This is a HI-FI tone control circuit with high Z input. This circuit is used to control HI-FI tone. Here is the circuit:
HI FI Tone Control Circuit High Z Input circuit schematic
To buffer op-amp operated feedback type tone control circuit, this circuit uses 2N3684 JFET. The 2N3684 JFET is implemented on this circuit because it has low noise characteristics and provides the function of a high input impedance. [National Semiconductor Application Note]