Mod Squad: Rossum Electro-Music Evolution

A classic lowpass filter design takes on new dimensions
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Back in the early ’70s, Dave Rossum was co-founder and “Chief Wizard” at E-mu Systems, a company that evolved from producing analog modular synthesizers to pioneering a range of digital technologies.

The Evolution Variable Character Ladder Filter ($369 street) marks Rossum’s return to the analog synth world with Eurorack products that are exceptionally well-engineered and solidly built. As a resonant lowpass filter, the Evolution is based on the classic E-mu 1100 design that, in turn, improved on Bob Moog’s patented ladder concept. Though there are already plenty of ladder-based filters on the market, Rossum’s design adds ingenious touches that give users unprecedented control over signal processing.

For example, the input and output are DC-coupled, allowing you to process control voltages as well as audio signals. Running CVs through this module—especially while modulating its resonant features—will really get your creative juices flowing.

The Evolution goes easily into oscillation—with or without audio input—and Rossum’s design lets the module be used as a wide-range VCO when the filter is in resonance. The 1V/octave input is designed to track through 10 octaves accurately, and the module displays an especially hefty low-end. The cutoff frequency has two additional CV inputs for control, one with an attenuator and one with a bi-polar attenuverter.

The Q setting also has a pair of CV inputs (one with an attenuverter), as well as Q Compensation. The latter gives you control over the level of the passband below the filter’s resonant peak (whether the module is resonating or not). When the knob is fully counterclockwise, the timbre of the audio thins out as the filter goes into resonance, just as expected. As you increase the level of Q Compensation, you reintroduce frequencies that were reduced as resonance increased. Use it to add girth to the output or set it so that you don’t hear any decrease in the passband spectrum when going into resonance.

One of the most exciting features of the Evolution is its Genus parameter, which let’s you dynamically control the filter’s cutoff slope from 3-pole (18 dB/octave) to 6-pole (36 dB/octave) with a continuous transition. The Genus knob sets the slope when no CVs are used, or weights the modulation toward one extreme or the other when placed under voltage control. Two CV inputs are provided, and Genus CV 2 includes an attenuverter that can be used to narrow the modulation range. Switching the cutoff slope is particularly handy for changing the resonant tone, and modulating it can provide a variety of bubbly, ringing sounds.

Species, on the other hand, sets the input level going into the filter ladder, and it provides enough gain to seriously overdrive the circuit. I ran vocals and drum tracks through the Evolution and used Species to creatively shred the sound. Increasing the level of Species when the filter is in oscillation decreases the resonant tone. This offers loads of creative possibilities and yields a wide range of full-spectrum, buzzy textures, especially when rhythmically modulated.

All of these features add up to create a powerfully versatile filter. And while it can be used as a smooth sounding LPF, the Evolution has a musically satisfying edge to it once you start flirting with its resonant qualities—and that’s before you even touch the Species control. Nonlinearity is at the heart of the design, tracing back to its early-’70s roots. Yet, the Evolution reaches beyond vintage emulation and can sound as modern as you need it to be.