R2 and C1 form one branch of an attenuator redrawn as XL1, and C4 and R5 form the second branch, equivalent to XL2. Across the audio spectrum, the impedance of XL1 and XL2 will vary, offering different amounts of attenuation at different frequencies.The impedance of C2 is chosen at the treble frequency, 20KHz to be small compared to R2, and at the bass frequency, 20Hz, C5 is chosen to have a high impedance. The result is a constantly changing impedance network, as the effective values of XL1 and XL2 vary. You can think of the network as a frequency dependant attenuator, and the frequency response of this single network is shown below:
Cascading two more of the same stages, results in a more pronounced peak at the bass and treble frequencies. As all three stages are passive, the op-amp is needed to provide the necessary gain. Gain in the op-amp is set by R9 and R10, C9 in parallel with R9 rolls off the high frequency response above 20kHz.Input Impedance
The op-amp is made by Analog Devices, type LT101. If not available, other low noise op-amps can be used instead such as LF071, NE5534, etc. All plots made with LTspice.
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