My x0x-heart

I bought the x0x-heart circuit board from Open Music Labs The x0x-heart is a clone of the TB-303 voice. It is just the synth voice, without a sequencer. That suits me just fine because I build my own sequencers and I am always looking for stuff to sequence. The x0x-heart is sold as a circuit board fully populated with surface mount parts. The only (ha, only) things missing are the pots, switches, jacks and an enclosure. Bottom line: I love it! It sounds just like a 303, and for $79 bucks, a DIYer can’t go wrong.

All of the information you need to assemble the x0x-heart is in the x0x-heart Wiki, so I won’t repeat all of it here. However, Open Music Labs included a lot of options in their design, and I will talk about how I implemented those options and how I would do it differently next time.


I used a gray case to sort of match the TB-303’s silver case. That enclosure also comes in black or tan. The printing on the control panel was done with my usual technique. I ink jet print the text and graphics on a sheet of vinyl sticker stock, cut out the holes with a Silhouette Cameo digital cutter, spray it with Krylon Preserve-It, then stick it on. For this design, I used a background image of brushed aluminum, which matches the gray case well and adds a subtle classiness. The typeface was chosen to match the Open Music Labs logo. To give it a splash of color, I chose red knobs and red switch caps. I am very pleased with the overall look of it.

The Enclosure

I have an obsession with fitting circuits into the smallest possible enclosure. So, I went with the SERPAC 251-I-G which is 5.63" x 3.25" x 1.54". I also like to have battery powered synths, and that enclosure has a 9V battery holder. Of course, all of the control panel elements and jacks also need to fit as well, so I did the layout for that stuff before settling on that enclosure. Here is how the x0x-heart PCB fits in the enclosure.

x0x-heart Case Fitting

It doesn’t quite fit, but it is close. The board covers two of the holes that the case screws go through. However, if two of the corners are cut off, then it does fit. An examination of the PCB reveals that it can be done without cutting any traces. Once the corners are cut off, it is then possible to get a screw through one of the PCB mounting holes into one of the mounting bosses built into the enclosure. One screw is plenty to hold it in place. This is what it looks like mounted in the enclosure with all of the wiring in place.

x0x-heart In Enclosure

If you look in the lower right hand corner of the above picture, you will notice that I used the holes provided for a eurorack power header to mount a 5V regulator. The x0x-heart uses a charge pump to generate +/- 12V from a 5 volt supply. It will run on as little as 3.3 volts, but no more than 5. I wanted to run it on a 9V battery or standard 9V pedal power supply, so the regulator was necessary.


As I mentioned, there are a lot of options built into the design of the circuit board, so if you want to build one, you have some decisions to make. You have options for knobs and switches, and lots of options for jacks. Some of the parameters in the x0x-heart are not CV controllable, but are PWM controllable. There is a eurorack adapter schematic on the Wiki that shows one way to handle voltage control of the x0x-heart’s various parameters.


The Tuning, Cut Off Freq, Env Mod, Decay, Accent, and Volume knobs have the same function as in the original TB-303. In the original, the Resonance control is a dual potentiometer. In the x0x-heart, you have the option of using two different pots, one of which is the actual filter resonance, and the other, which I labeled Accent Cuttoff, controls the amount the accent affects the filter cut-off frequency. In the original, those two pots are wired in reverse, so that when the resonance is highest, the accent cutoff is lowest. I used two separate pots, in large part to make the layout symmetrical. However, I find that the Accent Cuttoff has a minimal effect on the sound. Symmetry aside, I would recommend using a dual pot like in the original. You could even hard-wire it in the center by soldering a pair of 22K resistors from pins 1 and 3 to pin 2 of the RES2 connector.


There are several options here as well. In the original, there is a waveform switch for selecting sawtooth or square wave. I used a three-position switch here so that I could effectively disconnect the internal oscillator and just use the Mix In signal.

The x0x-heart gives you two options for slide, in and out. The difference is which side of the CV input buffer the slide capacitor is connected to, the input side, or the output side. In the original, it is one or the other, but chose to use a three position switch to set it to in, out or off. I can barely hear the difference between off and in, so I would recommend wiring this switch as in the original.

Accent and decay are interesting. In the original, when accent is on, decay is off, but the x0x-heart gives you the option of controlling them separately, and I think that is a real improvement. In my build, I used a separate switch for decay and accent. There is also a decay amount and an accent amount knob, so using switches there is kind of redundant. In the original, the accent can be set on individual notes, so really this should be a jack, not a switch.


The x0x-heart also gives you lots of options for inputs and outputs. They are covered well in the user manual, so I will not repeat all that information. As I mentioned in the above paragraph, I should have made the accent switch into a jack. I recommend that you do that if you are building one. The jacks that I did implement are Output, Gate, CV, and Mix In. All of them work like the original except the Mix In jack. The x0x-heart gives you several places on the circuit board where you can inject an external signal: into the VCF, the VCA, or the final mix. The original’s Mix In jack simply mixed an external audio signal (like from a TR-606) with the TB-303 signal so that they both show up at the output jack. I did not find that to be very useful, so my mix-in jack goes into the filter. That way I can use the x0x-heart’s filter and VCA to modify external signals. It works great for white noise, which you can hear on this track that I made. The rhythmic thumping sound is white noise through my x0x-heart with a Trigger Man sequencer on the gate input.


I am very happy with my little x0x-heart! Even with my relatively simple implimentation of the circuit, I can make some fantastic sounds.