Micro Filters (µAP, µNotch, µBP, µHP & µLP)

Cutoff Knob Sets the filter’s cutoff frequency.
FM Input Linear audio-rate FM modulation input, with FM depth knob.
Q Knob Sets the quality factor, or resonance.
Filter Order Selector 1, 2 or 4 for low-pass, high-pass and all-pass (6, 12 or 24 dB/oct slope), 2 or 4 for band-pass and notch (6 and 12 dB/oct slope)
Trim Knob Output level adjustment.
Inputs Signal to be filtered (stereo).
Outputs Filtered signal (stereo).

Overview ⚓︎

The micro (µ) filter collection covers all filtering needs with low-footprint panels and a straightforward feature set.

While the State Variable Filter and the Ladder Filter modules have a warm and musical saturation and provide creative tone shaping controls, the micro filters were designed for accuracy and transparency. They are ideal utility filters that can cleanly handle up to 100V peak-to-peak signals internally. Unlike the aformentioned filters, they don’t self-oscillate even with a high Q factor.

The micro filters have two channels, so they can easily be used in stereo patches. The two channels share the same settings. For mono signals, use the left input and output jacks.

All micro filters share the same features, except for the filter order selector:

The band-pass (µBP) and notch (µNotch) filters are available in second- and fourth-order topologies, as indicated by the 2 and 4 LEDs. The second-order filters have a slope of 6 dB/oct on either side of the cutoff frequency, while the fourth-order filters have a slope of 12 dB/oct. Click on the push-button to switch between the two topologies.
The low-pass (µLP) and high-pass (µHP) filters are available in first-, second- and fourth-order topologies. The first-order filters have a slope of 6 dB/oct, the second-order filters have a slope of 12 dB/oct and the fourth-order filters have a slope of 24 dB/oct. Click on the push-button to switch between the three topologies.
The all-pass (µAP) filter is available in first-, second- and fourth-order topologies. The filter order changes the amount and the steepness of the phase shift around the cutoff frequency.

The Q knob has no effect on first-order filters.

Implementation detail: the fourth-order filters are implemented by cascading two second-order filters.

Filter Types ⚓︎

µLP: Low-pass Filter ⚓︎

The low-pass filter attenuates frequencies above its cutoff frequency.

Low-pass filters are the main tone-shaping filters in most analog synthesizers.

µHP: High-pass Filter ⚓︎

The high-pass filter attenuates frequencies below its cutoff frequency.

High-pass filters are often used to remove lower frequencies from a signal to reduce low-end rumble and increase headroom for higher frequencies. When used like this, it is often called a low-cut filter. A high-pass filter set below 20 Hz is perfect for removing the DC component that can be introduced by rectification or by some waveshaping modules like the Saturator with high bias.

µBP: Band-pass Filter ⚓︎

The band-pass filter attenuates frequencies both above and below its cutoff frequency.

Sweeping a band-pass filter creates a wah-wah effect.

µNotch: Notch Filter ⚓︎

The notch filter is the opposite of the band-pass filter: it cuts the signal at its cutoff frequency, while frequencies above and below get gradually louder, thus creating a V-shaped notch in the signal’s frequency content.

The Q control is counter-intuitive on notch filters. Usually, increasing the Q creates a very obvious emphasis at the cutoff frequency and adds more character to the filter. For notch filters, increasing the Q makes the frequency notch thinner, so the effect of the filter becomes more subtle.

Sweeping a notch filter creates a nice effect similar to a phaser.

µAP: All-pass Filter ⚓︎

The all-pass filter lets all frequencies pass through at their original volume. This may sound completely useless, and it will probably be if you simply plop it at a random point in a signal path and never touch it. However, the all-pass filter introduces a phase shift centered around the cutoff frequency.

Modulating the cutoff frequency will introduce a vibrato effect for the frequencies around it. This can be used as a building block for a phaser effect.

DSP geeks can use the all-pass filter in mid/side or parallel processing to compensate for phase shifts introduced by other filters running in parallel.

Equalizers ⚓︎

There are no shelving or peaking equalizer filters in the micro filter collection, but the EQ Filter module fills this role with the same design philosophy.

FM ⚓︎

The FM input expects a ±5V signal. When the FM amount knob is set to 100% (fully clockwise):

a 0V input applies no modulation,
a -5V input takes the cutoff frequency down to 0 Hz (frozen output);
a +5V input doubles the cutoff frequency.

Filter FM sounds better when the Q is above 1, and becomes alive when an envelope or LFO is used to sweep the cutoff frequency.

Implementation detail: when the cutoff frequency is above 11 kHz, the maximum FM depth is gradually lowered to prevent it from going beyond 22 kHz.