gizm0x-x0x-heart-mods

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.

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