The equivalent impedance of the amplifier without adding R3

Capacitor cl shunts the output
signal of the feedback amplifier. In the middle and bass areas, ct is similar to open circuit t and does not
play a shunt role. As the frequency increases, the shunting effect of cl increases,
even causing cl to short-circuit Rt. In this way, the input resistance of the tone circuit will
be reduced from Ri + Rz to R20. Usually R1 is several times or even 1-o times larger than R2. Therefore,
the load is very different from the load in the treble, so that The gain of the amplifier decreases as the frequency increases. Obviously, at treble,
the gain reduction and treble boosting cancel each other out.