The APOLLO XX Spectral Resonator (EM-404) is an analog filter module utilizing a unique, non-linear feedback network derived from continuous-time state-space biquadratic topology. Rather than executing a symmetric integrator-loop configuration, the circuit functions in principle as two cascaded one-pole blocks, behaving as a partially decoupled pair of first-order energy domains or a distributed-feedback second-order system with asymmetrical pole participation.
By injecting the primary feedback loop downstream—specifically into the non-inverting structure surrounding the second operational transconductance amplifier stage—the initial integrator serves purely as a preconditioning pole isolated from direct participation in the variable-feedback loop. Built around classic CA3080 OTAs without modern linearization sub-circuits, this architecture manages non-idealities spatially through topology, eliminating DC-offset-induced runaway while preserving fundamental low-frequency energy.
Electro-acoustically, the design occupies a distinct territory between the fluid, geometric phase behavior of cascaded integrator filter topologies and the strongly dominant regenerative character of ladder architectures.
Because the resonance signal is reintroduced downstream, the regeneration feels internally accumulated within the circuit rather than externally imposed as a sharp, superimposed transfer function spike. Instead, the system exhibits distributed spectral curvature, rotational phase displacement, and gradual harmonic compression flanking the cutoff frequency.
APOLLO XX.
SPECTRAL RESONATOR
VOLTAGE-CONTROLLED FILTER
MODEL : EM-404
Asymmetrical pole distribution.
A continuous-time state-space biquadratic.
Perpetual geometric phase interpolation.
Crucially, the circuit layout ensures that soft limiting via a dedicated diode network occurs strictly after pole formation. This specific structural ordering permits the filter to glide into a condensed self-oscillation continuously rather than abruptly fracturing, preventing pass-band attenuation ("suck-out") and ensuring the simultaneous low-pass, high-pass, and independent band-pass outputs maintain structural coherence even at elevated regeneration levels.
The physical interface balances precision calibration with multi-parameter voltage control over the filter’s internal subsystems. The module's morphology circuit routes the low-pass and high-pass signals through a network of light-dependent resistors (LDRs) and a discrete, long-tail pair balancing circuit to facilitate continuous crossfading via the manual M. Shift Variable slider or the attenuated SHAPE C.V. input, yielding a deep spectral notch ($\sim XX\text{ dB} $) at its median position.
Cutoff calibration relies on a split-resolution system featuring a coarse F potentiometer and a high-resolution fine-tuning potentiometer designated by a Pyramid glyph, driven by large-geometry, monolithic 4-transistor NPN/PNP arrays for stable volt-per-octave tracking via the 1 V / OCT. channel. Designed for dynamic architectural control, the front panel is rounded out by manual DRIVE LEVEL and Q. / REGEN. sliders, a dedicated STRIKE channel with an internal fast-decay excitation circuit for percussive pinging, independent CV attenuverters for resonance and shape modulation, and a three-LED telemetry matrix monitoring real-time Peak, respective Morph amplitude dominance.
ALL RIGHTS RESERVED.
MMXXVI
|
©
SONIC
DESIGN
&
ARCHITECTURE
INSTRUMENTS
L.A.
CIRCUITS
CORPORATION
The APOLLO XX Spectral Resonator (EM-404) is an analog filter module utilizing a unique, non-linear feedback network derived from continuous-time state-space biquadratic topology. Rather than executing a symmetric integrator-loop configuration, the circuit functions in principle as two cascaded one-pole blocks, behaving as a partially decoupled pair of first-order energy domains or a distributed-feedback second-order system with asymmetrical pole participation.
By injecting the primary feedback loop downstream—specifically into the non-inverting structure surrounding the second operational transconductance amplifier stage—the initial integrator serves purely as a preconditioning pole isolated from direct participation in the variable-feedback loop. Built around classic CA3080 OTAs without modern linearization sub-circuits, this architecture manages non-idealities spatially through topology, eliminating DC-offset-induced runaway while preserving fundamental low-frequency energy.
Electro-acoustically, the design occupies a distinct territory between the fluid, geometric phase behavior of cascaded integrator filter topologies and the strongly dominant regenerative character of ladder architectures.
Because the resonance signal is reintroduced downstream, the regeneration feels internally accumulated within the circuit rather than externally imposed as a sharp, superimposed transfer function spike. Instead, the system exhibits distributed spectral curvature, rotational phase displacement, and gradual harmonic compression flanking the cutoff frequency.
Crucially, the circuit layout ensures that soft limiting via a dedicated diode network occurs strictly after pole formation. This specific structural ordering permits the filter to glide into a condensed self-oscillation continuously rather than abruptly fracturing, preventing pass-band attenuation ("suck-out") and ensuring the simultaneous low-pass, high-pass, and independent band-pass outputs maintain structural coherence even at elevated regeneration levels.
The physical interface balances precision calibration with multi-parameter voltage control over the filter’s internal subsystems. The module's morphology circuit routes the low-pass and high-pass signals through a network of light-dependent resistors (LDRs) and a discrete, long-tail pair balancing circuit to facilitate continuous crossfading via the manual M. Shift Variable slider or the attenuated SHAPE C.V. input, yielding a deep spectral notch ($\sim XX\text{ dB} $) at its median position.
Cutoff calibration relies on a split-resolution system featuring a coarse F potentiometer and a high-resolution fine-tuning potentiometer designated by a Pyramid glyph, driven by large-geometry, monolithic 4-transistor NPN/PNP arrays for stable volt-per-octave tracking via the 1 V / OCT. channel. Designed for dynamic architectural control, the front panel is rounded out by manual DRIVE LEVEL and Q. / REGEN. sliders, a dedicated STRIKE channel with an internal fast-decay excitation circuit for percussive pinging, independent CV attenuverters for resonance and shape modulation, and a three-LED telemetry matrix monitoring real-time Peak, respective Morph amplitude dominance.
ALL RIGHTS RESERVED.
MMXXVI
|
©
CIRCUITS
L.A.
CORPORATION
APOLLO XX.
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