Practical tone circuit with R4 attached

In practical circuits , another resistor core is often added as shown in Figure 1-75 . The purpose of adding R4 is to make the selection of treble
turning frequency more free and has little impact on the bass adjustment circuit. For example, for a tweeter circuit , Figure I-75 can
be simplified to Figure 1-76(a). For the convenience of analysis, Rl, R2, and R are converted into triangle connection mode in Figure 1-76(b)
.