Constructing Crossovers
 


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 former 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 lacquer or resin. After drying, wrap it with stretchable cloth tape and give it another coat of lacquer. 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 paralleled 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.