Intro to Eurorack Part II: The Enduring Character of Analog Filters

Knowing where to start is the biggest problem for most beginners to modular synthesis. Despite the overwhelming amount of modules available, only a few manufacturers offer complete systems. If cost is a concern, though, this is where the Eurorack format excels. When building up a system one or two modules at a time, I recommend getting a good filter as early as possible.

The single thing electronic musicians seemed to miss the most as synthesizers transitioned into fully digital instruments was the sound of analog filters. To address a complex discussion quickly, a filter is an audio processor that can attenuate and/or emphasize the timbre of a sound. Depending on the type of filter, this can be understood as altering brightness, darkness, or spectral content.

A page from Bob Moog's original 1969 Ladder Filter Patent

The gold standard of analog filters was invented by Bob Moog, otherwise known as US Patent 3,475,623, filed in 1966 and issued in 1969. Bob Moog's 24dB cutoff per octave transistor ladder design is the cornerstone of subtractive sound synthesis techniques. What began as the 904A Low Pass Filter and 904B High Pass Filter modules is the heart of Moog synthesizers to this date.

The beauty of a low pass filter for a modular beginner is that it is easy to understand with your ears. An audio signal is passed through the module. Turning up the frequency cutoff knob makes the sound brighter gradually until it is audibly the same as the original source. The resonance, or emphasis, boosts the signal specifically at the point where the cutoff is set. A traditional Moog filter also doubles as an audio source. By turning the resonance all the way up, the circuit will feedback and produce a sine wave, with the pitch determined by the position of the frequency cutoff.

As the synthesizer was increasingly embraced throughout the 1970s, upstart companies had to develop their own take on a filter design to not infringe upon Moog's patent. The resulting variety of filter circuits developed during that time period offer wide sonic variations, which is a boon to a sound designer and musician. 30 years later, as the Eurorack format began to likewise gain traction, early entrants made their mark adapting Moog alternatives into a contemporary context.

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Analogue Solutions and the SY02

Established with the necessity their name implies, UK based Analogue Solutions began offering equipment modifications, standalone instruments, and Eurorack modules in the late 1990s, making them the third Eurorack company behind Doepfer and Analogue Systems. By 2000 Tom Carpenter's company had released the Concussor system of analogue drum voices based on classic Roland instruments like the TR-808, TR-909, and CR-78, alongside MIDI to CV modules and synthesis building blocks. One of their finest pieces is the SY-02, a take on the Korg MS-20 filter.

Analogue Solutions SY02 Multimode Filter

Korg originally produced the patchable MS-20 between 1978 and 1983. Although this instrument is unique in a few different ways, it is the filter section that makes it stand out. Korg released two different versions of this filter in the MS-20.

The first, based around the Korg 35 proprietary chip, is notable for introducing noise as resonance levels increase, providing overdriven timbres with distinctively cutting results. The revision of the filter design used an off the shelf op-amp IC, which provides a smoother result. Both versions offer the same filter architecture, a 6dB/oct high pass and 12dB/oct low pass wired in series, allowing for a type of variable bandpass response. Both filters offer independent voltage control and resonance for incredible sound design possibilities. The signal could be overdriven via peaking the resonance at two different points, which paired with the not-quite-properly scaled CV inputs to yield wild sonic results.

The SY-02 utilizes this basic design with some variations, specifically that it consists of a 12dB/oct HPF for a more dramatic filtering of lower frequencies. There is an input level control which will overdrive the filter starting around 12 o'clock. The module also includes a simple VCA, originally hardwired to the filter output. This module was revised around 2005, adding a quarter inch output jack to the now independently controllable VCA, making the SY-02 a perfect main output for a compact modular system.

The Livewire Frequensteiner Revives the Synthasystem Filter

Eventually interest in the Eurorack synthesizer spread to the United States, and in 2005 the manufacturing and design consortium EAR Group was established. Pairing the designers Mike Brown and Peter Grenader and their respective brands Livewire and Plan B, EAR Group began offering fresh takes on more unusual historic circuits as well as innovative new designs. Grenader's association with engineer and musician Nyle Steiner led to the updating of a classic filter design. Steiner is most well known for inventing and performing on the EVI and EWI electronic wind controllers originally produced by his namesake company Parker-Steiner starting in 1975. The company also produced the Synthasystem modular and Synthacon fixed architecture monosynth. At the heart of both was Steiner's 12dB/oct multimode filter.

Livewire Frequensteiner

Steiner's design was deceptively simple, and could utilize just four discrete transistors in an array to produce multiple filter types. The schematic was published in the magazine Electronic Design in December 1974, and was subsequently taken up as a common project in the DIY community. Two things about this filter make it stand out. First, unlike the Moog low pass filter, bass frequencies are not attenuated at high resonance levels. Second, the filter offers switchable low, band, and high pass responses at the input, with a single output. It was determined that a revived and revised Steiner filter developed with the inventor himself would best fit the Livewire line, and the Frequensteiner was released in 2005.

The Frequensteiner is primarily derived from the Synthasystem. What is notable about this filter, and many other analog circuits for that matter, is that there are perceptible “flaws” in the design. In the case of analog synthesis, oftentimes a flaw is actually a feature, which gives the circuit the character that makes it stand out.

In the case of Steiner's filter, this manifests in the resonance portion of the design, which, much like the Korg 35 version of the MS-20, introduces noise into signal at high Q levels. This filter will go from a wet and saturated sound to full-on howl with the right combination of initial input levels and resonance settings. Sadly, Mike Brown passed in 2012 after a prolonged illness. Fortunately, the instruments and ingenuity live on—Monorocket's Steve Rightnour has recently reissued the Frequensteiner with other Livewire modules are soon to follow.

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The Harvestman Peers Behind the Iron Curtain

After their lauded debut with the Malgorithm voltage-controlled bit crusher, the Harvestman took a different tact with their equally reputation establishing second module. In October of 2007, the Polivoks VCF was released, and with it, the complete replication of an analog synthesizer of decidedly different pedigree. As the excitement for electronic music spread across the globe, so too did it come to Russia. Soviet economic policies kept Western synthesizers out of reach, and thus homegrown alternatives were developed. Between 1982 and 1990, at the Formanta factory in the Urals, the Polivoks was produced to meet the needs of the musicians behind the Iron Curtain.

The Harvestman Polivoks VCF

The Polivoks, regarded as the “Russian Minimoog,” is a two oscillator duophonic synthesizer. Built from Russian military parts alongside things like radios at Formanta, the filter section boasts both low pass and band pass responses. Scott Jaeger of Harvestman offered some insight into the character of the filter design: “The filter design is well-known for its minimal componentry and the lack of discrete capacitors in particular. The core of the filter uses two programmable op-amps, six resistors, and a current sink. The op-amps are NOS chips from former Soviet states, similar to the LM4250. The old Soviet amplifier ICs are crucial to accurately reproducing the sound of the Polivoks, and I’ve secured a supply of various devices for use in the series.”

The Harvestman Polivoks design came about through working with original designer Vladimir Kuzmin, a connection made by Shawn Cleary of Analogue Haven. Cleary, who briefly lived in Russia had made the acquaintance of Kuzmin, and was looking for a manufacturer to produce the Polivoks designs.

Although new to analogue circuit design, Jaeger helped transform the Polivoks filter into a tremendously successful module, and was soon adapting the rest of the Polivoks from new schematics provided by Kuzmin. The line of modules known as Iron Curtain Electronics form the analog half of Jaeger's instrument line, and with the release of the Stillson Hammer Sequencer, the company has rebranded as Industrial Music Electronics. As for the sound of the Polivoks filter itself, Jaeger said, “As the resonance is increased, the sound becomes more unstable, with a rapidly 'wobbling' characteristic that follows the harmonics of the input signal. Higher input gains result in a musically pleasing distortion of the bass frequencies. The Industrial Music Electronics module offers increased input gain and improved control laws for the front panel knobs.”

Patching It Up

A great way to utilize a filter at the outset is to get an additional input module. Eurorack audio levels are much higher than line level, so an amplifier module will be needed to get the best results. One of the best solutions after all these years is the Doepfer A-119 External Input module. The A-119 is a great balance of price and features, and includes an envelope follower which can be patched to the filter cutoff for easy autowah type effects.

Analog filters are the single most integral sonic component in traditional synthesis. A well selected filter or two will take the modular beginner quite far in establishing a signature sound.

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