Third order active filter

Passive crossovers have simple circuits, are more economical, and have better performance. They are currently
the crossover network used by most civilian speakers. However, the power loss of the Yuanyuan crossover is large, the levels of each frequency band cannot be
controlled independently, and the damping of the low-frequency speaker is affected by the inductor coil. What's more serious is that changes in speaker impedance
affect various characteristics of the crossover. These shortcomings of passive crossover limit its application in some highly demanding situations.
    Electronic frequency division divides the sound signal into several frequency bands, and then sends the signals of different frequency bands to the corresponding
power amplifiers to drive the speakers of the corresponding frequency bands. It can be seen that the cost of electronic frequency division is high and the system is complex, requiring the same number of frequency divisions. Multiple
power amplifiers, but electronic crossover also has many advantages. It has performance that is difficult or even impossible to achieve with passive crossover.
    Electronic frequency division has the following advantages compared with passive frequency division:
    (1) Electronic frequency division reduces losses and reduces intermodulation distortion between each frequency band. The level of each frequency band can be adjusted independently
, making it easy to match various speakers.
    (2) Electronic crossover is a speaker directly driven by a power amplifier, which has good damping and reduces distortion. This is more beneficial for low frequency band
playback.
    (3) The frequency dividing characteristics of electronic components are not affected by changes in speaker impedance, and the sound quality is significantly improved compared to passive frequency dividing.
    The electronic frequency divider is a probe filter, generally including Butterworth filter and Bessel filter. Among them, the Bessel filter
has good transient characteristics and ideal loudness characteristic curve.
    Figure 6-27 is the circuit schematic diagram and calculation formula of second-order, third-order, and fourth-order Butterworth and Bessel filters.
    In the same filter, the resistor R in the low-pass filter should take the same value. , for example, R should all be 4,7~
10k,fl, and the value of the capacitor C in the high-pass filter should also be equal. For example, C can be 4700pF~O.01p:F. In the calculation formula
, the unit of R is n and the unit of C is F.
    Bandpass filters can be obtained by combining highpass and lowpass.
    With the emergence of high-performance and high-power integrated power amplifier circuits, it is no longer difficult to produce electronic frequency-division power amplifiers, and the price is not very
high. When we make electronic frequency dividers, we must use high-quality components, and the values ​​of resistors and capacitors must be accurate to ensure
the performance of the frequency divider and reduce system distortion and noise.
    For the convenience of use, we can select a few more crossover points and switch them with switches to adapt to
speakers with different performances. For the selection of the crossover point and attenuation slope of the
    electronic crossover, please refer to the passive crossover .