BBD Resonator Synthesis Explained: How Echon 6 Works

Picture a bucket brigade at an old-fashioned fire scene: one person fills a bucket, hands it to the next, who hands it to the next, and so on down the line. Each handoff takes a moment, and a little water sloshes out along the way. That is almost exactly what happens inside a bucket-brigade device, except the "water" is an electrical charge and the "sloshing" is the subtle noise and filtering that gives analog delay its character. Morphor's new Echon 6 builds an entire six-voice polysynth around that principle, and understanding the mechanism explains why the instrument sounds like nothing else on the market.

What a Bucket-Brigade Device Actually Does

A BBD chip is a chain of capacitors and switches. An input signal gets sampled and stored as a tiny electrical charge on the first capacitor, then that charge gets passed to the next capacitor in line on every clock pulse, and the next, and the next, until it reaches the output stage. Each transfer isn't perfect. A small amount of charge is lost or distorted at every step, which is why BBD-based delays and choruses have that familiar darkening, slight pitch wobble, and gritty warmth that digital delay lines don't naturally produce.

The clock speed controls the delay time. Run the clock fast and the charge zips through the chain quickly, giving a short delay. Slow the clock down and the delay stretches out, but the signal also degrades more audibly because each capacitor holds its charge longer and leaks more in the process. This clock-dependent decay is the single most important thing to understand about BBDs: the delay time and the tone are linked in a way that has no real digital equivalent.

BBD Resonators Versus Digital Delay and Karplus-Strong Emulations

Digital delay lines store samples in memory and read them back with essentially zero degradation. You can set a delay time, feed it back into itself, and get a clean, repeatable, low-noise echo indefinitely. That predictability is exactly why digital delay dominates modern production, but it's also why it sounds comparatively sterile when pushed into resonator territory.

Karplus-Strong synthesis, the classic algorithm behind plucked-string emulations in software synths, works by feeding a short burst of noise into a digital delay loop with a filter in the feedback path. The filter gradually rolls off high frequencies on each pass, so the noise burst decays into something that resembles a plucked string. It's an elegant trick, and it's been done well in code for decades.

A BBD resonator does the same basic job, exciter into a feedback loop, but the loop itself is a physical analog circuit rather than a math operation. That distinction matters in three concrete ways:

• Noise floor and saturation: BBDs add their own hiss and soft clipping as the signal recirculates, so the decay isn't just filtered, it's textured.
• Clock jitter and drift: Analog clock circuits aren't perfectly stable, which introduces micro-variations in delay time that read as subtle chorus-like movement, something digital Karplus-Strong emulations have to fake with an LFO.
• Non-linear feedback response: Push a BBD resonator hard and the feedback path starts to compress and distort in ways that are difficult to model precisely in code, which is part of why analog resonator synths have carved out their own niche rather than being replaced by plugins.

Morphor didn't invent this idea from nothing. The company's earlier Eurorack module, Plectrum, was already a BBD-based Karplus-Strong design, and its chorus and delay modules established the sonic vocabulary. Echon 6 is effectively six of those resonating circuits scaled into a complete polyphonic instrument.

Why Six Resonators Per Voice Is the Whole Point

Here's where Echon 6 departs from a typical analog delay pedal or a single Eurorack resonator module. Each of its six voices runs its own dedicated 1024-stage BBD resonator, essentially a short delay line with feedback tuned to ring at a specific pitch. Because every voice has its own physical BBD chip rather than sharing a single processor or algorithm, the six voices don't sound identical to each other the way six instances of the same software plugin would.

Analog components carry manufacturing tolerances. Two BBD chips of the same part number will still behave slightly differently once you factor in clock drift, capacitor leakage, and thermal variation. Stack six of them into one instrument and you get a chorus-like richness across the voice allocation that no amount of digital unison detuning fully replicates, because the variation isn't a programmed offset, it's a physical one.

This is also why Echon 6 supports up to six-way multitimbral operation with individual outputs per voice. If each voice has its own analog signal path all the way through the resonator and exciter section, routing them separately makes sense. Producers can process, pan, or record each voice discretely, treating the synth almost like six independent BBD resonator instruments that happen to share a control surface.

The Exciter Section: More Than a Noise Burst

Traditional Karplus-Strong synthesis, analog or digital, typically excites the resonator with a short noise burst and lets the feedback loop do the rest. Echon 6's exciter section is considerably more elaborate. Alongside noise, there's a VCO with continuously variable waveshapes, plus the option to feed in an external signal, all blended through a crossfader.

That matters because the character of the excitation signal shapes everything that follows in the resonator. A noise burst produces the classic plucked, percussive attack associated with Karplus-Strong patches. Swapping in a VCO waveform instead gives the resonator a tonal, sustained input to chew on, closer to feeding a string resonator with a bowed or sustained source rather than a pluck. Routing in an external signal turns the entire instrument into a resonant filter-like processor for outside audio, similar in spirit to how a spring reverb or a comb filter can be used creatively on external sources, but with the BBD's characteristic grit attached.

Before the signal enters the resonator, it passes through a 12-dB input filter, and the feedback path includes a second filter for additional shaping. That second filter is doing the heavy lifting that Karplus-Strong's classic lowpass does, controlling how quickly high frequencies die out on each pass through the delay loop, which in turn controls the perceived "brightness" of the decay and how string-like or metallic the resonance feels.

What This Means for Sound Design in Practice

For a producer used to digital delay or convolution-based resonator plugins, the practical implication is that Echon 6 rewards patience with the feedback filter and exciter blend more than it rewards precise pitch-tracking expectations. Because clock-driven BBD resonance behaves more like a physical string than a quantized digital algorithm, small changes to feedback amount or filter cutoff can push a voice from a clean pluck into a screaming, self-oscillating drone, and that instability is a feature rather than a bug for anyone chasing textures that plugins struggle to fake.

The modulation architecture backs this up. An exciter envelope plus four LFOs, combined with mod wheel, velocity, key follow, and polyphonic aftertouch routed through a mod matrix, gives sound designers enough modulation depth to keep six independently drifting analog resonators under control, or to deliberately let them wander. MPE compatibility extends that expressive control down to per-note pitch and pressure, which is unusual territory for an analog resonator instrument and suggests Morphor built Echon 6 with expressive performance in mind, not just static patch programming.

With 72 factory presets and 144 user slots, and a price point expected around the €2,000 mark when it ships later this year, Echon 6 sits closer to a boutique instrument than a mass-market synth. That's consistent with its origin story: Morphor built it out of experiments running six Plectrum-style Karplus-Strong modules together, and the desktop synth is essentially that experiment refined into a shippable, six-voice production instrument. For producers who already lean on granular textures, physical modeling, or Eurorack-style resonant filtering, understanding the bucket-brigade mechanism at its core is the difference between treating Echon 6 as a novelty and actually knowing how to push its feedback paths, exciter blend, and per-voice analog variation toward genuinely new timbres.