000 | 7.1 0.22 | 3.55 0.45 | 1.77 0.9 |
| 4500 | 6.25 0.2 | 3.13 0.4 | 1.56 0.8 |
| 5000 | 5.64 0.18 | 2.83 0.36 | 1.41 0.72 |
| 5500 | 5.13 0.16 | 2.56 0.33 | 1.28 0.66 |
| 6000 | 4.72 0.15 | 2.36 0.3 | 1.18 0.6 |
| 7000 | 4.0 0.13 | 2.0 0.26 | 1.0 0.5 |
| 8000 | 3.55 0.11 | 1.8 0.23 | 0.88 0.45 |
| 9000 | 3.13 0.1 | 1.56 0.2 | 0.78 0.4 |
| 10000 | 2.83 0.09 | 1.41 0.18 | 0.71 0.36 |
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All capacitor values are in microfarads ( uF ).
All coil values are in millihenrys ( mH ).
NOTE: as the chart shows, values scale up and down directly, in
octaves, for changes in impedance of half or double, and by factors of
ten for frequency factors of ten. To determine crossover points higher
or lower than those shown on the chart, select the proper impedance (4,
8, or 16 ohms) then look up the frequency in the left column that is
ten times or one tenth of your desired frequency and simply move the
decimal points to the right or to the left to determine the correct
values. Higher frequencies require moving the decimal points to the
left, and lower frequencies require moving the decimal points to the
right.
EXAMPLE: 8-ohm crossover for 80 Hz subwoofer will require a 177 uF
capacitor and a 22.6 mH coil.
CROSSOVER INDUCTORS
Wire gauges used for winding air-core inductors should be chosen
so that crossover-associated power losses are minimized. Look at the
wire table to get an idea of the approximate added resistance you will
have when using a particular wire gauge to wind inductors. This added
resistance not only spoils amplifier damping factors, but can also de-
tune vented low frequency loudspeaker systems. Use the heaviest gauge
double formvar insulated wire possible for winding inductors. If you
are buying ready-made inductors, don't be misled; the coil in the 80 Hz
subwoofer example above would be 100-200 mm (4"-8") in diameter and
weigh from 2 to 8 kg (4-16 lb) depending on wire gauge. An RF coil
designed for use in radios may have the same inductance (at high
frequencies) and be wound on a pencil-sized bobbin with fine wire.
This type of inductor will definitely not work in a crossover with
loudspeaker loads connected.
Make sure your coils are wound tightly with neatly stacked layers
which produce a winding cross section that is approximately square.
The bobbin used to wind coils for crossovers should have an inside coil
diameter which is twice the coil width or thickness, and the outside
coil diameter should be wound out to four times the coil's width, or
twice the inside diameter. After winding, saturate the coil in laquer
or resin. After drying, wrap it with stretchable cloth tape and give
it another coat of laquer. Keeping the layers stacked neatly and the
coil cross section square, helps minimize the wire resistance for a
given inductor value.
Use a mounting board made of non-magnetic material such as
masonite or plywood. Place coils on your crossover mounting board so
that all coils are facing each other at right angles to minimize
magnetic inductive coupling. Some distance between coils helps reduce
coupling also. Keep any coils away from large metal surfaces. Place
insulating tubing over coil leadout wires to prevent shorts in case of
speaker system vibration. On long leadout wires, clamps or blobs of
hot glue should be used to dress wires securely to the mounting board.
CROSSOVER CAPACITORS
Capacitors used in crossovers MUST BE NON-POLARIZED and for best
sound must have low self-inductance. High-quality 100-200 volt rated,
foil wound Mylar capacitors should be used and should be individually
parallelled by 0.01 microfarad polypropylene or polystyrene capacitors
to short out the high-frequency self-inductance of coil formed by the
many layers of foil winding inside. Capacitors should be placed on the
crossover mounting board away from heat-producing power resistors.
Capacitor placement is not critical in terms of electrical operation of
the crossover, however, capacitors should be attached so that speaker
system vibration will not break their leads by bending and metal
fatigue.