Adjustable crossover point balanced subwoofer circuit

This circuit can mirror and improve the low-frequency attenuation of the subwoofer, thereby evenly broadening the low-frequency response to
about 20Hz.
    Figure 9-75 is the schematic diagram of this circuit.
    This circuit consists of input level selection, equalization, frequency division, and promotion. Level indicator and power supply are composed of five parts.
    This circuit is equipped with two input terminals AB. The former is used for high-level input and can be connected to the main channel speaker output; the latter
is used for low-level input and can be connected to a preamplifier. line output. The cutoff frequency of the crossover circuit is adjustable in 4 steps, limiting the speaker's
playback of frequencies above 70Hz.
    ICla is an adder that mixes the stereo left and right channel signals through Rl, R or R3, R and
amplifies them by ICla. The amplification amount can be adjusted by RPi connected in the negative feedback loop, so RPi is the subwoofer volume control.
    IC2d and IC2,c form a low-frequency boosting circuit . IC2a ​​and its peripheral components form a second-order low-pass filter. Its output
is sent to the inverting auxiliary input terminal of IC.2c through island and island, while its non-inverting input terminal passes through C4, Rn is augmented with the full frequency signal from the ICla output
, so that IIC2c outputs a low frequency boosted signal. C3 and C4 respectively limit the signal bandwidth entering IC2c, coupled with
the additional correction effect of IClb input terminal Rzs~‰ and island, the low-end frequency response is finally broadened.
    IC2b and its input part RC components form a third-order low-pass Butterfly Si filter, its cutoff frequency can be selected by Sl
, which are 40H, 50Hz, 60Hz, and 70Hz from top to bottom. Changing the corresponding resistance value can easily set the cutoff frequency
to other required values.