Since this is an audio device, it is important to ensure that the outputs of the filters remain in phase. Since Bessel filters have the flattest group delay, they are the filter of choice for this design. We will also be using Sallen-Key topologies to implement these filters as we can build a second order filter with a single op amp. For the bandpass filters, we will be using a VCVS topology.
The signal for this design is split into four channels, corresponding to four different bands important to hearing. The first band consists of signals below 300Hz, so we use a standard low pass filter. This band corresponds to low frequencies that can be heard. The second band starts at 300Hz and continues to 3kHz. This band is selected to correspond to the voice range. The third band begins at 3kHz and extends to 8kHz. This corresponds to high frequencies that can still be heard. Finally, the fourth band extends beyond 8kHz.
The filter specifications in the schematic above are recommended for operation. If you would like to change the bands for your own needs, component values will have to be changed. If you are unfamiliar with the Sallen-Key topology, please read through this
as it will guide you through the calculation. If you wish to change the bandpass filters, this
is a useful tool to guide you through the process. Please note that the Q-factor for a Bessel filter is 1/sqrt(3). Please refer to this
for Q-factors of other recommended filters.