Resonant filters are super popular in the sythesizer space, but aren't often seen in guitar effects outside of wah pedals. With resonance, a standard RC low pass or high pass filter can also boost sounds near the cutoff frequency. Adding resonance to a pedal's tone control can really intensify the pedal's character and provides a frequency-selective boost that you can tune with the tone knob.

  
A resonant filter is a multi-pole filter with the filter's output amplified and fed back to the first stage of the filter. Adding one to a pedal circuit is fairly straightforward. First, it requires a second identical copy of the tone circuit. Importantly, a dual gang pot should be used for the now duplicate tone circuits so that their cutoff frequencies remain the same. Second, it requires a low impedance copy of the filter's output, which may already be present in the circuit. With very lossy tone circuits, an active buffer or gain stage may be required between the two copies of the tone stack.

  
The first image is an example of a Big Muff with resonance added to its tone stack. You can see the two identical copies of the tone circuit built around a dual gang pot to ensure the filters in each have the same cutoff frequencies. Between them is placed a make-up gain stage to compensate for the amount of signal attenuation. Where the first tone stack would connect to ground, it instead connects to a voltage follower tracking the output of the second tone stack. It is this positive feeback that causes the filter to resonate.

  
The second image is a simpler implementation in the Rat tone control. Because the standard Rat tone control is a simple low pass filter followed by a high impedance buffer, a second pole can be added without additional buffering or amplification, again using a dual-gang pot. For this example the output buffer has been replaced with a gain stage, the output of which feeds back to the first pole of the filter to create the resonance. You can see the resonance on the scope output in the simulator, where instead of appearing as a normal square wave the voltage appears to "overshoot" on the rising and falling edges and oscillates a couple of times before settling. The result is that the filter continues to reduce high frequencies, but now also additionally boosts mids around a center frequency of \~400 to \~2000hz that changes dependent on the setting of the tone knob.

  
You can mess around with these in Falstad for yourself:



Rat: [https://www.falstad.com/s.php?s=EYhvu9](https://www.falstad.com/s.php?s=EYhvu9)



Big Muff: [https://www.falstad.com/s.php?s=FasfGP](https://www.falstad.com/s.php?s=FasfGP)

Big Filter doesn't want you to know this, but you can add resonance to most pedal tone controls