Adding extra oscillators to the Monotron


64377_366418110140592_1180467240_n-1In my previous post, I’ve explained how to add midi control to the monotron’s pitch, gate and frequency cut-off using a Teensy chip.

The good news is that the Teensy’s got plenty of raw power left leaving room for extra processing.

My first upgrade plan was to add an extra oscillator, generated by the Teensy, that would then be fet inside the monotron through its auxiliary port.

Since the Teensy is already the master controller for the monotron, the extra oscillator could easily be synchronised from the same midi control. I had an extra DAC output ready to generate a 12-bit signal out of it, it was time to try it out.

The aux input from the monotron is an interesting beast. It’s worthwhile to feed it with different signal at different gains and look at what happens.

Depending on how hard you drive it, two things are going to happen: first it overdrives naturally, which shapes whatever you put through it but, also, it will ‘choke’ the resonance of the filter making it nearly non-existing. You can think of it as the input signal taking all the headroom of the audio path, leaving no room for the resonance to move freely and add its own bite. The first one is highly desirable while overdoing the second one isn’t. I found in the end that using a 33k resistor gave me a good balance between overdrive and resonance choke.

From then on, it was just a matter of programming. I lifted my dirty digital oscillators from the old arduino piano squealer (thank got for gcc-based embedded and c++) and wired the midi control so that the oscillators from the teensy and the monotron’s internal one followed the same gate and pitch.

Here’s a quick take on how it sounds:

Next: Envelope and more oscillator controls !


  1. For me it is not clear, how you connect the 33k resistor to the output.
    Inline in series, or as a voltage divider from the output pin to ground?

  2. I absolutely love the sound of this thing! No amount of $$$ can buy a quirky, dirty sound like this…

    I am sure I can figure out the software part but I know very little about electronics and I am not able to build the hardware based on the information in your post. Is there any chance you could find the time to update the schematics from your previous post to include the digital oscillator?

    1. Thanks for the love. I do agree that nothing else could get the loving wierdness of it. I’ll look at it later and get back to you on this. It’s pretty much the original schema with one or two changes.

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