EM-404 | Spectral Resonator VCF

APOLLO XX is a voltage-controlled spectral resonator employing an unconventional second-order filter architecture built around two CA3080 operational transconductance amplifiers. Derived from continuous-time state-space biquadratic principles, its asymmetrical feedback topology separates the initial integrating stage from the primary regenerative loop, producing a distinctive relationship between cutoff, phase response, and resonance.

Unlike conventional symmetrical integrator-loop filters, the circuit introduces regenerative feedback downstream of the first pole, creating a distributed second-order response with asymmetrical pole participation. The resulting resonance develops progressively around the cutoff frequency, preserving low-frequency energy while introducing controlled spectral emphasis and harmonic compression. A dedicated diode-limiting network operates after pole formation, allowing the circuit to transition gradually into self-oscillation without the pronounced passband attenuation associated with certain regenerative filter structures.

Independent low-pass, band-pass, and high-pass outputs provide simultaneous access to the filter's principal responses. An additional morphology circuit combines the low-pass and high-pass signals through a network of light-dependent resistors and a discrete long-tail-pair balancing stage. The M. Shift Variable control continuously adjusts their relative contribution, moving between opposing spectral responses and producing a pronounced notch characteristic near the center of its travel. A dedicated SHAPE CV input with attenuation permits external modulation of this spectral balance.

Cutoff frequency is governed by separate coarse and fine controls, with the latter identified by the Pyramid symbol. A temperature-conscious exponential control architecture employing matched monolithic transistor arrays supports calibrated 1 V/octave tracking, complemented by independent voltage control of resonance and spectral morphology.

Dedicated Drive Level and Q / Regeneration sliders provide direct control over signal excitation and resonant intensity. A separate STRIKE input incorporates a fast-decay excitation circuit for percussive triggering, allowing the resonator to generate pitched, decaying responses without requiring a sustained audio input. Three LED indicators provide visual feedback of peak activity and the relative amplitude dominance of the morphology network.

01 / CIRCUIT ARCHITECTURE

Asymmetrical second-order filter architecture with distributed regenerative feedback
Dual 3080 operational transconductance amplifiers
Post-pole diode limiting for progressive resonant saturation
Self-oscillation with controlled low-frequency response

02 / SIGNAL INTERFACE

Independent low-pass, band-pass, and high-pass outputs
Continuous spectral morphology via LDR and discrete differential-pair circuitry
Manual M. Shift Variable control with dedicated SHAPE CV input
Coarse and fine cutoff frequency controls

03 / VOLTAGE CONTROL & EXCITATION

Matched transistor-array exponential conversion with calibrated 1 V/octave input
Variable Drive Level and Q / Regeneration
Dedicated STRIKE input with fast-decay excitation circuit
Independent resonance and shape CV attenuation

04 / VISUAL INDICATION

Three LED indicators for peak and morph activity

POWER & CONNECTIVITY

± 12V. 10-pin IDC / Keyed Headers. Red Stripe: − 12V.

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