Any coil used in a crossover network has a certain amount of dc resistance — and this resistance will dissipate a proportion of amplifier power.
Thus, the dc resistance of the coils should be as low as economically possible. A reasonable compromise > where the amplifier power does not exceed 30 Watts continuous power output per channel is to keep the dc resistance below one ohm. ln this drawing the radius of the circular winding bobbin is shown as 'x' and all other dimensions are related to this. Construction of the bobbin is not critical and it can be made from cardboard, or a combination of a wooden core and cardboard cheeks. Nletal must not be used. The design procedure is as follows:- 1. Determine the bobbin size required.
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This is done by using Graph l. This graph 1 indicates bobbin size (‘x' measurement) required to accommodate coils of different sizes and dc resistances. For example a 5.5mH coil wound on a O.75" former (remember that this refers to the measurement shown as 'x’) will have a dc resistance of 1.4 ohms if wound on a
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The nearest standard size is 16G so this is the wire size used. 4. The `coil should be layer wound using enammeled copper wire. As the operating voltage is quite low, no interiayer insulation is required. Graph IV shows the dc resistance of the _coil given ·the wire gauge and former size, providing the former is filled completely. ln our case the resistance shown is 1.0 ohm — but as we have only 290 turns whereas the filled- bobbin accommodates about 350, turns the resistance would be about 0.8 ohm. This is sufficiently close to our design requirement and would be acceptable for a low power amplifier.