Component

Synthex polysynth

v0.5.108

After the Elka Synthex: two oscillators per voice through a four-pole filter, with chorus.

Software v0.5.108

Overview

After the Elka Synthex: two oscillators per voice through a four-pole filter, with chorus.

A polyphonic voice after the Elka Synthex (1981, designed by Mario Maggi), with eight voices in one module. Each voice has two oscillators, noise, a four-pole multimode filter and two envelopes; one LFO, glide and a stereo chorus are shared. Joy X bends the pitch (V/Oct) and Joy Y moves the filter, like the joystick on the original.

Play it from MIDI-IN; Poly ▾ → Synthex ×8 sets it up. The knobs follow the original's 0 to 10 scale. The envelope times run from 2 ms to 10 s along that scale.

Controls

Control	Range	Default	What it does
Feet	16' / 8' / 4' / 2'	4'	Oscillator 1 footage, 16′ to 2′.
Wave	Ramp / Sqr / Pulse	Ramp	Oscillator 1 waveform: ramp, square or pulse.
Volume	0 … 10	10	Oscillator 1 level in the mixer (16 steps).
Feet	16' / 8' / 4' / 2'	4'	Oscillator 2 footage, 16′ to 2′.
Transp	-12 … 12 semi	0	Oscillator 2 transposed in semitones.
Detune	-50 … 50 ct	6	Oscillator 2 slightly detuned: beating and width.
Wave	Ramp / Sqr / Pulse	Ramp	Oscillator 2 waveform: ramp, square or pulse.
Volume	0 … 10	8	Oscillator 2 level in the mixer (16 steps).
Sync	off / on	off	Locks oscillator 2 to oscillator 1.
Ring	off / on	off	Replaces oscillator 2 with the product of both oscillators.
PW	0.05 … 0.5	0.3	Pulse width for the Pulse waveform.
Noise	0 … 10	0	Noise level in the mixer.
Freq	0 … 10	6	Filter: the brightness.
Reso	0 … 10	2	Filter: the resonance peak.
Env	0 … 10	4	How much the filter envelope opens the filter on each note.
Kbd	0 … 10	3	Keyboard tracking of the filter: 10 = follows the pitch fully.
Mode	LP / BP / HP	LP	Filter mode: low-pass 24 dB, band-pass 12 dB or high-pass 24 dB.
A	0 … 10	0.5	Filter envelope attack.
D	0 … 10	5	Filter envelope decay.
S	0 … 10	6	Filter envelope sustain.
R	0 … 10	4	Filter envelope release.
A	0 … 10	0.5	Volume attack: how fast a note comes in.
D	0 … 10	5	Volume envelope decay.
S	0 … 10	8	Volume envelope sustain.
R	0 … 10	4	Volume release after you let go.
Rate	0.05 … 20 Hz	5	LFO speed.
Wave	Tri / Sqr / Saw / S&H	Tri	LFO waveform: triangle, square, sawtooth or sample & hold.
→Osc	0 … 1	0	LFO to the pitch of both oscillators (1 = ±1 octave).
→PW	0 … 1	0	LFO to the pulse width.
→VCF	0 … 1	0	LFO to the filter cutoff (1 = ±2 octaves).
→VCA	0 … 1	0	LFO to the volume (tremolo).
Glide	0 … 1	0	Portamento time between notes.
Tune	-12 … 12 semi	0	Master tuning in semitones.
Chorus	off / on	on	The stereo chorus.
Level	0 … 1	0.8	Output volume.

Ports

Port	Direction	Signal	What it carries
voct_1	in	cv	Pitch of key 1, 1 V per octave (from MIDI-IN).
voct_2	in	cv	Pitch of key 2, 1 V per octave (from MIDI-IN).
voct_3	in	cv	Pitch of key 3, 1 V per octave (from MIDI-IN).
voct_4	in	cv	Pitch of key 4, 1 V per octave (from MIDI-IN).
voct_5	in	cv	Pitch of key 5, 1 V per octave (from MIDI-IN).
voct_6	in	cv	Pitch of key 6, 1 V per octave (from MIDI-IN).
voct_7	in	cv	Pitch of key 7, 1 V per octave (from MIDI-IN).
voct_8	in	cv	Pitch of key 8, 1 V per octave (from MIDI-IN).
gate_1	in	gate	Gate of key 1: high while the key is held.
gate_2	in	gate	Gate of key 2: high while the key is held.
gate_3	in	gate	Gate of key 3: high while the key is held.
gate_4	in	gate	Gate of key 4: high while the key is held.
gate_5	in	gate	Gate of key 5: high while the key is held.
gate_6	in	gate	Gate of key 6: high while the key is held.
gate_7	in	gate	Gate of key 7: high while the key is held.
gate_8	in	gate	Gate of key 8: high while the key is held.
Joy X	in	cv	Joystick X: pitch bend, 1 V per octave.
Joy Y	in	cv	Joystick Y: moves the filter cutoff.
L	out	audio	Left output (after the chorus).
R	out	audio	Right output (after the chorus).

How it works

On the instrument the oscillators are digital (counters on a 4 MHz clock), which is why it stays in tune. Each offers Ramp (sawtooth), Square or Pulse (with PW) at 16′ to 2′. Oscillator 2 adds Transpose and Detune; Sync locks it to oscillator 1, and Ring replaces it with the product of both. The mixer counts in 16 steps, like the 4-bit volume switches (binary-weighted resistors) of the original.

The filter on the original is four OTA stages in series with feedback for resonance; switches choose the tap, so the same filter is low-pass, band-pass or high-pass. Here it is four one-pole stages (TPT) with a soft limiter in the loop, and the modes mix the stage outputs: LP 24 dB, BP 12 dB, HP 24 dB per octave. Freq spans 20 Hz to 20 kHz over the knob; the filter envelope, keyboard tracking, LFO and Joy Y add to it. The chorus is the MusicBrain BBD chorus. The split and double modes and the sequencer of the original are not included.

Runs on

Teensy 4.1 (the MusicBrain Brain): about 12.2 % CPU per instance (one note, default settings), measured on firmware 0.5.108.
The browser: the editor's simulator runs the same C++ code as WebAssembly, so what you hear while patching is what the Brain plays.

Try it

Patches to play (open them in the editor from their page): Synthex ×8 (dramatic).

In the editor's example set: 🎛️ Synthex ×8 (Elka), 🎛 Synthex ×8.

Editor: https://editor.musicbrain.nl

Sources & licences

Own implementation (MIT licence), built from the topology in the Elka Synthex schematic diagrams (dated 1-6-83: sheet 1 block diagram, sheet 4 voice card 410CST5800) and the oscillator calibration instructions; not a component-level simulation.

Versions

First version (October 2026): eight voices, two oscillators with sync and ring, noise, multimode filter, two ADSRs, LFO routing, glide, chorus and joystick inputs.