Ladder Filter

1. Cutoff Knob Sets the filter’s cutoff frequency.
2. Slope Knob Selects the filter slope for the main output. Between 6 and 24 dB/oct. Filter sounds brighter and distorts more easily with lower slope.
3. Reso Knob Resonance control. Boosts the frequencies around the cutoff point. Filter starts self-oscillating above 80%.
4. Drive Knob Increases the input gain up to +20 dB to distort the filter.
5. FM Input Exponential audio-rate frequency modulation input, with FM depth knob.
6. Audio Input Signal to be filtered.
7. 6 dB/oct Output Signal filtered with 6 dB/oct low-pass filter.
8. 12 dB/oct Output Signal filtered with 12 dB/oct low-pass filter.
9. 18 dB/oct Output Signal filtered with 18 dB/oct low-pass filter.
10. 24 dB/oct Output Signal filtered with 24 dB/oct low-pass filter.
11. Main Output Signal filtered with the slope specified by the Slope knob.

Overview ⚓︎

The ladder filter is a 24 dB/oct low-pass filter topology found in some of the most popular vintage subtractive synths.

Our model adds 6, 12 and 18 dB/oct outputs by tapping the signal at different points in the simulated filter circuit.

Overdriving the input with a hot signal or by using the Drive knob will introduce a smooth and musical saturation, and will add some grit to resonant filter sweeps.

In Depth ⚓︎

Inputs and Outputs ⚓︎

The Ladder Filter expects an audio source to be connected to its In jack, and will output the filtered signal to the five output knobs.

The first four outputs have a fixed slope, while the slope of the main output (labeled Out) is controlled by the Slope knob. Modulating the slope has an interesting effect somewhere between a tremolo and a filter sweep.

You will notice that the outputs with a softer slope not only sound brighter, but also distort more easily. Use them when you are looking for a grungy sound.

Resonance ⚓︎

The Reso knob controls the amount of filter resonance. In a ladder filter, increasing the resonance reduces the volume of the pass band.

The Drive knob or the gain knob next to the Out jack can be used to compensate for the volume loss caused by resonance.

Starting at 80% resonance, the filter will self-oscillate. This produces a sine wave at the cutoff frequency, even when nothing is connected to the input.

Unlike the State Variable Filter which is very stable and precise, the Ladder Filter has a non-linear behaviour, especially with high input drive levels or high resonance. For example, increasing resonance past 80% has the side-effect of lowering the cutoff frequency. When that happens, the frequency and note values displayed by the Cutoff knob will be inexact.