VR2 is for low frequency while VR3 is for high frequency adjustments. You can add a balance control at the output, off the PCB, in my opinion this control is scarcely adjusted. VR1 is the volume control for adjusting the level of input signals simultaneously for both channels. A flat frequency response can be obtained at mid positions of the bass and treble controls. The mid range frequencies are not affected by the tone controls. The second 1/2 op amp is configured as an active filter amplifier whose feedback loop consists of passive components that adjusts the bass and treble frequency response. The circuit consists of the first 1/2 op amp as an input buffer amplifier providing a gain of about 2 and isolation from any source impedance. The schematic diagram is showing only one channel but the PCB layout was developed for 2 channels. This project uses 2 LME49720 op amps for both channels. In this project you can adjust the bass and treble frequencies simultaneously for both channels with the familiar cut-flat-boost positions of the controls. In my opinion, adding a tone control circuit into any audio system simply improves frequency response and provides more pleasing sound. The LME49720 combines extremely low voltage noise density with vanishing low THD+N (0.00003%) to easily satisfy the most demanding audio applications. Combining advanced leading-edge process technology with state-of-the-art circuit design, the LME49720 audio operational amplifiers deliver superior audio signal amplification for outstanding audio performance. Here's a brief description of LME49720 from Texas Instruments: The LME49720 is part of the ultra-low distortion, low noise and high slew rate operational amplifier series optimized and fully specified for high performance, high fidelity applications. 2-Channel Tone Control Circuit based on LME49720 Dual High Performance and High Fidelity Audio Operational Amplifier
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