The Erica Synths Syntrx is a semi-modular analogue monosynth developed from scratch with a user interface inspired by the legendary EMS Synthi AKS. While not a clone — no part of the circuitry is copied from EMS schematics — the Syntrx pays homage to its spiritual predecessor through its choice of modules, front panel layout, parameter naming, and distinctive patching system. Designed to encourage experimental electronic music, it captures the essence of the original Synthi's exploratory spirit while offering modern improvements and rock-solid stability.

At the heart of the Syntrx is a digitally controlled 16x16 patch matrix that replaces the traditional resistive pins with an LED grid controlled by two endless encoders. Unlike mechanically connected patch matrices, the Syntrx matrix utilizes 32 sixteen-channel analogue switch ICs (AS16M1, made in Latvia by Alfa RPAR) functioning as a mixer/buffered multiple with three-level attenuation (gain 1, 0.6, and 0.3) at connection points. The digital nature enables 256 patch memory slots with automatic patch switching in performance mode or via MIDI messages, allowing users to store templates and recall complex setups.

The sound generation capabilities are extensive. Three highly accurate voltage-controlled oscillators track exceptionally well over eight octaves, maintaining tuning stability far superior to vintage instruments. Oscillator 1 offers sine and sawtooth waveforms with wave-shaping on the sine, plus a ±1 octave switch. Oscillator 2 provides square and triangle waveforms with simultaneous pulse width and wave-shaping control, and features hard sync to Oscillator 1—a popular modification on the original Synthi. Oscillator 3 mirrors Oscillator 2's waveforms but operates at lower frequencies for LFO duties, with a switch to disable keyboard tracking for fixed-rate modulation.

Beyond the oscillators, the Syntrx features a noise generator with a colour filter, two external audio inputs with adjustable gain (microphone to line level) sharing a mic/line switch, and a ring modulator that delivers the characteristically clangorous tones essential to the Synthi sound. The resonant low-pass filter is an original design capable of self-oscillation, though it doesn't track 1V/Oct. Its generous gain staging makes overdriving the filter easy, causing the resonance to disintegrate in pleasing ways, and it responds beautifully to audio-rate modulation.

The spring reverb tank offers Mix and Feedback controls, with both input/output and CV control for mix amount accessible through the pin matrix. At high feedback values, the spring self-oscillates, creating otherworldly drones and feedback effects. The mechanical nature of the spring means it responds to physical interaction — striking the wooden side panels produces authentic spring reverb sounds, and the built-in speakers can create mechanical interference with the tank.

Modulation sources include a sophisticated trapezoid envelope generator with four stages: Attack, On, Decay, and Off. Operating in either AD or ASR mode, it offers more flexibility than traditional ADSR envelopes. In AD mode, it creates percussive sounds of fixed length. In ASR mode, it can function as a traditional AHR envelope or as a looping trapezoid shape for LFO-style duties. The envelope section contains a hidden VCA — routing signals to 'Envelope' in the pin matrix and then routing 'Env. Signal' elsewhere amplitude-modulates the signal by the envelope.

The sample and hold circuit features an individual clock and can use either VCO3 or the noise generator as its CV source, with physical controls for rate and output level. The joystick controller is perhaps the most transformative element, offering two dimensions (X and Y) freely assignable in the pin matrix, each with attenuators. Without a spring return or zero point, the joystick creates a sense of continuous sonic exploration, with small movements producing huge timbral changes.

Additional features include three VCAs, an output signal filter, an analogue CV/audio signal level indicator (VU meter), and built-in speakers. The synthesizer comes in a robust aluminium enclosure with ash tree side panels, measuring 309 x 457 x 115 mm, and can be rackmounted with optional ears.

The MIDI implementation, while basic, adds welcome functionality absent from the 1971 original. Note-on messages trigger the trapezoid envelope and oscillators track pitch (with VCO3 optionally in free mode). MIDI input is added to the 1V/Oct CV inputs, allowing an analogue sequencer to play melody while a MIDI keyboard transposes — a trick impossible on the original Synthi. Velocity can control filter cutoff (though the amount isn't adjustable), and two MIDI CCs can be assigned and converted to CV, added to the joystick X and Y outputs. The system accepts CV, Gate, Modulation Wheel messages, and Program Change messages for matrix control.

The Syntrx successfully captures the exploratory, off-piste approach to sound design that made the EMS Synthi legendary. The pin matrix encourages making multiple connections easily — single sources can connect to many destinations and vice versa—fostering the experimental workflow that defined the original. Where typical synthesizers guide users down familiar paths, the Syntrx begs for sonic exploration. Though it doesn't sound identical to a vintage Synthi, it retains every bit of the madcap, experimental character while offering superior stability, portability, and affordability compared to rare vintage units that command USD15,000 or new EMS units priced near USD9,000 with glacially slow production.

Technical Specifications

Oscillators

• Number of VCOs: 3 voltage-controlled oscillators
• Tracking Range: 8 octaves with high accuracy
• Tuning Controls: Three-turn vernier dials for precise tuning
• VCO 1:
  • Waveforms: Sine, Sawtooth
  • Wave-shaping on sine wave
  • Individual level controls for each waveform
  • ±1 octave switch
• VCO 2:
  • Waveforms: Square, Triangle
  • Shape control for simultaneous pulse width (square) and wave-shaping (triangle)
  • Hard sync to VCO 1
• VCO 3:
  • Waveforms: Square, Triangle
  • Lower frequency range for LFO operation
  • Wave-shaping capability
  • Switchable keyboard tracking (free mode for fixed-rate LFO)

Filter

• Type: Low-pass filter (original Erica Synths design)
• Features: Resonant, capable of self-oscillation
• Tracking: Does not track 1V/Oct
• Characteristics: Responds well to audio-rate modulation, pleasant overdrive/resonance disintegration

Modulation Sources

• Trapezoid Envelope Generator:
  • Four stages: Attack, On, Decay, Off
  • Two modes: AD and ASR
  • Looping capability
  • Trigger sources: MIDI note, CV gate input, Attack button
  • Hidden VCA functionality
• Sample & Hold:
  • Individual clock circuit
  • CV source: VCO3 or noise generator (selectable)
  • Controls: Rate, output level
  • Three-way switch: VCO3, S&H, or equal mix of both
• Joystick Controller:
  • Two dimensions: X and Y
  • No spring return or zero point
  • Individual attenuators for each axis
  • Freely assignable in pin matrix
• VCO 3: Can function as LFO with triangle and square waveforms

Additional Sound Sources & Processing

• Noise Generator: With colour filter control
• Ring Modulator: Two inputs, one output, level control
• Spring Reverb:
  • Controls: Mix, Feedback
  • CV control for mix amount (via pin matrix)
  • Self-oscillation at high feedback values
  • Responds to mechanical interference
• External Audio Inputs: 2 inputs
  • Individual gain controls
  • Shared mic/line switch
  • Adjustable gain from microphone to line level

Pin Matrix

• Type: Digitally controlled 16x16 matrix
• Technology: 32× 16-channel analogue switch ICs (AS16M1, Alfa RPAR, Latvia)
• Function: Mixer/buffered multiple with 3-level attenuation
• Attenuation Levels: Gain 1, 0.6, 0.3
• Control: Two endless encoders for navigation (X/Y axes)
• Patch Points: 256 total (16×16 grid)
• LED Indicators: Brightness indicates signal level
• Patch Memory: 256 slots
• Features: Automatic patch switching, temporary pin insertion (with Shift button)

VCAs

• Number: 3 voltage-controlled amplifiers

Additional Features

• Output Signal Filter
• CV/Audio Signal Level Indicator: Analogue VU meter
• Built-in Speakers: Stereo
• Octave Switch: For VCO1
• Gate Input

MIDI Implementation

• Ports: MIDI In, MIDI Thru
• Functions:
  • Note-on triggers trapezoid envelope
  • Pitch tracking for oscillators (1V/Oct)
  • MIDI input added to CV inputs (enables transposition)
  • Velocity control for filter cutoff (on/off only)
  • Two assignable MIDI CC numbers (converted to CV, added to joystick X/Y)
  • Program Change messages for matrix control
  • Modulation Wheel messages
• Configuration: Small push button for MIDI preferences (channel, CC assignments, velocity response)

CV/Gate Inputs

• CV Inputs: 2× 1V/Oct inputs
• Gate Input: 1 input
• Integration: MIDI and CV inputs are summed

Audio Inputs/Outputs

• External Audio Inputs: 2 (rear panel)
• Audio Outputs: Dual outputs (rear panel)
• Headphone Output: 1× 1/4" jack (rear panel)

Power

• Type: 12V DC
• Connection: Rear panel socket

Physical Specifications

• Dimensions: 309 × 457 × 115 mm (W × D × H)
• Enclosure: Aluminium with ash tree side panels
• Mounting: Can be rackmounted with optional ears
• Portability: Semi-portable, suitable for live performance

Construction

• Circuit Design: 100% original (no cloned EMS schematics)
• Development: Erica Synths with Riga Technical University engineers
• Origin: Designed and manufactured in Latvia
• Build Quality: Solid, professional construction

Design Inspiration

• Based on: EMS Synthi AKS interface concept but not a clone%u2014entirely new circuitry and implementation
• Improvements over vintage: Superior oscillator stability and tracking, buffered matrix design, digital patch storage, MIDI support, S&H circuit, oscillator sync