Simplified form of practical tone circuit with R4 attached

Figure l-75 can be simplified to Figure l+77 0. It can be seen that although a resistor R4 is added to the input end of the amplifier ,
the input impedance of the amplifier device is very high and the open-loop amplification factor is very large, so the current ij of the input loop All will flow through the feedback loop , that is, ij = ff, and the current flowing into the device will be almost zero. Therefore, the feedback effect of R4 on the bass adjustment circuit can be ignored. Note: The above assumptions are based on the extremely high amplification factor of the amplifier and the extremely high input impedance of the device . Integrated operational amplifiers can easily meet this condition. If a transistor amplifier is used, Gaul tubes should be used as much as possible.