Acid synthesiser, Model AG-78, v0.2.0

Agena

Acid monosynth and step sequencer

Agena is a monosynth for acid lines. It plays through a diode ladder, or through a model of the filter in a battery-powered British synth from 1978, which used logic chips as its amplifiers. A 32-step sequencer with accent and slide plays it, and nine distortion models, a delay, a plate reverb, an EQ and a limiter come after it.

The Agena panel: the voice unit with its CRTs, knobs, LED display, output meter and effects switches above, and the sequence unit with the step computer and line computer below
Fig. 01 The Classic Acid preset, playing the first pattern at 125 BPM.

Listen25 presets on four of the factory patterns, at 125 BPM

Pattern 1, the classic shape

Preset

Classic Acid

The Ladder board, with Overdrive blended in at 45 % and a low-passed delay.

Pattern 2, a drifting lane

Preset

Rubber Duck

Band pass, with a ramp LFO on the cutoff, a quarter note long.

Pattern 3, 13 steps against the bar

Preset

Basement

Cutoff at 150 Hz on the Ladder board, with the Amp model at 50 % Mix.

Pattern 4, a dub line

Preset

Dub Plate

The 1978 board through Tape, band-passed echoes at 70 % feedback, and the plate.

Pattern 1, away from the squelch

Preset

Bell Keys

Band pass at 2 kHz, high-passed echoes and the plate.

Each clip is a factory preset playing one of the factory patterns, rendered by a test build of the plugin and left at the level the preset plays at. Pick another preset while a clip plays and it comes in on the same beat.

The voicefilter, oscillator, envelopes, scope, LFO

The Filter section on the Ladder board: a CRT with the filter's response, Cutoff, Resonance and Env mod, Accent, Sweep, Drive and Key track, the Mode selector, the voltmeter reading no battery, Battery and the Board switch
Fig. 02 The filter

The filter

Low pass, band pass, high pass or notch, on three boards

The filter comes from a battery-powered British synth of 1978. Its designer used CD4069 chips, digital inverters made for logic circuits, as the filter's amplifiers, and Agena models that filter part by part, on the original 5 V board or the later 12 V modular one. The third board, Ladder, is a four-pole diode ladder built part by part from a bass synth's service notes, with the couplings around it, its way into the amp, its pulse and its Env mod, and it is where a new instance starts. Drive sets how hard the oscillator hits the filter. Drag across the screen to set Cutoff and Resonance.

The Oscillator section: a CRT with the waveform, Wave, Width, Sub and Tune, the Octave selector, Glide and the glide Mode switch
Fig. 03 The oscillator

The oscillator

Sawtooth to pulse, with a sub

Wave blends a sawtooth into a pulse, and Width sets the pulse's width. On the 1978 and modular boards 50 % is a square. On the Ladder board a single transistor shapes the pulse from the sawtooth, as the circuit it follows does, so it narrows as notes rise: with Width at 50 %, it is high for about 61 % of each cycle at 35 Hz and 41 % at 200 Hz. Sub adds a square an octave down, locked to the oscillator. The sawtooth, and the pulse on the other two boards, stay clean on high notes. The Ladder board's pulse switches fast enough to alias a little: before the filter, the loudest alias below 20 kHz is 61 dB under the note at C4 and 47 dB under it at 1 kHz. Glide sets how long a slide takes, the same for any interval, and starts at 66 ms.

The Filter envelope and Amp envelope sections, each a CRT with the envelope's shape over Attack, Decay, Sustain and Release
Fig. 04 Envelopes and accent

Envelopes and accent

Two envelopes, each with its own screen

The filter and the amp each have an ADSR, drawn on its own screen with a dot where it is now. An accented note starts louder, by up to 8 dB with Accent at full. Sweep sets how an accent moves the cutoff: low, a sharp blip; turned up, a swell through the note, and accents close together climb higher.

The Scope section with the output's waveform, and the LFO section with its Shape selector, Rate, Amount and the Sync switch, on the Wide Open preset
Fig. 05 The scope and the LFO

The scope and the LFO

Six shapes, free or in time

The scope shows the output as it plays. The LFO moves the cutoff, up to four octaves either way, with a sine, ramp, sawtooth, square, noise or random shape. Free, it runs from one cycle every 20 seconds to 100 Hz; locked to your host, from four bars to a thirty-second note.

The effectsdistortion, delay, plate, EQ, limiter

The effects section: the switches for Dist, Delay and Plate thrown up under lit lamps, Limit down with its lamp out, and the Edit pushbutton
Fig. 06 The effects section on Peak Hour: the distortion, the delay and the plate on, the limiter off.

The distortion, the delay, the plate and the limiter each have a switch on the front, under a lamp that is lit while it is on. Switched off, the delay and the plate fade out over 10 ms, and the distortion eases out as it does when its Drive is turned down. In order, the chain runs distortion, delay, plate, EQ, Output and limiter.

Edit opens the effects out over the voice unit, a page for each behind a tab, with all their controls. Each tab has a lamp, lit while its effect is on: click the lamp to switch the effect on or off from any page, or the rest of the tab to open its page. The presets, the EQ, Output and the sequencer stay in reach.

Beside the tabs are the effects' own presets: Dry and 31 chains of two effects or more. Each sets the four pages and nothing else, so the EQ and Output stay as they are. Lock keeps the effects as they are while you try other sounds through them.

The distortion page, its tab open and the effects presets beside the tabs, reading No preset: the nine model keys with Amp lit, the on switch and the Position switch, and Drive at 85 %, Tone, Level, Mix, Bias and Bass keep
Fig. 07 The distortion page on the Broken Speaker preset: the Amp model at 85 % Drive.

Distortion

Nine models. CMOS is the fuzz Agena started with, a CD4069 inverter wired as a fuzz. Overdrive clips softly and keeps the clean signal in. Distortion is an op-amp distortion with up to 67 dB of gain into silicon diodes. Crunch is a booster and an op-amp into diodes, with a scooped tone. Fuzz has two clipping stages and a scoop around 1 kHz. Amp is two triode stages, sag and a 4x12 cabinet. Tape saturates between pre- and de-emphasis, Fold is a wavefolder, and Crush takes away bits and rate.

Each has Drive, Tone, Level and Mix. Bias moves the clipping off centre for even harmonics. Bass keep sends everything under its frequency round the distortion clean, through a crossover that adds back up flat, so the bass line keeps its weight. Position puts the distortion after the amp, as a pedal after the synth would be, or before it, so the grit stays the same through each note's fall.

The delay page, its tab open, with the lamps on the Distortion, Delay and Plate tabs lit and the effects presets on Echo Vault: the on switch, Send at 45 %, Time at 1/4, Feedback at 75 % and Width, and the taps' filter on Band pass at 1.80 kHz with Res at 70 %
Fig. 08 The delay page with the Echo Vault effects preset on Echo Squelch: echoes a quarter note apart, band-passed at 1.8 kHz, with 75 % feedback.

Delay

A ping-pong delay at a note value of your tempo, from a sixteenth to a half note, with dotted values and an eighth-note triplet. Each repeat loses a little top and bottom and is gently saturated, so long feedback darkens as it goes. Width narrows the bounce towards the middle. The Delay lane in the sequencer sends single steps into it.

The taps' filter is a resonant low, band or high pass where the echoes are written, so the first echo goes through it once and each repeat once more. However high the resonance, the echoes never build up: each mode is scaled so its peak stays at unity, and more resonance narrows what comes round.

The plate page, its tab open: the on switch, Send at 60 % and Low cut, Decay at 4.60 s, Bass, Crossover and Treble, and Predelay, Size at 4x, Depth and Diffusion
Fig. 09 The plate page on Acid Cathedral: a 4.6 s decay at four times the plate's size.

Plate

The Drum Plate from our Mantissa reverb, in its 1978 voice, on a send after the echoes. Decay runs from 0.6 to 6.5 s, and Bass, Crossover and Treble shape how the tail falls, as Mantissa's faders do.

Low cut keeps the bass out of the plate, from 150 Hz to start. Predelay holds the start back by up to 107 ms, Size scales the plate from half to four times, and Depth and Diffusion set how far away it sounds and how dense its start is.

The limiter page, its tab open and its lamp lit: the on switch, Threshold at -12 dB, Ratio on Limit, Knee, Attack, Release, Makeup and Ceiling, the Soft clip switch, and the gain reduction meter lit
Fig. 10 The limiter page on Tight and Clicky, its threshold taken down to -12 dB for the picture, so the meter shows it working.

Limiter

Last in the chain, after the EQ and Output, with Threshold, Ratio up to Limit, Knee, Attack, Release, Makeup and a Ceiling. Soft clip leaves everything up to 6 dB under the ceiling as it is and bends what is over it round into the ceiling. A row of lamps shows the gain reduction, 1 dB each up to 20 dB.

The step computer32 steps, 8 patterns

The step computer zoomed to fill the window below the top bar: the notes on a large CRT, with the accent and slide keys under it
Fig. 11 The step computer zoomed in. Each slide is drawn as a glide into the next note.

Notes, accents and slides

Click a step to place a note, or drag to draw a line. The lamp keys under the screen set accents and slides. Notes snap to the scale you pick, and changing the scale moves only the notes that fall outside the new one.

Ten lanes, each its own length

Seven lanes change the sound on each step: cutoff, resonance, env mod, decay, wave, gate and the delay send. Octave, Chance and Ratchet change the steps. A lane shorter than the notes drifts against them, and the line changes shape from bar to bar.

In time with your host

Steps are placed on the host's beat, and a pattern always lines up with the bars. Chance skips steps at random, but the same point in the song always rolls the same way, and a bounce matches what you heard on playback.

Play it from the keys

With Keys on Transpose, the last key you hold moves the line. Hold does the same, and plays the line only while you hold a key. Set Play from to MIDI to play the voice yourself. Overlapping notes slide, and a velocity of 100 or more plays an accent.

The Cutoff lane at 11 steps, drawn as bars above and below the centre, against 16 steps of notes
Fig. 12 The Cutoff lane at 11 steps against 16 steps of notes. Each time the notes repeat, the lane starts five steps earlier.

The line computer

The line computer: Busy, Acid, Range, Accents, Slides and Stir, the guarded Cook button, New notes, New rhythm, Spice and Stir, undo and redo, and the punched card
Fig. 13 The line computer, with the Cook button under its guard.

Press Cook for a new line; its guard swings up as the pointer comes near. Acid sets how it is phrased. At Classic it uses a few pitches, mostly the root and its octave, with a fifth, minor third, flat seventh or flat second for colour; at Wild the intervals are free. Busy, Accents and Slides set how many of each a bar gets. Acid lines usually have six to ten notes a bar, with two or three accents.

New notes keeps the rhythm and New rhythm keeps the notes. Spice redraws the accents, slides and octaves, and Stir changes a few steps. Undo brings back the previous line.

Drag the punched card into your DAW to take the pattern as MIDI. Slides become overlapping notes and accents full velocity. Played back with Play from on MIDI and Transpose at 0, the clip sounds like the pattern, except that a slide from the last step into the first starts a fresh note.

The EQlow shelf, bell, high shelf

The EQ view over the step computer: the curve on the CRT, with a gain, frequency and Q knob for the low shelf, mid bell and high shelf
Fig. 14 The EQ view, with the bell at 1.2 kHz, +4.5 dB and a Q of 1.4.

A low shelf, a bell and a high shelf come after the plate and before Output, each with ±12 dB and its own frequency and Q. Turn an EQ knob and the curve comes up over the step computer. The sequencer comes back a moment after you let go.

The EQ adds no latency, and moving or automating a band doesn't click. With all three gains at 0 dB it leaves the sound untouched. Beside Output, a meter shows the peaks leaving the plugin, left and right, with a red light at the top that stays lit after a sample reaches full scale, until you click it.

Board and battery

The Filter section on the 1978 board, with Battery at 35 %, Drive at +6 dB and the voltmeter showing the lowered supply
Fig. 15 The Low Battery preset: the 1978 board, Battery at 35 % and Drive at +6 dB.

Board picks the filter. The 1978 board runs on 5 V, as the original synth did. The modular board is the later 12 V version of the same circuit, with more headroom and so little damping in its resonance that Resonance at full turns its pot only 62.5 % of the way. The Ladder board is the diode ladder, with the pulse, Env mod and way into the amp of the circuit it follows.

On the 1978 and modular boards, Battery lowers the supply to as little as 60 %, as a battery running down did in the original, and the meter shows it. As it drops, the filter has less headroom and distorts sooner. The Ladder board runs on no battery.

The rear paneleight trimmers

The voice unit turned round to its green circuit board: eight worn ceramic trimmers in four groups, each with its value, the board's presets on Lazy, and the Stock and Front keys
Fig. 16 The rear panel on its Lazy preset: the filter envelope's capacitor at 2.2 µF and the accent's at 4.7 µF.

Rear turns the voice unit round to its board, where eight trimmers set capacitors in the Ladder board's circuit: two shape its pulse, two its ladder and resonance, two the way into the amp, one sets the filter envelope's length and one the accent's sweep. The accent's works on every board, the rest on the Ladder board. Stock puts them all back.

The board has 13 presets of its own, named for what they do, such as Short Envelope, More Bass and Steady Square. Each changes the trimmers and nothing else, and Lock keeps them as they are while you change sounds.

Presets100 factory presets, plus your own

  • Basics12
  • Squelch20
  • Deep14
  • Space16
  • Grit12
  • Odd14
  • Keys12

Presets change the sound only. Your patterns and sequencer settings stay with the session when you change preset. Most presets use the effects: 69 a distortion model, 37 the delay and 58 the plate. The effects panel and the rear panel have presets of their own as well, 32 and 13, and you can save your own of each.

Specifications

Formats
VST3, AU and Standalone
Filter
A diode ladder, or a model of a 1978 CMOS filter on its original 5 V board or the 12 V modular one; low pass, band pass, high pass or notch
Oscillator
Sawtooth to pulse with a sub an octave down; band-limited, apart from the Ladder board's transistor-shaped pulse
Sequencer
Up to 32 steps and eight patterns, ten lanes with their own lengths, swing, a scale and four play directions
Distortion
Nine models, with Drive, Tone, Level, Mix, Bias, Bass keep, and Position after or before the amp
Delay
Ping-pong at note values of the tempo, with a resonant low, band or high pass on its taps and Width
Plate
Mantissa's Drum Plate on a send after the delay, decay from 0.6 to 6.5 s, with the program's own controls
EQ
Low shelf from 20 to 500 Hz, bell from 100 Hz to 10 kHz, high shelf from 1 to 16 kHz, each ±12 dB, no latency
Limiter
Threshold, ratio up to Limit, knee, attack, release, makeup, ceiling and soft clip, last in the chain
Rear panel
Eight trimmers for the Ladder board's capacitors (the accent's works on every board), with 13 presets of their own
Metering
Output peaks, left and right, from -48 dB to full scale, with a clip light
MIDI
Play the voice from MIDI, transpose the line from the keys, pick patterns with notes 0 to 7, drag a pattern out as MIDI
Price
To be announced

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