For many, their first encounter with a chord oscillator was, well… the original Qu-Bit Chord. Although fairly limited and large, for a long time there weren't many other options that made generating chords as quick and easy as the Chord. But with the revised Chord v2, Qu-Bit vastly expanded on the original while cutting the size in half, making it much more appealing as other options appeared on the market. Most importantly, it kept the super straightforward interface intact, meaning that even if music theory isn't your strong point, lucious wavetables and suspended 7th chords are only a few knob turns away. The individual outputs for each chord tone and the optional true polyphonic mode are nice touches as well.

Chords can be found in other modules though, not just the Chord, and might be more appealing depending on workflow and compositional preferences. The ever versatile Plaits by Mutable Instruments (as well as its many third-party successors) contains a chord mode that works rather similarly to the Qu-Bit Chord, among the many other things it can do. Another option is the 4ms Ensemble Oscillator, though it's worth mentioning that it will quantize internal pitches to scales rather than chords. Some scales do actually take the shape of triads, and of course chords can be formed by carefully setting the spread control within your desired scale, but the Ensemble Oscillator also goes a step further and breaks free from the confines of western 12-tone equal temperament tunings. Just intonation, 5TET and 7TET, and even some of the tunings explored by composers like Wendy Carlos make an appearance.

Closeup of the Minimoog's three-part Oscillator Bank

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Contrasting with chord oscillators, a lot of multi-oscillators are truly (or mostly) analog through and through. Consider the arrangement of oscillators on a classic synth like the Minimoog—while its three VCOs share internal pitch control input from the keyboard, each oscillator can be set to a different waveform and two of them offer fine tuning controls to deviate from the base pitch of the first oscillator. In this sense, while there's still only one note being played, the frequencies of each oscillator can be offset to create power chords, major or minor triads, or whatever three-note chord voicing you'd like. Many of the multi-oscillators in Eurorack can function in similar ways, but the open nature of patching greatly expands on the possibilities!

As far as actual modules go, Doepfer's A-111-4 Quad Precision VCO is about as straightforward as it gets. Each of the four oscillators offers tuning and independent 1V/Oct input, routings for frequency or pulse-width modulation, sync inputs, and dedicated outputs for triangle, sawtooth, and pulse waveforms. But it's the master section at the bottom which ties everything together, providing the base pitch reference for all four VCOs, cascading pitch and modulation CV inputs onto the other oscillators, and summed mix outputs for all the various waveforms. Sure, this could also be done with individual modules, but at only 18hp and under the $500 price point, you'd be hard pressed to accomplish the same tasks as effectively, considering you would need not just four oscillators, but also a buffered multiple and mixers to account for the internal routings.

Of course, because sequencers and 1V/Oct pitch sources are abundant in Eurorack, refining the focus of the module and removing physical knobs while retaining CV inputs can reduce HP size without sacrificing on sound or function. Instruo has another multi-oscillator named Saïch, boasting four sawtooth oscillators for pseudo-supersaw sounds, but the module contains an internal quantizer that can lock oscillators two through four into intervals beyond the base frequency set by the coarse and fine tuning controls. In a way, Saïch thrives in the middle ground between multi-oscillators and chord oscillators.

It should also be noted that in some scenarios, complex oscillators can behave as multi-oscillators, with the added benefit that their sophisticated internal routings can aid in generating some wild timbres. Modules like the Make Noise DPO and Frap Tools Brenso even provide their respective Follow and Integrator circuits to impart variance in the response of both oscillators to one shared voltage source. But most complex oscillators are only two VCOs, so if you want to have your multi-complex oscillator cake and eat it too, there's perhaps no better choice than the Trident from Rossum Electro-Music. With its three oscillators and generous modulation options, you never have to choose between harmony and complex intermodulation, and using both at the same time will lead you to compelling sonic destinations.

Among the more prolific choices, Rings by Mutable Instruments could be a large contributing factor for the popularity of physical modeling in Eurorack. Not only does it offer impressive versatility as a sound source and as a processor, another reason for its abundant success can probably be attributed to designer Émilie Gillet's clever implementation of polyphony, with distinct modes that provide one, two, or four voices. By patching a V/Oct source into the pitch input without using the external gate input to excite the resonator(s), Rings will automatically generate a trigger when it detects a change on the incoming pitch voltage, making it a great choice for smaller systems where there's only room for a small sequencer or just a pitch quantizer.

Turning up the Damping control will increase the decay time for each active voice, so with careful construction of sequences it's totally possible to get interesting strumming and harp-like patches. And Rings isn't limited to string-like sounds as a real harp or guitar would be—different arrangements of the resonator's harmonics can be changed with the Structure control (of course, this isn't even considering what other things can be done with the sympathetic strings or non-linear string models). Regrettably, as of December 2022, Mutable Instruments has permanently ceased production—but their open-source designs mean that alternative versions of Rings are still available. After Later Audio's Resonate and Tall Dog's uRings SE are both clones of the original Clouds design.

By far the simplest approach to sample-based polyphony would be playing back full chords from a single-channel sampler module. Triggered playback and pitch transposition are essential for integration into most patches—luckily, modules like the 2hp Play or even Winterbloom's Big Honking Button after some firmware tweaks can do all of these things and more. Both the One and Play also offer the ability to select different samples with CV, so with proper curation and organization of audio files, harmonically coherent sequences of chords could be composed when paired with a sequencer.

But sampler modules in Eurorack are only getting more powerful as time goes on, and today anyone that makes extensive use of samples in their patches is likely to utilize one of the highly capable multi-channel samplers that are available. The largest differences are going to come down to personal workflow and patching needs, but the common thread between many of our favorites like ALM's Squid Salmple, the Rossum Assimil8or, and 1010's Bitbox Mk2 and Micro are that they each offer eight channels (16 in the case of Bitbox Mk2), which is plenty of room for sufficient exploration of drums, layering external sounds, or, for our purposes, polyphony. And of course, if your composition doesn't require the use of every channel for your chords, the sampler can load up any other audio file in those channels to fill whatever roles are missing from your patch.

Forming chords with stacked fifths

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But another advantage of multiple channels is that we don't need to rely upon samples of full chords to achieve polyphony, and reducing our sample content down to one or two voices per audio file offers more flexibility in generating chords and progressions on the fly. For example, using the same sample of a synth pad playing a perfect fifth on two channels but transposing one up a major third (or four semitones) results in a major seventh chord. Exploring stacked intervals in this way not only results in full fledged harmonic chords like the major seventh example, but several dissonances and neutral chords suggesting neither major or minor tonalities can be found and used to great effect.

Likewise, layering drones of single notes or making use of single-cycle waveforms is a further extension of this concept. This does effectively turn your sampler into a multi-oscillator, but with the distinct advantage of being able to use any sound as source material.

But for the ultimate in polyphonic sample playback, the Eurorack module that's perhaps among the best suited for the task has to be the Disting EX from Expert Sleepers, specifically running its SD Multisample algorithm. If you have any experience with using sample libraries and software instruments like Native Instruments Kontakt, the behavior on Disting EX is largely the same—a collection of related samples from an instrument like a piano, usually one audio file for each individual key, are mapped across a range of notes to be played by a MIDI controller or file. In this case, the Disting EX can read up to three incoming pitch and gate sequences to trigger notes with your favorite control voltage sequencer, and combining this with the internal chord and arpeggiator modes can result in some magical musical ideas. But being the highly flexible and wonderful device that it is, Disting EX can also respond to MIDI data when used with the MIDI Breakout Expander module or messages on the I2C bus, should you be using something like the monome Teletype.

Disting EX comes preloaded with stellar sample libraries from Spitfire Audio specifically curated by Expert Sleepers for the multisample mode, but loading in your own audio files is definitely encouraged as well. In fact, the Disting EX also offers a WAV Recorder algorithm with a multisample recording mode, so capturing audio within your system for the purpose of resampling into a polyphonic instrument is totally feasible and can lead to some highly personalized results.

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Perhaps the easiest way to approximate polyphony with a granular sampler or processor is to start by recording an arpeggio or sequence into its buffer. The key here is to experiment with grain duration and density while the buffer is being read back at different points, for although multiple grains might not be generated at the exact same moment in time, overlapping tones can stack and form harmonic structures. For more consistent or predictable performance, this may be more easily achieved using pre-recorded audio on modules that support granularization of external files, but improvised polyphony using continuous live input on a module like the (now-discontinued) Beads from Mutable Instruments can be a ton of fun as well!

As an extension of this approach, the Arbhar from Instruo contains six different buffers (referred to as layers) that can be dynamically recorded to and read from on a whim, almost like a granular version of our previous discussion regarding single and multichannel samplers. If you were to then arbitrarily assign one note to each layer, each note has a ton of additional breathing room to explore beyond holding a static sound at a particular frequency, such as using timbre or amplitude modulations. A dynamic control voltage source into the Layer CV input on Arbhar's included expander can offer a fun way to scan through grains of chord tones. In some ways, this would be no different from dividing one large buffer into several sections, but the pre-prepared delineation of recording space in such a way immediately offers alternative perspectives to inspire new ideas.

This method of creating instant chord stacks lends itself well to generative modular workflows, but companies like ACL and Instruo have taken the idea of a chord quantizers to a new level. ACL’s Sinfonion features a startling amount of pitch control, boasting three complete quantizer channels in addition to a chord generator with four outputs and an arpeggiator. Although the Sinfonion’s interface may seem intimidating at first, with a little practice it can be easily mastered and used to perform and create beautifully complex and musical sequences. While the Sinfonion certainly has enough options to generate any chord or sequence you could possibly dream of, Instruo’s Harmonaig approaches polyphony from a simpler, intuitively harmonic angle. The Harmonaig provides four voltage outputs each corresponding to a voice in a chord stack, users can then freely explore Instruo’s modal music system which allows for notes and voicings to be dialed in and restructured at the turn of a knob.