SynthCraft: A Free Online Synthesizer (and How Subtractive Synths Work)

SynthCraft is a free online synthesizer that lives inside SoundCraft, our browser DAW. Two oscillators, a noise source, a multimode filter, two envelopes, an LFO. That happens to be the classic parts list of subtractive synthesis, which makes it a good place to learn how synths actually work: you can read a paragraph, then reach over and turn the exact knob it was talking about. Nothing to install, no demo timer, no watermark on your exports.

This post walks through each section of the panel, explains what it does to the sound in plain words, then puts it together into a bass patch you can build in about two minutes.

A free online synthesizer you can open in a tab

SynthCraft is one of SoundCraft's built-in instruments. Open the editor, click the ☰ rack in the top corner, switch to the Instruments tab, and click the SynthCraft card. You get a new synth channel with the compact panel docked in the workspace below the timeline. The ⛶ Full view button pops the same panel out into a draggable window with a two-octave keyboard under it, which is the view I'd use while you learn. The editor itself is free (here's the full SoundCraft tourif you're new), and a free account is all you need to save what you make.

Oscillators: where the sound starts

An oscillator makes a raw tone, and its wave shape decides the character. SynthCraft gives you four per oscillator, and it's worth clicking through them while holding a note so your ears learn the difference. A saw is buzzy and bright, the fullest of the four, and it's the default for a reason: it has every harmonic, so the filter has plenty to carve away. A square sounds hollow and reedy, a bit like a clarinet or an old video game. A triangle is soft and round. A sine is a pure tone with no harmonics at all, which makes it useless to filter but perfect for sub bass.

There are two oscillators so you can stack them. Each one has Oct (±2 octaves), Semi (±12), Fine (±50 cents) and a Level knob. The oldest trick in synthesis is two saws a few cents apart: the slight pitch clash makes the sound wide and alive instead of flat. SynthCraft ships with that trick pre-loaded. On the Init patch, oscillator B already sits at +7 cents fine detune with its Level at zero, so raising one knob gives you instant width. There's also a Noise level in the filter column, which is what you reach for when a patch needs breath or hiss.

The filter is the tone shaper

Subtractive synthesis means you start with a bright wave and subtract harmonics from it. The filter does the subtracting. Cutoff decides where it starts cutting (SynthCraft's low-pass runs 40 Hz to 18 kHz, default 9 kHz), and sweeping that one knob while a note plays is the most recognizable move in electronic music. Close it and the sound gets dark and muffled; open it and the buzz pours back in. If you only ever learn one synth control, learn cutoff.

Res (resonance) boosts the frequencies right at the cutoff point. A little makes the sweep sound sharper and more vocal. A lot, and SynthCraft's goes up to 20, turns into a whistling ring that's a genre choice of its own. The filter also has an Env amt knob, which lets an envelope push the cutoff open by up to four octaves every time a note starts. That's the secret behind almost every squelchy or plucky synth sound you've heard.

Envelopes, in plain words

An envelope is the shape of a note over time: Attack (how long it takes to reach full volume), Decay (how long it takes to fall back), Sustain (the level it holds while you keep the key down) and Release (how long it rings after you let go). SynthCraft has two of them. The amp envelope shapes loudness. The filter envelope shapes brightness, through that Env amt knob.

Here's the practical version. A pluck is a fast attack, a short decay, sustain at zero and a short release; the note is over almost as soon as it starts, no matter how long you hold the key. A pad is the opposite: the Soft Pad preset uses a 450 ms attack and a 900 ms release, so every note fades in late and breathes out long after you let go. Between those two extremes is basically every synth sound there is.

The LFO adds the movement

An LFO is an oscillator too slow to hear as a pitch (0.05 to 20 Hz here), used as a hand that wiggles another knob for you. SynthCraft's can target three things. Point it at Pitch and you get vibrato, capped at ±1 semitone so it stays musical. Point it at Filter and you get wobble, the dubstep staple, especially with the rate around 2 to 4 Hz and the cutoff low. Point it at Volume and you get tremolo. Depth controls how far it wiggles, Rate controls how fast, and slow settings are almost always more useful than fast ones.

Steal from the presets

The Presets menu in the panel header has six starting points: Init, Sub Bass, Fat Bass, Saw Lead, Soft Pad and Pluck. Each one replaces the whole patch, so flip through them and watch what the knobs do. Two are worth dissecting.

• Sub Bass is one sine wave dropped an octave, oscillator B off, and the filter barely doing anything at 900 Hz because a sine has nothing to cut. The whole patch is the wave choice. That's a lesson on its own: sometimes the right sound is the simplest possible routing.
• Pluck is the envelope lesson. Sustain sits at zero on both envelopes, decay is fast, and Env amt is cranked to 75%, so the filter snaps open at the start of the note and slams shut. Hold a key as long as you like; the note dies anyway. The shape is the sound.

Build your first bass patch

Time to make one from scratch. Start on Init and dial this in (it lands close to the Fat Bass preset in the screenshot above, which is the cheat code if you get lost):

1. Oscillator A: saw wave, Oct to -1, Level around 90%.
2. Oscillator B: square wave, Oct to -1, Fine at +12 cents, Level around 60%. The square fills in the low mids under the saw's buzz.
3. Filter: low pass, pull Cutoff down to about 700 Hz, Res up to 4. It should sound dark and thick now.
4. Filter envelope: Attack 2 ms, Decay around 220 ms, Sustain 15%, Release 120 ms, then raise Env amt to about 45%. Now every note gets a bright little bite at the start before it settles.
5. Amp envelope: fast attack (3 ms), Sustain around 70%, Release short at 140 ms so bass notes stop when you stop. Long releases turn basslines into mud.

Play it low, around C2 to C3. If it clips or crowds the mix, pull the Volume knob down a couple of dB; the preset sits at -5 dB for exactly that reason.

Playing it: on-screen keys, MIDI, or the piano roll

The Full view window has a C3 to C5 keyboard at the bottom. Click and hold a key and you get the real attack and sustain; let go and you hear the release tail, which makes it honest enough to audition envelope changes with. A hardware MIDI keyboard plays the synth directly whenever its track is focused, and there's a ▶ C4 button in the header for a quick one-note preview while you tweak.

For actual writing, right-click the synth's lane to insert a MIDI clip and draw notes in the piano roll. Our guide to making beats online builds a whole bassline on this exact synth if you want the sequencing half of the story. And if you own real synth VST3s like Massive X or Omnisphere, the free ClipCraft Bridge hosts them inside SoundCraft too; here's how that works.

Everything in this post runs on the free plan, and the editors stay free on every tier; paid plans sell storage and server compute, not instruments. Open the synth, load Init, and spend ten minutes sweeping the cutoff while you switch wave shapes. That one exercise teaches more than any diagram.