Active subwoofer circuit

This circuit uses an LM13700 operational amplifier connected to a second-order Butterworth low-pass filter. Its corner frequency can be adjusted by RPi to match the low-frequency lower limit frequency of
the main channel speaker
.     The schematic diagram of this circuit is shown in Figure 9-770 The audio signals input
    from the left and right channels are processed by a low-pass filter composed of Icl and J Xiao and then sent to the M21812, and then
divided into two channels. One channel shapes and limits the signal and converts it into a digital signal and then sends it to the two-way frequency divider circuit. The frequency of the signal is reduced by
half, that is, f1/2, and then converted into an analog signal through a low-pass circuit. The other signal is sampled for signal amplitude by the envelope detection circuit
in order to control the amplitude of the output f1/2 signal. After envelope detection, exponential transformation is performed, and
the voltage-controlled amplifier VCA is controlled by linearly increasing amplitude changes. The output of the VCA is a
signal with the same amplitude as the input signal and a frequency of f1/2. The subwoofer signal output by M21812
is mixed with the original sound source signal by a hybrid amplifier composed of ICID.IC2D and then output. The mixing amount is adjusted by the potentiometers WR and WL. If there is a subwoofer speaker
, the subwoofer can be taken out directly from pin 18 of M21812 Signal. If you want to form a 3D system, you can connect according to the dotted line in Figure 9-79.