Filters are one of the most common and useful module types. They typically dampen broad groups of (or, with feedback turned up, accentuate a select few) harmonics to help focus or clarify a sound. The frequency of these harmonics can be swept by a front panel knob or under voltage control, adding dynamic movement to that sound.
However, if you haven’t used filters before (or had to think about picking one for yourself, as most non-modular synths have a pre-selected one or two built in), they can be a bit bewildering: What mode or response do you want – high pass, low pass, all pass, band pass, notch, comb, or…? What slope or “number of poles” do you desire? (This determines how strongly they dampen harmonics the further they get away from the “cutoff” frequency.) And what the heck is “topology” (the internal circuit design – transistor or diode ladder, state variable, Sallen-Key, etc.) and what does that do to the sound?
Some of these choices make a huge difference in the final sound you’ll get; some result in slight variations that come down to matters of taste. I typically have 20(!) or so different filter in my large studio modular, so that I have quick access to a wide swath of these choices that match my personal tastes. But how does a beginner choose their first filter or two?
The following is what I recommend. It is very model-specific, and inevitably based on personal taste, but I think it provides a couple of strong starting points:
Your First Filter
I recommend beginners start with the Intellijel Polaris, which I use in my live performance case. It is small, reasonably priced, sounds good, and allows you to switch between a wide selection of filter modes and slopes. It has a nice selection of voltage control inputs, and has optional overdrive to explore clean vs. distorted sounds. Playing around its options should help you learn what you prefer in a filter (such as two poles versus four, bandpass versus high pass, etc.), which will inform future purchase decisions.
If for some reason the Polaris does not appeal to you or is hard to get, there are a few alternatives worth looking into. Many recommend the Doepfer A-106-6 XP Filter, as it emulates the 15 different filter mode and slope selections available in the legendary Oberheim Xpander. It’s only slightly larger than the Polaris, but lacks voltage control over resonance; the Polaris is cheaper in the US, while the XP Filter is cheaper in Europe. I covered it in my Eurorack Expansion course; click here for the playlist of movies I created for it If you prefer something a bit more esoteric, check out the Random*Source Serge Variable Slope filter, as it also has multiple modes, and a continuously variable slope. It is also available as a kit, if you like to build your own modules.
Your First Two Filters
If you have more room in both your modular case and your budget, I have two filters in particular that I tend to use the most often (at least as a starting point), and which would be my “desert island” filters if I could only choose two:
Doepfer A-106-5 SEM Filter: This is one of the best emulations I’ve heard of the filter in the much-loved Oberheim Synthesizer Expander Module (SEM). It is multi-mode (low pass continuously variable through high pass, as well as bandpass), with a two pole (12 dB/octave) slope. It’s also small and inexpensive. Most importantly, it just plain sounds good with a wide variety of sources. It was the “alternative” filter I demonstrated as part of my original Learning Modular Synthesis course; I also covered it in my Euroack Expansion course.
Rossum Electro-Music Evolution: This is a significant, high-quality update on the timeless Moog transistor ladder filter design. Although it offers “only” low pass mode, it features continuous control over the number of poles (slope) from 3 (18 dB/octave) through 6 (36 dB/octave), as well as voltage control over cutoff frequency, feedback, slope, and overdrive. It is not small and not cheap, but is akin to a Mercedes in that it just oozes quality. Click here for the playlist of movies I created for it; I also covered it my Eurorack Expansion course.
But Don’t Stop There
There are a wonderful world of many different filters to explore – and I encourage you to do so. As I mentioned earlier, I have ~20 different filters, and I love each of them for a particular reason; if you talk to users, each will have their own favorites as well. And new ones are coming out all the time.
Therefore, consider the suggestions above to be a starting point, not a final destination: The section on filters in Patch & Tweak (pages 162 through 171) will also give you an excellent overview of the different kinds available and how you might use them; the Eurorack Expansion course covers 10 different filters in depth and that list will be growing in the future.
Have fun exploring!