This is an introduction and overview of the x0x-heart, available at Takazudo Modular. The x0x-heart is a replica module that recreates the iconic TB-303 sound using currently available components, housed in a Eurorack format. You can play 303 sounds using your preferred sequencer.
This product was assembled by Takazudo from sourced components, with panel designs created in collaboration with designer SAUCER. Five panel design variations are available. Your chosen panel is installed on the unit, and the remaining four are included as interchangeable panels.
This product features a modification that improves the VCF behavior over the original x0x-heart design. In the original x0x-heart, the Cutoff response via CV differed significantly from the original TB-303 character, and this has been improved.
Panel-only sets (all 5 panels) are also available separately.
*Note: x0x-hearts sold by our shop prior to 2024 follow the original design and do not include this modification.
This product is available for purchase below.
- Product Photos
- The x0x-heart and DIY Modules
- Features of the x0x-heart
- How to Use the x0x-heart
- VCF Improvement Modification
- Panels
- Manual
- Included Items
- Reference Videos
- Specifications
- Product Warranty
Product Photos
The x0x-heart and DIY Modules
The x0x-heart was designed by Open Music Labs. However, the company only sells x0x-heart PCB panels (through-hole PCB + SMT-assembled PCB) as a DIY project. Therefore, any completed x0x-hearts in the world are modules that someone has built using those PCBs—not mass-produced by a large manufacturer. The design is open source and can be viewed on the following website:
At Takazudo Modular, we purchase the x0x-heart PCBs, source components according to the published BOM (Bill of Materials), and assemble them ourselves.
Before the x0x-heart, there was x0xb0x, an open-source standalone TB-303 clone developed by Adafruit Industries. The x0x-heart inherits x0xb0x's sound design while removing the sequencer section, using currently available components, and fitting it into a Eurorack format—making it a spiritual successor to the x0xb0x.
While the x0x-heart is a TB-303 replica, its components and circuits were researched and designed to reproduce the original TB-303 sound. Some components have been substituted with affordable IC chips to reduce costs. Nevertheless, the distinctive squelchy filter and characteristic envelope of the TB-303 are well reproduced, and the unique bass character from VCA-induced resonance compression is reminiscent of the original TB-303.
Features of the x0x-heart
From a typical modular synth perspective, this module can be classified as what's called a synth voice. Other modules we carry that fall into the same synth voice category include:
- Weston Precision Audio: B2 Kick / Bass Synth Voice
- ADDAC System: ADDAC107 ACID Source
- RYK Modular: Vector Wave
These modules contain a VCO (Voltage Controlled Oscillator), VCF (Voltage Controlled Filter), VCA (Voltage Controlled Amplifier), and EG (Envelope Generator) all within a single module. Since they can produce synth sounds simply by receiving Gate and pitch (1V/Oct) signals, they're often called "synth voices."
If you were to recreate the x0x-heart from individual modules, you'd need to prepare and connect a VCO, VCF, VCA, and EG separately. Having everything in one module makes it very convenient to use.
Faithful Control Recreation
Is the x0x-heart simply a bundle of multiple modules in one? Not exactly. For example, in the TB-303, the built-in Envelope Generator is connected to both the VCF and VCA. When Env Mod (Envelope Modulation) is at 0, the VCF doesn't move, but as you increase Env Mod, the CutOff changes along with the Envelope.
Additionally, the TB-303's Envelope has a non-adjustable Attack and a characteristically short Decay. The Accent affects both Decay and VCA, and Slide affects pitch transitions—the x0x-heart recreates these inter-component control relationships that are recognized as "TB-303 character."
Circuit Design for Authentic Sound
Would recreating all these TB-303-like behaviors produce a TB-303-like sound? Not necessarily. Even if you faithfully reproduce everything described above, it alone wouldn't create an authentically TB-303-like sound. So what else contributes? The components and circuits used.
For example, while VCFs vary widely in design approach, VCAs—primarily used for volume control—also produce different characteristics depending on components and circuits, such as intentional clipping effects. Many of the original TB-303 components have been discontinued and are difficult to source today. However, the x0x-heart was created to reproduce the same sound using currently available components, aiming as close to the original output as possible.
The x0x-heart in Eurorack format doesn't include the TB-303's distinctive sequencer. However, this means you can use the TB-303 sound with your modular synth's flexible sequencers and other tools. By controlling the x0x-heart with CV from your modular system, you gain a more flexible environment for creating original TB-303 sounds.
How to Use the x0x-heart
The x0x-heart's internal architecture naturally resembles the original TB-303, with some additional features added throughout.
The basic signal flow is VCO → VCF → VCA. There's one Envelope that affects both the VCF and VCA. As mentioned earlier, when Env Mod is at 0, the Envelope only controls the VCA, but as you increase Env Mod, the same Envelope is applied to the VCF's CutOff, producing the TB-303's characteristic squelching sound. The Envelope is triggered by CV at the Gate jack.
Sending CV to the Accent jack shortens the Decay, slightly increases VCA Gain, and strengthens the VCF effect. This also mimics TB-303 behavior, producing the distinctive accent effect.
The VCO offers Saw wave and Square wave selection, with pitch controlled by CV from the 1V/Oct jack and the Tuning knob. You can also mix both waveforms together. (Saw:Sqr switch)
Alternatively, external audio can be received via the Ext In jack to replace the VCO, in which case you'll be using the x0x-heart's filter section onward. You can also mix Ext In with the internal VCO (Ext:Int switch), expanding sound design possibilities by mixing with external oscillator sounds.
While CV is received at the Slide jack, pitch changes become gradual, creating an effect similar to the TB-303's slide.
Audio output is available from two jacks: VCF Out and VCA Out. Normally you'll use VCA Out for the authentic TB-303 sound, but you can also use VCF Out for the signal before VCA routing, without the VCA's characteristic clipping character.
The Volume knob controls the output volume and can also produce a soft distortion-like effect.
VCF Improvement Modification
Products sold by Takazudo Modular from March 2025 onward include a modification that improves the VCF behavior over the original x0x-heart design.
Products made and sold before March 2025 followed the original x0x-heart design, which had a characteristic where controlling the Cutoff via CV significantly altered the original sound character.
A modification method to address this issue has been published, and the following changes have been applied to create this improved x0x-heart.
Panels
The panels for this product feature design variations by SAUCER, who also designs the decorative blank panels we sell. At purchase, you select one panel to be installed, and the remaining four are included as interchangeable panels. Swap them out to enjoy your own personalized x0x-heart.
- A: Black background with gold decoration
- B: Gold background with black decoration
- C: Black base with relief-style grouping
- D: Aluminum base with black decoration
- E: Black base with border-style grouping
Panel-only sets of all 5 designs are also available separately.
Manual
The x0x-heart manual is available below, though it is primarily developer-oriented technical content (since only PCBs are sold). You probably won't find much documentation elsewhere on the web, so we recommend referring to the content on this page or resources about the original TB-303.
Included Items
- x0x-heart module
- Power ribbon cable
- M3 screws x 4
- M3 plastic washers
Reference Videos
The x0x-heart was originally funded through crowdfunding, and below is the promotional video from that campaign.
Additionally, I recorded a session playing two x0x-hearts. (These are the older pre-2024 version x0x-hearts.)
Below is another session with two x0x-hearts—this time the modified version, with CV controlling various parameters. From around the 49-minute mark, the output of RYK Modular: ALGO is connected to the x0x-heart's Ext In, replacing the oscillator section.
Below is another recording by Takazudo, playing the x0x-heart while controlling it with various CV sources. This one features a simple combination of x0x-heart and ADDAC104 VC T-Networks, which may give a clearer picture of the x0x-heart's sound character.
Specifications
- Width: 18HP
- Depth: 45mm
- Power consumption: 10mA +12V / 10mA -12V / 50mA 5V
Product Warranty
This product was assembled by Takazudo Modular and does not carry a manufacturer warranty. We will address initial defects within one month of purchase. Since we built it ourselves, we can likely diagnose issues relatively well, so please don't hesitate to contact us if you find any problems.
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That concludes our introduction of the x0x-heart.
The TB-303 has been replicated in many ways, and there are plenty of more affordable alternatives available. However, many of them don't quite nail the authentic TB-303 sound. Having assembled each unit by hand, I've come to feel that the Accent, Env Mod, short Decay, pitch Slide, and VCA clipping all contribute to that distinctive TB-303 character.
If you're seeking that iconic TB-303 sound, I highly recommend giving this module a try.
We hope you find this helpful.