The Bass Boost section provides two switchable levels of Bass enhancement. The input audio is first low-pass filtered by a second-order LPF, whose cutoff frequency is 100Hz. The LPF is built around IC2C and IC2DThe output of the LPF is given to a MUX based on a CD4052. This mux can choose between ground, an attenuated version of the LPF, and the full LPF output. The selected MUX output is summed with the original audio, yeilding a Bass Boosted signal. The outputs of this section are fed to the final power stage.
The trimpots R21 and R25 are used to set the bass level for the low-bass mode. This is usually not just the midpoint (50%) level, it’s somewhere like 70%. It can be adjusted to personal taste. IC2A and IC2B act as the summing amplifier and output stage driver. 100Î© resistors R11 and R16 are used to prevent the load capacitance from causing instability in IC2A and IC2B. 2Î¼F coupling capacitance is provided.