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Getting Started: Filters

If you’ve been following our Getting Started series, you’ll know that we’ve covered a lot of topics. However, most of them have been focusing on utilities: utilities are the tools we use to make our patches behave the way we want them to. However, timbre, or how an instrument sounds, is just as important as how a patch behaves, so today we’ll be looking at how we can use filters to change how things sound in our patches.


If you read our first Getting Started article about oscillators, you’ll know that we generally start any sound with one of a few basic waveforms. However, if you listen to the sound examples of those basic waveforms, you’ll hear that they can be somewhat limited, and may not fit into a piece of music unchanged.

Enter filters: if we have a harmonically complex wave like a saw or a square, we may want to take part of its harmonic information away to make it more appealing. That’s a whole lot of words to say that if something is too brash, harsh, or buzzy, we can use a filter to make it less brash, harsh, or buzzy, and fit into our sounds better.

In Eurorack, there are tons and tons of different filter modules, but they generally build off of one of a few basic types. Most commonly, you’ll see lowpass, highpass, bandpass, and notch filters. And luckily for us, their names describe what they do: for instance, a lowpass filter lets low-frequency content through and cuts out high frequencies. A highpass filter lets high-frequency content through, but blocks low frequencies.

Image shows 4 graphs that demonstrate frequencies created by Lowpass, Highpass, Bandpass, and Notch filters

Let’s listen to what these sound like. Here, we’ve run a saw wave through a filter, and swept the filter frequency, with increasing resonance each time (more on resonance later).





However, just eliminating a set of frequencies won’t always be what we want. Parameters vary, but most traditional filters you’ll find will have at least two basic controls: frequency and resonance. Frequency control sets the point that the filter gets rid of things: for instance, on a lowpass filter, the frequency will set the maximum frequency allowed through. If you set it to 500hz, everything lower than 500hz will pass through, and everything above 500hz will be silenced. Most filters will also have CV inputs that allow you to control their frequency. Often, we’ll control this with something like an envelope or an LFO, or multiple modulation sources using a mixer.

A resonance control emphasizes sounds around the cutoff frequency, and is part of the reason filters of the same type sound different. At its core, resonance is a feedback loop: as you turn up the resonance on a filter, more of the output is fed back into the input and the frequency that the filter is set to is emphasized more and more. Since each filter is built a little bit differently, that feedback will sound a little bit different from module to module.

Some filters can even self oscillate: that is, when their resonance control is turned up high enough, they can generate sound on their own. Generally, this will produce a sine wave -- this can be surprisingly useful if you need an extra oscillator in your patch.

Other things that go into making filters sound how they do, like how sharp the curve of filter things off: this is known as slope, and is measured in “decibels per octave”. Basically, all that means is that some filters cut off frequencies more aggressively than others do, and that changes how they sound.

Image shows four graphs that show the slope of different frequencies caused by filters

There’re also a lot of different filter designs, and they all have a different character to them -- even if two filters share the same specs, they’ll often sound different. I have lots of filters in my system simply because, well, I really like them -- they all sound different and picking out a different filter for every patch brings me a lot of patching joy. (Why, yes, I am a huge synth nerd!)

Now, that was a lot of talk, so let’s get to some sounds.


We can talk technical details about modules all day long (I know I have once or twice…), but, of course, these instruments are made for music. Let’s explore what a filter can do with a classic brass patch. Emulating brass instruments perfectly is difficult, but we can take inspiration from brass timbres and emulate some of their most distinctive characteristics with a saw wave and a lowpass filter.

First, let’s hear what our sequence would sound like without any sort of filtering:


Saws are my favorite type of waveform, but we could definitely make things more interesting with a modulated lowpass filter. Take a listen:


Here, we’re using an envelope to modulate our filter’s frequency, and we’re also sequencing its cutoff frequency to add some extra expression to our sequence. The flavor and expression of this sound comes from both the filter cutting out part of our saw wave, but also its resonance: we’re using about 30% resonance to make our sound a little bit more dramatic.

Why’d we use a lowpass filter here, instead of, say, a bandpass filter? A bandpass filter rejects frequencies above and below the cutoff point, and lets a thin band of audio through. Let’s hear what this exact patch would sound like using a bandpass filter instead:


This is still a really cool sound, but as you can hear, it’s very different from our previous result: this might fit well into our patch, or maybe the lowpass version is a better choice. As with anything in modular, there are lots of creative choices to be made when using a filter!


While filters are fantastic creative effects, they can also be used when mixing elements in a patch. Many traditional EQs feature low or high cut bands, which are basically highpass and lowpass filters, respectively. For example, if you’ve got a rockin’ bassline, but your kick just isn’t coming through, try using a highpass filter on your bassline to just take out enough sub to let your kick through. It can make a big difference!

Looking for modules?

Here are just a few modules that can perform some of the functions we've been chatting about. There are always lots of choices in modular!

Noise Engineering Viol Ruina: 24dB lowpass filter and distortion

Doepfer A-124 Wasp Filter: 12dB LP/HP/BP/notch filter, with a very unique sound. 

Bastl Cinnamon: A small LP/BP/HP filter with overdrive. 

Intellijel Polaris: An extremely versatile filter that can be configured to be a huge number of filter types and slopes. 

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