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Circuits

The transistor ladder filter

Ten transistors stacked like the rungs of a ladder, a capacitor across each rung, and one current that sets how fast they all follow: a four-pole low-pass that rings when you turn up its resonance. Here it is as a circuit, every part in its parts list.

The transistor ladder filter's schematic: a pair of transistors at the bottom on a shared tail, four pairs stacked above them with a 68 nF capacitor across each, the bias resistors down the left, the output buffers and difference amp at the right, and the exponential converter under the tail.
The schematic draws it as a textbook does: the pairs stacked, the driver at the bottom.

1Part by part

  1. Step 1: Two transistors, one current. The driver pair, Q1 and Q2: the sound tips their shared current from one side to the other. (larger)
  2. Step 2: A capacitor across each rung. Each rung passes the current up; each capacitor fills only as fast as it lets it. Four of them: 24 dB an octave. (larger)
  3. Step 3: Resonance: out back in. P1 feeds the output back into Q2 through Rfb. Past about 83% it sings on its own. (larger)
  4. Step 4: A volt an octave. The exponential converter: Q12 copies Q11’s reference current; 18 mV more at its base doubles it. (larger)

2It plays live

Its transistors are written 2N3904 simple, and it runs at oversample 1x: three voices take about a fifth of a core on a fast computer.

Try it

  1. Open Ladder Sweep and hold a chord.
  2. Turn Resonance past 83%: it whistles at its cutoff.
  3. Turn Cutoff: the whistle follows.
Open the example sound: Ladder Sweep ↗ (opens in a new tab)

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