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.

CONSTRUCTION

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.

a) The sonorous object is not the instrument that was played.  b) The sonorous object is not the magnetic tape.  c) The magnetic tape can contain a number of different sonorous objects.  d) But the sonorous object is not a state of mind.  Sonorous objects can be clearly described and analyzed.

Listen to this recording.  Do you hear sonorous objects?  Do the sonorous objects have any reference beyond themselves?  To how they were made?  To other things that come to your mind when you listen?  It is called Untitled A, because otherwise I cannot think how to give a title to a recording unless it either 1) refers to how it was made, 2) refers to something it reminds me of, or 3) is just whimsical nonsense.  Schaeffer believed that the sonorous objects can be clearly described and analyzed.  I wonder why I’d want to do that.  I’d rather just listen.

The DuoFonik is a dual SSM2044 VCF from Neutron Sound.  The SSM2044 is an integrated circuit designed for electronic music, for making 4-pole low-pass filters.  This module contains two of them, A and B, with features to enhance their use in tandem.

The PC board is small enough to fit many different modular synthesizer format panels.  This one is built with the Front Panel Express panel created by the designer, which features switches that allow the A pots to control the B filter in tandem with the A filter.  Unfortunately, there wasn’t room on the panel for signal input attenuators.

The panel design squishes the MOTM format, so that four pots can fit vertically above three jacks.  The board is affixed to the panel by 2-inch standoffs that fasten to the front panel using flush-mounted black screws (visible from the front).  Here’s a photo comparing it to a standard MOTM module (the 485).

Construction

The PC board is chock-a-block full.  Most of the resistors stand on end.  The bypass capacitors are those tiny surface mounts, soldered to the back of the board, that require the occasional expression of colorful language during soldering.  Here’s a photo of my assembled board.  Note the 10n polystyrene capacitors – Mouser #23PS310.  They are a tight fit.  Note that I chose normal resistors for R17 and R117 instead of tempco ones.

There is a circuit flaw on the REV 1 board that requires replacing C9 with a jumper.  I found out about it after I finished the board, so my C9 is installed.  Here’s a photo of my fix, including the addition of a C9 on the bottom of the board.  The red circle shows the shorting where C9 is originally installed, and the yellow cap above is the replacement.  Note that it goes between U2 pin 13 and the opposite end of R12, rather than to pin 13.  I did that to match the circuit at R112 and C109.  And it was convenient.

All connections to the panel are made via headers along one side of the board, making it a fairly simple affair to remove the board completely for repairs, if needed.  I made one change to the panel wiring scheme:  B IN is normally connected to the A IN by the jack switch lug.  That way whatever is patched to A IN also goes to B IN, if nothing’s patched there.

Calibration

Each filter has three trimpots:  scale (1V/octave), initial frequency, and self oscillation.  A seventh trimpot adjusts the balance of the two outputs into the on-board mixer.   Calibration was straight-forward.

1. I adjusted the self-oscillation trimpot, at a frequency of about 1 KHz, until the oscillation started around 75% rotation of the RESONANCE pot.  (The filter oscillates well at all frequencies.)
2. With the filter oscillating (high resonance setting), I adjusted the 1v/octave trimpot.  I found that the tracking was not wide range, so I chose a middle frequency and calibrated the 1V/octave input to jump from 400 Hz to 800 Hz when 1 volt was applied to the 1V/octave CV input.
3. I adjusted the initial frequency trimpot so that with the FREQUENCY pot fully anticlockwise the lowest frequency was 32 Hz.  The top end then fell between 15 and 16 KHz.  There was a little difference in the frequency range for the A and B filters.  Since the scale trim also impacts the FREQUENCY panel pot, it should be possible to get them both fairly close, if you take the time, but with the caveat that this interacts with 1V/octave.
4. The null trimpot adjustment is made by applying a 10V p-p triangle wave from an external VCO to both the A and B inputs.  Turn the FREQUENCY pot to the max and the RESONANCE pot to the min.  Switch the A input to INVERT.  The two signals should cancel each other.  Adjust the trimpot to get the smallest signal from the MIX OUT.

Sample Sound

I made this during the calibration procedure.  It’s a sawtooth wave, attenuated to about 5V p-p through the A filter.  I used the Binary Zone to make a stepped CV sequence, sending the output to the FREQ CV input, and the inverted CV output to the VCO FM input.  So when the VCO steps to a lower frequency, the VCF steps to a higher.  I twiddled the FREQ and RES controls during the recording.  This is not definitive!  See the Neutron site for the good demos.