A Modern Twist on the Synth That Defined Techno

Marcus Dunn is a big fan of the classic Roland TB-303 Bass Line synthesizer and the unique sounds it can produce. The TB-303 has become a fixture in the house and techno music scene, where it is prized for its unusual capabilities. But if you want to play with one at home, you’ll find that they don’t come cheap. So, rather than opening up his wallet, Dunn made a clone of the TB-303 called Acid Drip — but unlike the original, Dunn’s creation is easy to play.

While the original TB-303 relied on analog circuitry packed with op-amps, transistors, and perfectly tuned filters, Acid Drip takes a different approach. It is built around an RP2040 microcontroller running custom firmware that uses the Mozzi audio synthesis library. Instead of recreating the original circuitry component by component, Dunn recreated the experience in software, combining classic acid bassline sounds with a modern user interface designed to make experimentation quick and intuitive.

Dunn chose to use a Raspberry Pi Pico-compatible RP2040 board, a 320×240 ILI9341 TFT display, and sixteen Cherry MX mechanical switches arranged in a two-row grid. Three potentiometers provide real-time control over filter cutoff, resonance, and decay, allowing users to shape the characteristic squelchy acid sound while a sequence is playing. The synth outputs audio through a standard 3.5 mm jack and can synchronize with external drum machines and other hardware through a dedicated sync input mode.

Acid Drip also includes a built-in drum machine running alongside the bass synthesizer. The drum section provides eight percussion instruments spread across sixteen preset rhythm patterns, ranging from straightforward house beats to breakbeats and drum-and-bass-inspired grooves. Because the firmware runs across both RP2040 cores, the device can simultaneously handle audio generation, sequencing, display updates, and drum playback.

The sixteen keys double as both sequencer steps and menu controls. Users can enable notes, add accents and glides, adjust note values, load preset riffs, select different synthesizer voices, apply step effects, and alter playback behavior without navigating complicated menus. Additional features include save slots stored in EEPROM, adjustable accent characteristics, evolving note-walk modes, and a hidden collection of preset patterns inspired by classic acid house tracks.

Custom PCBs form both the electronics and the front panel, while the display is mounted directly onto the panel for a polished appearance. Builders can assemble the project using mostly through-hole components, making it accessible to hobbyists with basic soldering experience. Power comes from a rechargeable lithium-ion battery paired with a charging circuit and voltage boost converter, allowing the synth to operate as a portable instrument.

Acid Drip captures much of the spirit of the legendary TB-303 while adding features that would have been unimaginable on the original hardware. If you’d like to learn more about it, check out the step-by-step build guide.