How this mid scoop works

This is a fully passive, variable-depth midrange scoop, and it is a cousin of the two bridged-T EQ calculators in this family. The signal enters through C1, a coupling cap, into node A. From there two routes run down to an internal node: R1 straight across (the bridge), and C2 in series with R2 (the top arms of the T), with the output tapped at the junction between them. Unlike the EQ versions, the T's far end is not the output: instead a leg of R3, C3, and the depth pot runs from it to ground, and the output sits in the middle of the top arms.

The behavior falls out of what the caps do at each end of the band. At low frequencies C2 is open, so the only path to the output runs through R1, and C3 is also nearly open, so the leg shunts almost nothing away: bass passes at close to full level. At high frequencies C2 is a short and couples the input straight across to the output tap: treble passes at unity no matter where the knob sits, above the corner set by R2 and C2 (about 1.6 kHz stock). In between, C2 is partly conducting and C3 has dropped enough that the leg pulls the internal node toward ground, and the output dips. That dip is the scoop, and with the stock values it lands right on the low-mid honk region.

The pot is wired as a rheostat, wiper tied to one end, so its in-circuit resistance sweeps the full track from 0 Ω up to the marked value (a tied wiper does not halve the range; the shorted section simply carries no current). The signal never passes through the pot on its way out: the pot only decides how firmly the leg pulls that internal node down, which is why it works as a depth control instead of a volume or a sweep. It is not a pure depth control though. With the stock parts the notch runs from about 7.8 dB deep at 522 Hz with the pot at zero, to 3.6 dB at 330 Hz at half track, to 2.4 dB at 261 Hz at the full 25k, so backing the depth off also slides the center down. The "C" on the schematic's pot is a reverse-audio taper, which changes how the knob feels through its travel but not its endpoints.

Everything here assumes the output feeds a high-impedance stage, a buffer or an op-amp input, which is how passive networks like this are used; hang a low-impedance load on it and the numbers shift. There is no fixed part on this page: all seven components are yours to move, and the three resistors all run down to true 0 Ω jumpers. The dashed curve is the nearest-E12 build with the nearest standard pot value.