Doepfer A-124 VCF5 Wasp Filter

Modeled from the Doepfer A-124 hardware module, based on a vintage “Wasp” circuit using a non-linear 2-pole SVF (State Variable Filter) with OTA filter stages and CMOS buffers.

Demos

Solo bass with various cutoff and resonance:

Bass and lead with drum machine:

Features

• Light (Doepfer-theme) and dark (Wasp Special Edition) panels.
• Low-pass and high-pass with mix knob, with separate band-pass output.
• Extended cutoff range up to 30 kHz compared to 12 kHz of original circuit.
• +12/+24 dB input gain modes for increased filter drive.
• Polyphonic.

DSP

Doepfer A-124 for VCV Rack simulates a fully analog signal path including over 30 discrete components with over 60 component parameters. All 3 CMOS buffers are modeled using PMOS/NMOS complimentary pairs, allowing hardware-accurate saturation when driven at high levels.

Medium and high-quality DSP modes are available, using 1.5-3% and 2-4% CPU, depending on your machine.

Usage

To filter audio, patch an audio signal into the Audio In port, and listen to the BP Out (band-pass) or LP/HP Out (mixed low-pass / high-pass) output.

Adjust Lev. to set the input gain level, Frq. to set the filter cutoff frequency, and Res. to set the filter resonance. The Frq parameter tooltip displays the exact cutoff frequency in Hz.

To control the cutoff with CV, patch a CV signal into the CV1 or CV2 ports. The amplitude of the CV2 signal can be adjusted with the CV2 knob. The CV1 input tracks roughly 1V/octave, but due to the chaos of the analog circuit (especially with high resonance), the tuning does not perfectly match equal temperament. Both CV1 and CV2 inputs can be used simultaneously.

The Mix knob mixes the low-pass and high-pass signal for unique filtering effects. At intermediate values between the knob’s extremes, the LP/HP output functions as a variable shelf filter, with optional resonance. At the middle “5-5” position, the LP/HP output functions as a notch filter, creating interesting effects when resonance is used.

Settings

Right-click the panel to tweak module settings.

• Boost input: Adds +12 dB or +24 dB input gain to achieve filter saturation without the use of additional gain modules.
• Boost resonance: Allows self-oscillation when resonance is high by adding two resistors to the circuit model.
• High detail mode: Uses a more accurate filter model at the expense of a bit of CPU. Useful for patches that specifically highlight the sound of the Wasp filter.

Doepfer Changelog

2.0.3 (2023-08-21)

• Use Rack’s global dark panel setting to set theme.
• Build on Mac ARM64.

2.0.2 (2023-08-08)

• A-124 VCF5 Wasp Filter
  • Add “Hardware clone” tag and link to ModularGrid.

2.0.1 (2022-05-12)

• A-124 VCF5 Wasp Filter
  • Add “Resonance boost” mod setting.

2.0.0 (2022-05-01)

• Initial release.