gizm0x x0x-heart MODs
Analog TB-303 acid bass voice for Eurorack
18 HP Eurorack module
The x0x-heart MODs is a Eurorack analog bass synthesizer voice based on the classic Roland TB-303, featuring a VCO, resonant low-pass VCF, and VCA with accent and slide functionality. Built by gizm0x on redesigned PCBs from Open Music Labs open-source schematics, it captures the iconic squelchy, acid bassline character with added switches for extended range and tonal shaping.
Features
- Analog TB-303 clone bass synthesizer voice
- Based on Open Music Labs open-source design with additional improvements
- Three stacked PCBs: Pacemaker (front panel) and Heart (sound generation)
- Dual-row pin headers for improved connection reliability
- Protection against power cable polarity reversal
- HI-RANGE switch extends cutoff frequency range upward
- RES.BOOST switch extends resonance to self-oscillation
- ENV.M.INC switch expands envelope modulation depth range
- BASS BOOST switch for strong low-frequency enhancement
- VCF OUT provides pre-VCA filtered signal
- VCA OUT provides final output with volume control
- External VCO input with EXT:INT mix toggle
Controls & Jacks (25)
- CUTOFF CV — CV input for cutoff frequency control (0V to ~5V).
- SLIDE TRIGGER — Trigger input to activate slide function.
- 1V/OCT — 1V/oct pitch CV input (+1V to +5V, 4 octaves).
- RES⇒ACC — Adjusts filter modulation depth applied during accents. Turning counter-clockwise increases modulation depth. Set to same position as RESONA
- CUTOFF — Adjusts the cutoff frequency of the low-pass filter. Turning clockwise increases cutoff frequency and opens the filter. Can be modulated via
- VCA OUT — Final output after VCA processing.
- GATE — Gate/trigger input for controlling envelope and note triggering.
- DECAY CV — CV input for decay time control (0V to ~5V).
- ACCENT TRIGGER — Trigger input to apply accent to output.
- ACCENT — Controls accent effect including volume and filter modulation. Turning clockwise increases accent intensity. Apply CV (0V to ~5V) with knob
- TUNING — Tunes the VCO pitch. Module follows 1V/oct standard. Center position outputs approximately C1 to C5 with CV range +1V to +5V (4 octaves).
- EXT IN — Input for external VCO signal.
- VCF OUT — Filtered waveform output (before VCA).
- ACCENT CV — CV input for accent depth control. Range: +0V to approximately +5V.
- RESONANCE — Adjusts the resonance of the filter. Turning clockwise increases resonance, emphasizing the filter's resonant frequency.
- ENV MOD — Adjusts the envelope modulation depth. Turning clockwise increases modulation amount. Apply CV (0V to ~5V) with knob fully counter-clockwise
- DECAY — Adjusts the decay time of the envelope. Turning clockwise increases decay time. Apply CV (0V to ~5V) with knob fully clockwise for CV contro
- ENV MOD CV — CV input for envelope modulation depth (0V to ~5V).
- SAW:SQR — Selects VCO waveform: Left = Sawtooth, Right = Square, Center = Mix of both (slightly reduced output).
- HI-RANGE — When ON (up), extends the cutoff frequency range upward by one step.
- ENV.M.INC — When ON (up), expands the adjustment range of the modulation depth.
- RES.BOOST — When ON (up), extends resonance range up to self-oscillation.
- BASS BOOST — When ON (up), boosts low frequencies across the entire signal. May produce undesirable effects with some connected modules, amplifiers, or s
- EXT:INT — Selects VCO source: Left = External VCO signal, Right = Internal VCO, Center = Mix of both signals.
- VOLUME — Adjusts the output level. Turning clockwise increases volume.
How-To Guides
- Classic Acid Bassline
- Self-Oscillating Filter Sine Bass
- Wide-Range Filter Sweep
- External Synth Through 303 Filter
- Fat Mixed-Waveform Bass
- Slow-Slide Dub Bass
- Screaming Acid Lead
- CV-Controlled Filter Automation
- Internal + External VCO Layer
- Parallel VCF and VCA Processing
Explore the interactive x0x-heart MODs panel on BleepDex — tap any control to learn what it does.
