Modular Tetrazzi

After I received my Ciat-Lonbarde Tetrazzi Organ, I spent a lot of time delving into its design.  I wanted to understand it as well as possible in order to be able to play it well manually.  By design, Ciat-Lonbarde instruments are intended for play, as in fun, and their operation is to be joyfully discovered, rather than coldly reasoned out.  I like to play, but my brain also likes to figure out.  I used the paper circuit diagram, together with the schematic diagrams provided by Peter Blasser, and conducted my own experiments.  These then were my starting points.

Shown above is Peter B’s schematic for one of the four Tetrazzi oscillators.  It is an ingenious design for a voltage-controlled sawtooth/ramp oscillator, where the up and down rates are separately voltage controlled.  Not only are the rates under VC, each VCO has two modulation inputs, one for up (UPMOD) and one for down (DONMOD), that are hard-wired internally to come from the other VCOS.  The internal patch looks like this:

UPMOD cross connections:  [1 -> 4; 4 -> 3; 3 -> 2; 2 -> 1]
DONMOD cross connections: [4 -> 1; 4 -> 2; 2 -> 3; 2 -> 4]

The depth of modulation for UPMOD and DONMOD are under voltage control, all four being simultaneously controlled.  The different topologies for these two internal patches is the reason behind the different sound experienced for UPMOD vs DONMOD.   Did I just reveal a secret?  No, it is discoverable to anyone who takes the time to understand the paper tetrazzi.  If you click this image, save the GIF, and print it, you will have the paper Tetrazzi.

Functional Structure

The Tetrazzi consists of the following circuit elements:

  • Four VCOs with VC up and down ramp, plus a solid state switch for additional modulations from node space
  • Four “scroll” generators, track and hold circuits for establishing initial frequency and wave shape for each VCO
  • Stereo VCA with inputs to each channel from each of the four VCOs, controlled by touch-sensitive piezo sensors
  • A matrix of connections called node space for patching additional cross-modulations

The Tetrazzi Organ, built into the lovely wood case, provides the following means of manual control.

  • Initial pitch and wave shape (the scroll) are set randomly by a red button, one for each VCO.
  • UPTON pot controls the rate of rise, all VCOs
  • DONTON pot controls the rate of fall, all VCOs
  • UPMOD pot controls the depth of modulation for the rise rate, all VCOs
  • DONMOD pot controls the depth of modulation for the fall rate, all VCOs
  • Four wooden bars with piezo sensors attached control the stereo VCA
  • Node space connections are made by touch or jumper wires

I took note that the pot wheels simply provide a voltage source.  I also noted that the scroll circuit simply provides a voltage.  And that the piezo electric sensors are voltage generators.   What I’ve done is to create a modular package for the Tetrazzi in which all of these voltage-control inputs are interfaced to standard modular levels through panel jacks.

I modified the scroll circuit on the board.  The scroll is now disabled.  The CV input goes into the on-board op amp where the scroll voltage used to go.  I used the XRS pad for a resistor that brings out the individual VCO output.  The SCR pad, originally for a capacitor, was used for a 100K terminating resistor.  See details by clicking on the schematic.

Adapter Circuitry Schematic

I designed interface circuits for

a) mixing CV from a panel pot with an external CV (8 of these),

b) buffering the VCA outputs up to modular level,

c) buffering the individual VCO outputs up to modular level,

d) creating a mix of all four VCO saw/ramp outputs,

e) creating a mix of the four pulse outputs that go into node space.

The CV input buffers also have the job of limiting the CV to the Tetrazzi board to safe ranges.  My design provides the following features.

  • Pots for UPTON, DONTON, UPMOD, DONMOD, plus an attenuated CV input for each
  • Pots to replace the scroll, one for each VCO, plus an attenuated CV input for each
  • Four attenuated, AC-coupled CV inputs to replace the bars for controlling the VCA
  • High/low range switch for each VCO
  • Individual output for each VCO at 9 v p-p, bipolar
  • Left and Right VCA outputs, bipolar
  • Averaged sum of all the individual outputs (9 v p-p), bipolar
  • Mix of the node space pulse outputs, scaled for modular use (nominally 10 v p-p, adjustable), bipolar
  • Banana jack patch bay for the node space outputs (red) and inputs (blue)

So many patching possibilities exist I can’t describe them all now.  I will make MP3 posts soon.  I may also make a video demonstration.  Some things I’ve already tried that work well include patching an external LFO into the frequency inputs for vibrato or into the VCA inputs for stereo panning.  An envelope generator into the VCA control inputs works pretty much as expected.  And now it’s possible to tune the VCOs together with pots instead of randomly.


Here’s the Tetrazzi board (purchased by itself, assembled but I had to add the power components) with only the power input wired up for testing with the prototype adapter board.  Look closely and you can see the eight 1/8-watt resistors I added near the button connectors.  The circuit modifications were minimal, but I did have to cut some traces to disable the scroll circuit and bring out the individual outputs.

Here’s my adapter board, wired up on a prototyping board in the classic manner.

The back of the board with wiring.  It’s all hand-soldered, folks.

I built a jig from a piece of plywood to mount the two boards next to each other for the prototyping phase.

Panel assembly and wiring of the panel.

Wiring to the Tetrazzi board.  Some go to MTA connectors that hook to the adapter board and some go to the panel.

The Tetrazzi board installed and wired up to the panel.

Final assembly.


10 Responses to Modular Tetrazzi

  1. Hex says:

    Woah!!! This is an amazing build, Richard! I’m blown away! Great idea with using the banana plugs for the nodes too. I can’t wait to see this in action. Video please!

  2. heau says:

    This is really awesome. I own a Tetrazzi and would never imagine it to be integrated into a modular system.
    Bravo !

  3. Pingback: musikgear » Blog Archive » Richard Brewster’s S Modular Tetrazzi

  4. emdot_ambient says:

    Richard, you are NUTS…but I like that! I’m like totally envious of your organizational powers when it comes to wiring. My efforts so far are highly chaotic. You’re an inspiration…but don’t expect me to jump into doing one of THESE! **thumbs up**

  5. Richard says:

    When it comes to panel wiring, it helps that I learned harness wiring for prototypes when I worked in a General Electric avionics plant in the 70s. None of my DIY modular wiring comes close in complexity to the work I did at that time. I’m talking wire bundles with 50+ wires.

  6. decaying.sine says:

    Richard, this is superb. What skill and patience you have!!!

  7. Scott_G says:

    Can you provide any insight into the barre function? It’s my understanding that piezos function great as transducers for momentary deformation, but won’t offer steady (DC) voltages when pressed continuously. But with the Tetrazzi, you can continuously press on a barre, and it will affect the sound, seemingly indefinitely, suggesting they are somehow outputting DC in indefinitely stressed states. How does this work? Could you point me at a schematic indicating the operation of this “block” of the Tetrazzi? I’m puzzled, and any info you could provide would be super helpful!

  8. Richard says:

    Your description of the piezo function is correct. The piezo generates a voltage proportional to the rate of change in the pressure. It’s the same on a Tetrazzi. It may seem you are continually pressing on a barre, but you are actually continually changing the pressure. The schematic image of one Tetrazzi channel is here, just above. The piezo is indicated by the concentric circles at the bottom. Notice that it connects to two amplification circuits. One responds to positive pressure, the other to negative (release). These are applied to control the amplitudes in the left and right channels.

  9. Scott_G says:

    Man, that’s weird that such steady, DC-seeming modulation is coming straight out of a piezo with some simple conditioning. Still having trouble with Peter B’s heiroglyphical schema. Having played with a Tetrazzi, I was just sure that there had to be something special to how the piezos were being used. So, basically, you are saying that it’s really just buffered signals straight from the piezos and into the CV in of the VCAs?

  10. Richard says:

    That’s right. In fact I use the inputs where the piezos connect as the input for CV over the VCAs. I put attenuator pots on them, plus a DC-blocking capacitor. Modular envelope generators work just fine.

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