The Undertoner

The latest issue of the Make magazine from January 2024 includes a very nice article (“Synthesizer mit perfekter Stimmlage”) about a simple synthesizer (“The Undertoner”) based on NAND gate oscillators. Watch the video for a first impression.

I have not built the synthesizer yet (albeit I am planning to). Since the circuit runs with 9V I was concerned that the high voltage could damage my active speaker. I hence decided to do a simulation with ngspice in KiCAD first. The article unfortunately does not include the circuit diagram, but only a description of how to build it on a breadboard. The circuit diagram may hence be wrong.

Unfortunately, the voltage signals do not make sense to me yet. The primary oscillators around U1C and U1D seem to do what they are supposed to do. U1C is responsible for the actual tone, while U1D creates the staccato effect.

U1C out (VO2)

U1D out (VO1)

The purpose of the feedback circuit involving U1A and U1B is not entirely clear to me. From the article I understood that they modulate the pulse width up to creating octave jumps. In any case, I do not seem to get a meaningful voltage signal at the speaker inlet.

U1A out (VO3)

U1B out (VO4)

Now I am not sure whether I
a) made a mistake in reconstructing the circuit from the article,
b) am running ngspice with the wrong parameters, or
c) the ngspice model is simply not sufficiently accurate to simulate the reality.

Anybody having experience with this type of synthesizers or KiCAD/ngspice? Would be nice to exchange on the topic.


Finally, I have built it! Still do not understand how, but it works. And it sounds awesome! Check out the video for a demo.