TT-303 (JSFX) — User Manual

01The concept

The Roland TB-303 is a single-oscillator bass synthesizer whose filter, envelopes and sequencer were never meant to be a bass synthesizer — and became the sound of acid house. TT-303 is a tribute to it as a REAPER instrument: a model of the signal path, without samples and without a sample ROM. There is one voice (monophonic), one oscillator, one filter, and the two circuits that give the original its character: accent and slide.

note → oscillator (saw / square) → 18 dB ladder filter (VCF) → VCA
     → drive → ping-pong delay → reverb → compressor (dry only) → VOLUME → soft clip → stereo out

The plug-in has no audio input and two outputs (stereo). Its MIDI input is the only way to play it from the outside; in SEQ mode an internal 16-step sequencer with four banks A–D plays the notes itself. The whole front is drawn by the plug-in, scales with the window, and every control can be automated.

02Installation

TT-303 ships through the TimTechlor ReaPack repository. In REAPER: Extensions → ReaPack → Import repositories…, add

https://raw.githubusercontent.com/timtechlor/reapack-TimTechlor/main/index.xml

then Browse packages, pick TT-303 Acid Machine and apply. The effect is listed under JS: TT-303 Acid Machine in the FX browser. Manual installation works too: the plug-in is one file, TT-303_Acid_Machine.jsfx; copy it into Effects (or a sub-folder) in REAPER’s resource path (Options → Show REAPER resource path in explorer) and add it from the FX browser.

There is nothing else to install: the plug-in needs no samples and no additional files. REAPER 6 or newer is assumed. The wordmark uses the font Zilap Orion if it is installed; otherwise a bold sans-serif is used. The front measures which fonts exist and falls back by itself.

After an update REAPER may keep showing the old name or version in the FX window title — see Troubleshooting. The real version is always written in the header of the front (v1.2).

03Quick start

1. Create a track and add JS: TT-303 Acid Machine (it is an instrument: it needs no audio input, only MIDI). A new instance starts in HOST mode with the INIT sound.
2. Give the track MIDI: record-arm it and play a keyboard, or put a MIDI item on it. Play a low note (C2 is the original pitch of the sequencer, the keys around C1–C3 are the bass register). Overlapping notes slide, a note with velocity ≥ 100 is an accent.
3. Shape the sound with CUTOFF FREQ, RESONANCE, ENV MOD and DECAY (the four knobs that make the acid sound) and ACCENT. Pick a starting point from the preset bar in the header (INIT, DEEP, SQUELCH, SCREAM, HOOVER, DUB).
4. Want a pattern without MIDI? Switch the MODE (top right) to SEQ and press play: the 16-step sequencer below the effects panel plays the start pattern of bank A on the project timeline. FREE RUN lets it run while the transport is stopped.
5. Add character in the effects panel: DRIVE on with FOLD or FUZZ, the dotted-eighth DELAY (the INIT default), some REVERB; COMP adds punch to the dry signal.

04The front & how to operate it

The front is drawn in a fixed grid (900 × 732 units) and scaled to the window: drag the FX window larger to enlarge everything (HiDPI is honoured). All built-in REAPER sliders are hidden; you operate the drawn front only. The areas, top to bottom:

Knobs

Drag a knob vertically (up = more). Hold Shift while dragging for fine adjustment (about five times finer); the mouse wheel over a knob moves it in steps of 2 % of its range; right-click or double-click resets it to its default value.

Buttons and the EQ strips

Buttons switch on click (mode, waveform, drive model, delay division, FREE RUN and so on). The three EQ strips under DRIVE, DELAY and REVERB carry two handles: the left one is the low cut (LO), the right one the high cut (HI), on a logarithmic scale from 20 Hz to 20 kHz. Drag a handle horizontally (Shift = five times finer, mouse wheel = steps), right-click restores the factory value. At the outer edge a handle snaps to OFF (low cut 0, high cut 20 kHz = true bypass of that filter). The low cut cannot cross the high cut.

05Operating modes: HOST and SEQ

The MODE switch in the header selects who plays the voice. Switching modes releases all notes cleanly.

HOST

The plug-in is a pure sound generator, played from the piano roll, a keyboard or another plug-in’s MIDI output. Rules, as printed on the front:

• Velocity ≥ ACCENT VEL = accent. The threshold is set with the - / + buttons under ACCENT VEL (1–127, default 100). It is only shown and used in HOST mode.
• Overlapping notes = slide. A note that starts while the previous one is still held glides to the new pitch without retriggering the envelopes (legato).
• Last note wins. The voice is monophonic. Notes that are still held are remembered (up to 32): when you release the newest one, the voice glides back to the one before.
• The keyboard at the bottom (C1 to C4) lights the held notes; HELD: n counts them.

SEQ

The internal 16-step sequencer takes over and plays the pattern of the active bank (A–D). MIDI notes no longer play the voice: they transpose the pattern (section 12). The bottom panel turns into the sequencer.

06Oscillator & filter

The filter is a three-pole ladder, 18 dB per octave (like the original), built in zero-delay-feedback form, run at twice the sample rate and decimated with a 1/4-1/2-1/4 filter. A 20 Hz high-pass sits in the feedback path as a DC block, so the resonance stays alive even at a low cutoff. The amplitude envelope (VCA) is an attack of about 3 ms (1.8 ms on accented notes) followed by a slow sag with a fixed time constant of about 1.1 s, independent of the DECAY knob, and a release of about 12 ms when the note ends. The filter cutoff is limited to 30 Hz at the bottom and to 16 kHz (or 0.45 of the working rate) at the top.

07Accent & slide

Accent

An accented note is louder, has a sharper attack and opens the filter further — and the effect depends on the knobs, as on the hardware:

• ACCENT (0–100 %) scales both the level boost and the extra filter sweep of accented notes.
• The sweep is coupled to resonance: the more resonance, the stronger the accent sweeps the cutoff (and a little to the cutoff setting).
• Accented notes use a fixed filter decay of 0.2 s and an extra sweep that dies away in about 85 ms; the VCA attack shortens to 1.8 ms.
• The accent capacitor models the recharge of the original: accents in quick succession build up (the charge is capped) and are stronger than a single accent after a rest. The ACCENT bar in the header shows the charge and flashes on every accent.

In HOST mode an accent is a velocity at or above ACCENT VEL; in SEQ mode it is the ACC row of a step.

Slide

A slide ties two notes: the pitch glides from the old to the new note and the envelopes are not retriggered, so the filter and amplitude carry on. The glide is an RC-style approach in logarithmic frequency: constant time for any interval (a fifth takes as long as an octave), fast at first, then settling. The SLIDE knob (0–300 ms, default 60) is the time constant: about 63 % of the way after one time constant, about 95 % after three. At 0 the pitch jumps. (The knob sits in the REVERB column of the effects panel, to the right of SIZE.)

An accented note that is tied by a slide still fires its accent sweep, but the amplitude envelope is left alone — that is how accents and slides combine on the hardware. In HOST mode a slide is an overlapping note, in SEQ mode the SLD flag of a step ties it to the following step.

08Drive

A distortion stage between the voice and the delay. ON/OFF switches it (off = the stage is bit-exact transparent). Four models:

DRIVE (1 … 40) is the gain in front of the shaper; the output level is compensated with the amount and, per model, so that changing the model does not change the loudness much. The shaper runs at twice the sample rate to keep aliasing down.

The EQ strip selects what gets distorted. It is not in series: the band between LO and HI goes into the shaper, what the EQ removes runs past it undistorted and is added back afterwards. Raise the low cut and the bass is taken out of the distortion, not out of the output: the fundamental stays clean, only the mids get dirty. The factory setting is OFF / OFF (the whole signal is distorted).

09Delay

A stereo ping-pong delay, synchronised to the project tempo.

• 1/8, 1/8. (dotted eighth, the INIT default), 1/4, 1/2: the delay time as a note value at the current tempo.
• MIX 0–100 %: the level of the echoes. Below 0.5 % the delay is bypassed completely.
• FEEDBACK 0–90 %: the number of repeats. The feedback path is low-passed (about 2.4 kHz), so each repeat gets darker. The echoes bounce between left and right.
• EQ strip: filters what goes into the delay (factory 250 Hz … 3.0 kHz). The dry signal is unaffected.

The echoes also feed the reverb, so they get a tail of their own. The tempo is read from REAPER; below 10 BPM (or if none is reported) 120 BPM is used. The delay buffer holds 3.3 s, which is only reached with the 1/2 setting below about 37 BPM.

10Reverb

A Freeverb-type room: eight comb filters and four all-pass filters per channel with a short pre-delay (about 12 ms), lengths scaled with the sample rate.

• MIX 0–100 %: level of the tail. Below 0.5 % the reverb is bypassed, and its buffers are cleared once on the way down (no old tail comes back when you turn it up again).
• SIZE 20–100 %: room size. Small rooms give a short, tight tail, large ones a longer tail.
• EQ strip: filters what goes into the reverb (factory 300 Hz … 2.3 kHz). The tail itself is additionally band-limited (a 250 Hz high-pass and a 3.2 kHz low-pass), which keeps it out of the bass and off the top.

11Compressor & master

The COMP column holds a compressor that acts on the dry signal only — delay and reverb are added afterwards, so the punch of the 303 stays in front of the room. Fixed values: threshold −14 dB, ratio 5:1, soft knee (30 dB wide), attack 3 ms, release 250 ms, plus a make-up gain of about +5 dB.

• ON/OFF: default ON. Switching it off also removes the make-up, so the dry signal is about 5 dB quieter (compensate with VOLUME). The detector keeps running, so switching is click-free.
• MIX 0–100 %: a linear blend between the uncompressed (0 %) and the compressed signal (100 %) — parallel compression. At 100 % it is identical to the plain compressor.

Master. Dry, delay and reverb are summed and scaled by VOLUME; then a soft clipper takes over: linear up to 0.7, a tanh knee above it, with a limit of exactly 1.0, so a resonance overshoot rounds off instead of clipping square. A hard limit at ±1 follows.

12The sequencer

In SEQ mode the plug-in plays a pattern of up to 16 steps. One step is a sixteenth note at the project tempo. Each step has a note, a gate (sounds or rests), an accent flag and a slide flag.

Editing steps

• Note lane: click or drag in a column to set the note of that step (two octaves, C1 to C3); the name of the note (for example D#2) is printed under the lane. Right-click a column to switch the step’s gate on or off.
• GATE / ACC / SLD rows: click a cell to toggle it; drag across a row to paint the same state onto several steps — the quickest way to lay out a run of accents or slides. GATE off = rest, ACC (white) = accent, SLD = the step is tied to the next one (slide).
• The column of the step that is playing is highlighted. Steps beyond LEN are dimmed and do not play.

Playback controls

Pattern tools

• COPY: click COPY (it lights and SELECT TARGET BANK appears), then click a bank letter: the active bank is copied into it. The active bank does not change.
• RND: a random pattern in the active bank. Notes come from a minor pentatonic over C2 with now and then an octave down; about 78 % of the steps sound, 22 % are accented, 16 % slide. Step 1 always sounds, accented.
• MUT: a mutation: three random steps are changed (a new note, or gate, accent or slide flipped), the rest stays recognisable.
• CLEAR: all steps become rests on C2.
• UNDO: takes back the last RND, MUT or CLEAR of the active bank (one level; the button lights while an undo is available). It does not cover manual step edits.

All four banks, the length, shuffle and gate settings are saved with the project. RND, MUT, CLEAR and UNDO work on the active bank only.

Transport and MIDI transpose

The sequencer runs while the project plays (and, with FREE RUN, also while it is stopped). On start it picks up the step under the cursor; a loop jump, a position change or a tempo change re-syncs it, but the ordinary running does not, so the timing stays steady. Incoming MIDI notes transpose the pattern: C2 (MIDI note 36) is the original pitch, every semitone above or below shifts the pattern accordingly while the key is held; MIDI TRANSPOSE +n shows the shift. When the key is released the pattern returns to its original pitch.

13MIDI

Notes

In HOST mode incoming notes play the voice (velocity ≥ ACCENT VEL = accent, overlapping = slide, last note wins). In SEQ mode they transpose the pattern. The plug-in listens on all MIDI channels. Notes are consumed by the instrument: unless MIDI THRU is on they do not appear at the output.

Controllers

Pitch bend, clock and all other controllers are not used by the voice; they are passed to the output.

THRU and SEQ OUT — two different sources

• MIDI THRU (header) forwards the incoming notes to the output. CC, pitch bend and clock always pass through.
• SEQ OUT (sequencer panel) sends the self-generated sequence to the output.

In SEQ mode incoming notes are transpose commands — whoever also passes them on with THRU sends them out next to the sequence. MIDI events carry their sample offset: notes start sample-accurately, and the sequence leaves the plug-in with the same accuracy.

The pattern mirror (gmem)

The plug-in mirrors the displayed bank into shared memory named tt303_acid. A ReaScript can read it and turn the pattern into a MIDI item; the script tt303_pattern_to_midi.lua does exactly that (accent = velocity 127, otherwise 100; slides tie legato into the next step). It is a separate file that is not part of the plug-in package. With several instances in a project, the last one processed wins.

14Preset bar & user bank

The preset bar sits in the header, directly left of OPTIONS: < name > SAVE .... It replaces the six patch buttons of earlier versions.

The factory presets

The six FACTORY entries are the former patch buttons. A factory preset sets the sound controls listed below and leaves everything else (tuning, fine, volume, accent velocity, slide time, the delay division, the EQs, the compressor, the pattern and its banks) alone — except INIT, which also restores slide time 60 ms, dotted-eighth delay, room size 70 %, the three EQ strips, compressor ON / 100 %.

All values are knob positions (0–100), except drive (1–40) and feedback (%). They are starting points: every control stays editable.

What a user preset contains

All sound controls (waveform, tuning, fine, cutoff, resonance, env mod, decay, accent, volume, slide time, accent velocity, drive on / model / amount, delay mix / division / feedback, reverb mix / room size, the three input EQs, compressor on / mix), the pattern settings (length, shuffle, gate, active bank) and the four pattern banks A–D with all steps. The pattern is always part of a user preset (for an acid machine the pattern is part of the sound), so loading one replaces the pattern too. Not part of a preset: the operating mode (HOST / SEQ), FREE RUN, SEQ OUT, MIDI THRU, the colour scheme (OPTIONS) and the transport.

Where the user bank lives

A plug-in cannot write files, so the 32 user slots are part of the plug-in state: they are saved with the project, with FX-chain and track-template copies, and in every REAPER preset of this plug-in (the + button in the FX window toolbar). To take your bank to another project or computer, save a REAPER preset once (+ → Save preset) and load it there: the whole bank comes with it. Projects saved with earlier versions (v1.0, v1.1) load unchanged with an empty bank.

15Options / Color

The OPTIONS button is at the right end of the header (up to v1.1 it sat between THRU and MODE; since v1.2 the preset bar is on its left). Click it, then a swatch. A click outside the box closes it.

COLOR offers ten schemes: ACID, CYAN, ORANGE, MAGENTA, YELLOW, RED, BLUE, VIOLET, MINT and ICE. A scheme changes the accent colour (knob rings, active buttons, step bars, LEDs, traces), the dimmed accent and, for RED to ICE, the slightly tinted panel background of the whole front at once. The choice applies per plug-in instance and is saved with the project and with FX-chain copies. The factory colour is ACID, the classic acid green (#39FF14). Loading a preset (factory or user), RND, MUT or UNDO does not change the colour. It is stored in the hidden parameter 36 (UI Color, 0–9); the sound is not affected.

16Automation & parameters

All sliders are hidden in the plug-in window, but every one is a normal REAPER parameter: use the Param menu of the FX window (or the automation list of the track) to put any of them on an envelope or to map it to a controller. The values are saved with the project. Names as REAPER shows them:

REAPER adds its own Bypass, Wet and Delta after these. Moving a knob on the front, a CC (section 13) or a preset load all move the same parameters.

17In practice

• The classic squelch: preset SQUELCH (saw, resonance 80, env mod 82, decay 22, accent 80), then play with CUTOFF FREQ while the pattern runs; set ACC on a few steps and SLD on two or three.
• A bass that sits in the mix: DEEP or DUB with drive off. The delay EQ high cut around 3 kHz and the reverb EQ low cut at 300 Hz (both INIT defaults) keep echoes and tail out of the low end.
• Dirt without losing the fundamental: DRIVE on, FOLD or FUZZ, and raise the drive EQ low cut (LO) to 150–300 Hz: the bass stays clean while the mids are distorted.
• Play it from MIDI, tie the notes: in HOST mode let notes overlap a little in the piano roll for slides, and use velocity 100+ (or lower ACCENT VEL) for accents.
• Pattern ideas: press RND until something sticks, then MUT for variations; COPY into the next bank before you go on; UNDO takes back the last RND / MUT / CLEAR.
• Songs from banks: put the parts in A–D and automate SEQ pattern bank (parameter 25).
• With TT-303PATGEN: the TimTechlor pattern generator writes acid patterns as MIDI; insert it before the TT-303, keep the TT-303 in HOST mode and use velocity for accents and overlaps for slides (see the TT-303PATGEN manual).
• Play the sequence on another synth: SEQ mode, SEQ OUT on, and route the track’s MIDI output; turn VOLUME down if the TT-303 itself should stay quiet.
• Parallel compression: COMP at 40–60 % mix keeps the accents lively while adding body. With COMP off, raise VOLUME to make up the roughly 5 dB.

18Troubleshooting

• No sound. TT-303 makes sound only from MIDI: in HOST mode it needs notes on the track (record-arm or a MIDI item; the keyboard display lights when notes arrive), in SEQ mode it needs the project to play or FREE RUN on. Check VOLUME and that the track is not muted.
• The voice does not play my notes / the pattern plays transposed. In SEQ mode MIDI notes only transpose (C2 = original pitch). To play the voice from MIDI, switch to HOST.
• Notes come out twice. MIDI THRU passes incoming notes, SEQ OUT sends the sequence: with both on in SEQ mode you send both. Switch off what you do not need.
• No slide. In HOST mode the notes must actually overlap (the second starts before the first ends). In SEQ mode set SLD on the first of the two steps; the flag ties a step to the following one. With SLIDE at 0 ms the pitch jumps.
• No accent. HOST mode: the velocity must reach ACCENT VEL (default 100). SEQ mode: set ACC on the step. Also raise the ACCENT knob; at 0 % an accent has no effect.
• The sound got quieter by about 5 dB. Compressor OFF also removes its make-up gain; turn COMP on or raise VOLUME.
• No echoes / no tail. Delay and reverb are bypassed below 0.5 % MIX. The delay EQ and reverb EQ are input filters: a high low cut or a low high cut can leave nothing to delay.
• Tempo feels wrong. The plug-in reads the project tempo; if REAPER reports none (under 10 BPM) 120 BPM is used. The delay and the sequencer both follow it.
• The FX window still shows the old name or version. REAPER caches the description of a JSFX (the desc: line) in reaper-jsfx.ini in its resource folder, so after an update the window title can keep the old text. It is cosmetic: the version in the header of the front is the real one. To refresh it, close REAPER, delete reaper-jsfx.ini (REAPER creates it again and re-reads all JSFX at the next start) and start REAPER.
• The first click in the plug-in window does nothing. A click that only gives the window the focus can be swallowed by REAPER; click again.
• The front is small. Drag the FX window larger: the front scales to the window.
• A project from v1.0 or v1.1 shows an empty user bank. Expected: older projects have no bank; it starts empty and all sounds and patterns of the project load unchanged.
• Names cannot contain a space / Esc closes the window. See the box in the preset section: use Shift+Space and CANCEL.

19Release notes

v1.2 — 2026-09-30: preset bar

• New preset bar in the header (< name > SAVE ...): six FACTORY presets and a 32-slot user bank; RENAME, COPY, DELETE, INIT. A user preset contains all sound settings and the four pattern banks. The bank is saved in the project, FX chains and REAPER presets.
• The six patch buttons are gone (their sounds are the FACTORY list); OPTIONS moved to the top right of the header.
• New hidden test parameters 37–40 for the preset bank. The audio is unchanged against v1.1 (bit-identical render).

v1.1 — 2026-09-30: Options / Color

• New OPTIONS box with COLOR: ten colour schemes (ACID, CYAN, ORANGE, MAGENTA, YELLOW, RED, BLUE, VIOLET, MINT, ICE), stored per instance in the hidden parameter 36.
• No change to the sound: a render of v1.1 is bit-identical to v1.0.

v1.0 — 2026-09-07: first full release

• The sound engine was fitted to recordings of a TB-303 software reference: 3-pole 18 dB ladder with new cutoff and resonance curves, filter decay of 30–570 ms, a fixed VCA sag, an RC slide in log frequency, a narrower square duty, a level trim.
• Factory presets retuned for this engine; start pattern PUMP in bank D; INIT with the working defaults (delay and reverb EQs, compressor on).
• Compressor on/off with a MIX knob, in its own COMP column of the effects panel; version badge in the header; the CUTOFF and DECAY read-outs show the real values of the engine.
• Pattern mirror in shared memory for a companion ReaScript that exports the displayed bank as a MIDI item.

20Technical data & honest notes

Honest notes

• TT-303 is a tribute model. The filter, envelope, slide and square-duty values were fitted to reference recordings of a TB-303 software emulation; the match is close for level, decay and the filter curves, not exact. In the filter, resonance peak and corner frequency sit somewhat higher than in the reference (a limit of the three-pole ladder structure); the cutoff range was set a little darker on purpose, by ear.
• The square wave has fewer strong high harmonics (from about the 6th) than the reference.
• The default slide time (60 ms as time constant) has not been confirmed against a reference; the reference glide was estimated at about 100 ms in total, so a setting of 30–40 ms comes closer.
• The user bank was verified through the project and FX-chain round trip. The REAPER preset file path relies on the same plug-in state, which the other plug-ins with this preset module were tested with, but it was not run separately for TT-303.
• Pitch bend is not applied to the voice.