Crossovers are generally used with loudspeakers having multiple drivers. Loudspeakers with single drivers are unable to cover the entire frequency range i.e from low to high frequency. Each driver cater to a particular frequency band. Crossover circuit splits the signal into different frequency band which is then send to 2 parts of a loudspeaker. High frequency signal is send to tweeter part and low frequency signal is send to the woofer part.
There are two types of components which can be used in crossover circuits. One of them is passive components which include inductor along with resistor and capacitor. Passive components are generally not preferred at high ratings as the inductor becomes bulkier. For low frequency application, more number of turns of wire must be used which in turn adds to the series resistance degrading inductor's performance, that is, low Q, resulting in high power loss.
Active components, on the other hand, uses operational amplifier along with resistor and capacitor. The active filters, by enclosing a capacitor in the feedback loop, avoid the use of inductor. Active crossover always require 2 amplifier for each output band which proves to be costly but the advantage given by it is far more better than of passive crossovers. Operational amplifier has the advantage that it provides gain. So the input signal is not attenuated as in case of passive filters. Operational amp offers high input impedance and low output impedance. This improves the load drive capacity and the load is isolated from the frequency determining network. Because of the high input impedance large value resistors can be used, thereby reducing the size and cost of the capacitors required in the design. By using operational amplifier, one can easily vary the frequency for a particular driver. Transient response of a system is improved by using active crossover as amplifiers are directly connected to the drivers of the speaker. But main disadvantage of active crossover is that the high frequency response is limited by gain bandwidth product and slew rate of operational amplifier ( Choudhary, 2007 ).
2nd order crossover circuit is better than 1st order circuit for the loudspeaker. This is because they can produce maximum flat amplitude response and requires relatively few components as compared to first order filters. First order filters can produce minimum phase response but main disadvantage is that its 6 db/octave slope is too often shallow to prevent modulation distortion, especially at a tweeter's resonance frequency. Second order filters have the phase difference of 180 degree between the outputs of high pass and low pass filters. This 180 degree phase shift is accommodated by reversing the polarity of the tweeter as it produces minimal or no lobbing in the coverage pattern. Second order filters have 12 db/octave slope (or 40db/decade). The crossover frequency for the high pass filter and low pass filter should be same. This is done to achieve a smooth overall response and to avoid lumps or dips.
In case of a low pass filterdamping plays an important part. For a heavily damped filter, the response is very stable. However, the roll-off begins very early to pass band. As damping coefficient is reduced, the response exhibits overshoot and ripple begins to appear at the early stage of pass band. If damping coefficient is reduced too much, the filter may become oscillatory.