Pole Mixer

One ladder filter, every slope. The Pole Mixer is built on the same 4-pole transistor ladder as the Ladder Filter, but instead of only listening to the last stage it taps all four and blends them — so a single Response knob morphs continuously from a steep 24 dB/oct low-pass, up through bandpass, all the way to a 24 dB/oct high-pass. Reach for it when you want to sweep the character of the filter, not just its cutoff, in one gesture. (Oberheim Xpander / Doepfer A-107 territory.)

How it works

Four one-pole low-pass stages are cascaded inside a single tanh-saturated resonance loop, exactly like the Ladder Filter. The trick is that each successive stage is one more 6 dB/oct of low-pass, so mixing the stage outputs in different proportions reconstructs whole new filter shapes — a bandpass is the difference of two stages, a high-pass is the input minus the low-passed stages, and so on. These are the classic Oberheim "pole-mixing" combinations, and they're scaled so every response comes out at roughly the same loudness.

The Response knob crossfades across five anchors — LP4 → LP2 → BP2 → HP2 → HP4 — in four equal segments. At each quarter turn you get the pure response named on the readout; in between you hear a smooth blend of the two neighbours, giving a continuous low-to-high tilt through bandpass.

Controls

  • Cutoff — the corner frequency the whole filter is built around; tracks 1V/oct, so it can be played like a pitch. Default sits around 4 kHz.
  • Res — emphasis at the cutoff. Low for gentle shaping, high for a vocal peak, all the way for self-oscillation (cleanest in the low-pass zones).
  • Drive — input gain into the ladder. Clean at 0; as you push it the filter saturates and thickens, with makeup gain keeping the level roughly steady.
  • Response — the pole-mix morph. Sweeps LP4 (steep low-pass) → LP2 → BP2 (bandpass) → HP2 → HP4 (steep high-pass); the readout names the nearest zone.
  • FM — attenuverter for the fm CV input; depth and polarity, centred is off.

Inputs & outputs

  • In L — the audio (or CV) signal to filter. Patch this alone and it's treated as mono: one ladder does the work and the same result appears on both outputs.
  • In R — the right side of a stereo pair. Patch it and the right channel gets its own ladder — its own poles and resonance feedback — driven by the same Cutoff, Res, Drive and Response settings, so the two sides stay in step instead of smearing.
  • V/Oct — 1V/oct pitch control of the cutoff, for keyboard- or sequencer-tracked filtering.
  • FM — cutoff modulation; depth and polarity set by the FM attenuverter.
  • Res — resonance modulation; sums straight into the resonance amount.
  • Out L / Out R — the mixed, morphed filter output. If you feed stereo in but only cable Out L, the two sides are folded back down to mono so nothing goes missing.

Tips

  • Patch an envelope or LFO to Response territory by automating it in the rack — sweeping from low-pass through bandpass to high-pass is a sound a single fixed filter can't make.
  • Sit Response mid-way (around BP2) and add Res for sharp, resonant bandpass tones — great for wah-style movement and percussion.
  • Keep it in the LP4/LP2 zone with high Res and nothing on In L, then play V/Oct: like the Ladder Filter, it sings as a sine voice.
  • A touch of Drive in the high-pass zones adds bite to hats and snares without muddying the low end.