About This Circuit

This is the capacitor-topped bridged-T, the mirror image of the resistor-topped version that has its own calculator. Here the capacitors C2 and C3 form the top arms of the T, R4 loads the middle of the T to ground, and the bridge across the pair is resistive: Pot1 in series with R3. The extra trick in this circuit is where the op-amp listens. The inverting input taps the pot's wiper, so the feedback point slides along the bridge as you turn the knob.

Both ends of the curve are pinned at 0 dB by the same physics as the resistor-topped version, just with the parts swapping roles. At DC the caps are open, nothing in the network can conduct, and the stage is a unity follower. At very high frequencies the caps are shorts, which ties both ends of the bridge to the output, so the wiper has to sit at the output voltage and the stage is a follower again. In between, R4 steals current through the capacitor arms, the op-amp has to work to hold the wiper at the input voltage, and you get a boost.

The division of labor is clean, and it's why this version earns a pot. The parts set the center frequency, and at these defaults the peak sits near 123 Hz whether the knob is at 0 or 100. The wiper sets the depth: at the C2 end you get the full boost, about +10 dB with the stock values, and as you sweep toward the R3 end the peak flattens smoothly to nothing. One knob, one job. That's a boost control you can put on a pedal's face. Like its resistor-topped sibling, this topology only boosts; the passive network can't push the gain below unity.

Response modeled with an ideal op-amp and the bridged-T network alone (Pot1, R3, C2, C3, R4), solved with the wiper tap at its actual position. The header formulas are the closed forms for the wiper at the C2 end; the plotted curve and readouts are computed from the full network at the wiper position you set. The fixed coupling and bias parts set the stage's own low-cut corners and are not part of this curve.