An acid bass is one oscillator, a resonant lowpass filter, and an envelope biting at the cutoff. The Roland TB-303 did nothing else, and that limitation is the sound. Serum 2 can copy it in about five minutes.
The patch is the easy half. The 303 feel actually lives in slides and accents, so I’ll cover how to fake those properly too.
The core patch
- Init a patch. One oscillator, plain saw. The 303 also had a square, so try both. Saw is nastier, square is rounder and more rubbery.
- Unison off. One voice. An acid line is skinny by design, and that’s what lets it cut through a busy techno mix.
- Lowpass filter, cutoff pulled way down, around 200–400 Hz to start.
- Resonance high, around 60–80 percent. High enough to whistle, just short of self-oscillation.
- Send an envelope to cutoff: fast attack, decay around 150–300 ms, zero sustain. Amount fairly strong.
Now every note is a little filter squelch that opens and snaps shut. That squelch is the entire genre.
One non-obvious detail: the 303’s filter was a diode ladder that slid around 18 dB per octave, softer than a typical 24 dB lowpass. If your acid patch sounds too clinical, try a gentler filter slope. The resonance peak gets less icy and the low end keeps a bit more body.
Slides: the part everyone skips
The 303 slid between tied notes instead of retriggering them. In Serum 2 you fake it with two settings:
- Mono voicing with legato on, so overlapped notes don’t restart the envelopes.
- Portamento around 60–100 ms, set to glide only on overlapping notes.
Then the slide is a playing technique, not a patch setting. In your piano roll, overlap the tail of one note into the next by a tick or two. Overlapped notes glide, separated notes snap. A line where every third or fourth note slides instantly sounds ten times more like acid than any filter tweak.
Accents: velocity into cutoff and volume
The 303 had one accent button, and it did two things at once: louder note, brighter filter. In the mod matrix, route velocity to cutoff and velocity to volume. Then draw most notes at low velocity, around 60–70, and push accented notes to 127.
The common mistake is mapping velocity to volume only. That gives you dynamics but no bite. The accent has to open the filter or it doesn’t read as acid.
Here’s a squelchy tech house take on the idea from one of our packs, built exactly this way: square wave, resonant filter, velocity doing the work.
Sequencing an acid line
Patterns matter more than the patch. What works for me:
- Straight 16th notes as the base grid, mostly one or two pitches.
- Octave jumps on a few steps. The 303’s up/down octave transpose is why classic lines leap around.
- Accents on off-beats, not on the kick. That’s what makes the line push against the groove.
- Keep it short. Real acid lines are one bar, maybe two, mutating slowly via the filter, not the notes.
Then automate cutoff and resonance across the whole 16 or 32 bars. Acid tracks are basically one pattern plus twenty minutes of filter automation, and that’s not a criticism.
Dirt: distortion after the filter
Clean acid is a myth. The classic records ran 303s through overdriven mixers and pedals. In Serum 2’s FX chain, put distortion after the filter, not before. Before the filter, drive flattens the resonance peak you just dialed in. After it, the peak hits the distortion and you get that screaming top end.
Soft clip or tube-style drive at low amounts for house, harder clipping for techno. I also nudge a little EQ boost around 1–2 kHz when the line needs to fight a busy percussion bus.
If you want working starting points instead of building from init, grab the free Serum 2 techno presets and check how the acid-style patches route velocity in the mod matrix. Reverse-engineering a working accent setup is the fastest way to learn it.
Where it sits in techno today
Acid lines show up in modern techno as a mid-layer riff over a rumble bass, not always as the lead. Keep the acid line highpassed around 100 Hz and let a separate sub carry the weight. The 303 never had much real low end anyway, which is partly why it layered so well. I usually pull that sub from a techno pack rather than build it twice, and our Serum 2 presets sorted by genre make that a ten-second job.
Here’s a harder, modern techno cousin of the sound in context.
And when free packs stop being enough, the Serum 2 preset bundles bundle our techno and house libraries at the lowest per-preset price. That’s the sensible upgrade path, no urgency implied.
For the wider low-end picture, see tech house bass in Serum 2 and the full techno in Serum 2 guide.
FAQ
Saw or square for acid bass?
Both are authentic. Saw is brighter and more aggressive, square is rounder and hollow. Classic acid records used each; try the square first if you’re layering over a big sub.
How do I make notes slide like a 303?
Mono legato voicing plus 60–100 ms portamento set to glide on overlapping notes only. Then overlap notes in the piano roll where you want slides.
Why does my acid line sound flat?
Usually missing accents. Route velocity to both cutoff and volume, keep most notes quiet, and spike a few to full velocity.
Does resonance need to self-oscillate?
No. Sit just under self-oscillation, around 60–80 percent. Full self-oscillation adds a whistle that fights the mix on most systems.
One oscillator, one filter, an envelope, and a pattern with slides and accents in the right places. Resist adding a fourth layer. The emptiness is the sound.
I run Monosounds.studio and make the packs myself. Ten-plus years of producing, 97+ releases, 16,000+ downloads. Every Serum 2 preset we ship passes a CPU check on a mid-range laptop and gets used in a real track before release. If it does not survive a mix, it does not go in the pack.
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