about CHRT

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The content of this page is relevant to CHRT rev.1.19. From rev.2.0, the page requires some updates. I am working on them, though circuits and customers have priority. Thank you for understanding.


CHRT is an open-design synthesizer. I started building it after a long unsuccessful search for an analog which would be played on a WX5 wind controller with satisfactory control over dynamics and phrasing, having natural analog electric sound at the same time. Experimenting and research led to a simple logical conclusion:
If synth sound -> then analog.
If analog -> then valve.

Another thing to explain is the unusual architecture of the synth. It appeared quite clumsy to try to reproduce the established structures like ladder or Curtis filter in a valve circuit. I rather followed the path of finding "what the valves want to sound like" and tried to allow them to do so.
The fact that there is not a single "right way" to design a circuit like this is also the reason why it is distributed as an open-design. I would be more than delighted if someone finds this design - or any part of it - useful in some way and explores more possibilities. If you do, please let me know through CONTACT link on HOME page. All documentation on this website is available under GNU or CreativeCommons licenses, please follow the rules of open source.

In order to save reading time to someone who would not be interested in CHRT anyway, here is

  • sharp filter sweeps
  • bubbly basses
  • helicopters, UFO landing, etc...
  • upredictable sound effects

Valve circuits are often thought of as unstable, unpredictable, crazy sounding, always out of tune, humming creatures. The CHRT Synthesizer tends to be just the opposite: perfectly in-tune, clear and natural sounding predictable instrument with timbre possibilities that never go off the edge, an instrument you either like or not.

not a cheap synth

The CHRT synthesizer design aims at function and quality. It avoids low-cost compromises in all aspects and was created "by ear", not by any circuit simulation program, which took some time and experimenting effort. All that is reflected in the price, yet there are DIY variants which are more friendly to your budget. On the other hand, for those who are willing to pay a bit more to get something special, there is the offer to build the synth into any reasonably sized case of your choice. I have done the same for myself.

how to play

CHRT is primarily designed as a sound module for wind controllers. When the trigger levels are properly set up, it has a range of at least 5 octaves (the whole range of e.g. Yamaha WX5). It perfectly responds to your phrasing and dynamics (slurs, staccato, crescendo/decrescendo, sforzato, accents). The accentuation can be spiced up with the EG functions with adjustable velocity dependency (in wind controllers, this message byte does not always follow the breath-controller byte). The synth certainly responds to lip pressure and PitchBend control.

If you are a keyboard player, I would recommend an alteration of the control firmware (free of charge), otherwise the dynamic abilities are wasted unless you play with a breath controller. There is not one standard keyboard firmware because I need to know first how you prefer to control the dynamics (e.g. ModulationWheel or AfterTouch)


The principle of synthesis adopts slightly different approach from mainstream subtractive analogs or virtual analogs, where a mathematical wave (saw, square) is passed to the filter which gives the instrument its character. CHRT generates its characteristic sound already in the oscillator, a triggered thyratron relaxation circuit, with a waveform that is far from a perfect triangle. It is passed to a damped Abraham-Bloch multivibrator which serves the same purpose as a resonator body of an acoustic instrument. Then follows another resonating body - a low pass LC. The last stage is a VCA, modulated by breath CC and/or EG.

The unique character of the CHRT synthesizer is achieved by these two features:
  • The oscillator is triggered, which means each articulated note begins with a characteristic transient of a starting oscillator
  • The timbre is a result of gain, load and impedance settings around the resonator, you will not find the traditional cutoff or resonance controls on this synth.

digital control and the valve myth

There is a frequently encountered myth that a valve circuit, especially a synthesizer, has to be a crazy behaving unstable animal with a lot of background hum. Can be, if it is done so. The CHRT design follows the simple rule that valves sound better and digital circuits are better at control. This means that the sound signal path is not touched by anything else than a valve grid and the tone frequency and VCA CV are generated by a microcontroller-based circuit. The digital circuit based on an ATMega328 interprets the MIDI messgage pitch-related bytes as the frequency of oscillator trigger pulses and the dynamics-related bytes as the VCA control voltage.

hum and noise

The CHRT synthesizer is designed with respect to possible hum and noise isssues:
  • solid chassis and case shielding
  • proper grounding topology of PSU and analog board
  • the analog board is not a PCB in order to have a solid ground console and shortest wiring paths
  • signal leads to front panel controls are properly shielded without ground loops
  • the heater supply is DC elevated with a hum dinger
  • the output jack has a safe ground lift option

high voltage and safety

Whatever you do with the CHRT hardware, whether you are assemblig a DIY kit or experimenting with circuit alterations, please do keep in mind, that:


If you are even in a slightest doubt that you are doing the right thing, CONTACT me.