The Dannysound Cali Osc is one of two new, analog VCOs available from Thonk only as DIY kits.

The Cali oscillator is a new and improved version of the classic Buchla 258 waveshaping linear FM oscillator with an additional pulsewidth modulated output.

It’s hard to believe that it has been seven years since I build two J3RK 258J oscillators in MOTM format.  The Dannysound Cali Osc is in Eurorack format and has some nice features.

• Waveform modulation is selectable by a big red three position switch.  Modulations available are sine-to-square, sine-to-saw, and pulse to triangle.  (By contrast my J3RK version has only sine-to-saw modulation.)
• 1V/octave CV input with good tracking
• Linear FM input with attenuator
• Log (i.e. exponential) FM input with attenuator
• Pulse output with initial width pot and PWM CV input with attenuator
• The four attenuated CV inputs feature an LED that monitors the incoming CV prior to attenuation
• Coarse and fine frequency pots are driven by internal 9 volt regulators, isolating them from the power supply

Construction

As we commonly see in Eurorack modules, the circuitry is separated onto two PC boards, one with most of the circuit and a power header facing the back, and one holding all the panel parts facing front.  These are connected by male and female headers that join the two boards.  Cali Osc is a relatively simple analog design with through-hole components.  A few of the integrated circuits are legacy ones, i.e. CA3140 and CA3080.  The core of the circuit is almost exactly like the one posted on my J3RK build.  Schematics and build documentation are available publicly from Thonk (see the above link).  The build documentation is almost too detailed, with many photos of the board showing exactly where each component is to be placed.  I didn’t follow the build order exactly, because I like to get as many components as possible soldered with washable organic solder.  So I place all of those, then solder and water wash.

Notice that R10, 220K, is omitted in the picture of the main board.  I’ll get to that later.

The matched transistors in the exponential converter circuit need temperature compensation for stability.  I went a little farther than the manual instructs (which is to make sure the tempco resistor is touching the transistors).  I bent the lead so that the transistors would touch each other, adding a bit of thermal compound (heat sink compound) between them and then shrink wrapping them tightly together.  I also put a bit of the compound between the transistors and the resistor to insure maximum thermal conductivity.  You can see a bit of the white compound there.

When it came time to assemble the panel, I was very careful to examine the fit of all the parts, prior to soldering anything.  With everything sitting in place, I soldered the coarse frequency pot at the top, and then the DPDT switch.  These two, with added nuts, establish the board to panel spacing at 8mm.  The other pots do not fasten to the panel, just protruding through from their seating on the board.  However the jacks presented a small problem: they are 7mm in height from their feet to the panel.  The jacks cannot sit flush to the board without pulling the panel down, causing the board to flex. Flexing the board would put stress on the pot and switch that support the panel over the board.  The solution is simple enough, but not made clear in the assembly guide.  You must first snug the jacks to the panel.  You will notice a bit of flex; you can press the panel down to make the jack feet hit the board, but we don’t want that.  Measure 8mm spacing at the bottom of the panel, along the four jacks at the edge.  You can then solder a couple of jack ground lugs at the outer jacks to set the spacing so that the board and panel are parallel.  Then the remaining panel components can be finally soldered into the board.

Circuit Modification

I made one modification to the circuit.  R10 set the maximum depth for linear FM.  It’s given as 220K.  I was suspicious of this value, because I had used a 39K resistor in the corresponding place on the J3RK.  The associated attenuator is log taper, so you have plenty of attenuation available.  I experimented by putting in a 100K pot for R10 and it turned out that a 39K value worked fine here, too, and that’s what I went with.  This allows for a deeper linear FM, which is demonstrated below.

Calibration and Initial Frequency Setting

Calibration went very nicely, following the excellent instructions in the build guide.  The last trimpot to consider is the Master Tune.  You can use this to set the initial frequency and the range available from the coarse tune pot and CV inputs.  A quirk (to me) of this oscillator is that the 1V/octave input is normalized to +3 volts.  This means that patching 0V to it will drop the frequency three octaves.  I set the Master Tune pot to yield the following ranges (all in VCO mode, not LFO mode).

• With nothing patched to 1V/octave, the coarse tuning goes from 38Hz to 4KHz.
• With the coarse tune at the lowest setting, 0 to +5V on the V/oct input goes from 6Hz to 182Hz.
• With the coarse tune at the highest setting, 0 to +5V on the V/oct input goes from 605Hz to 20KHz.

That’s right!  In VCO mode this oscillator has a range from 6Hz to 20KHz.

Demo Recordings

NOTE:  In all of these recordings, the Cali Osc initial frequency is 326 Hz, and the audio rate modulating oscillator is a sine wave, +/-5V, at 248 Hz.

Wave Shaping Demo

The first recording demonstrates voltage-controlled wave shaping.  There are three sections and three sub-sections within each of those.  In the sub-sections we hear each position of the red switch:  up for sine-to-square, center for pulse-to-triangle, and down for sine-to-saw.  (They might not be in that order, but the middle position is always the second one.)

The first part is a manual sweep of the initial waveform pot, with switch up, middle, down.  In the middle, the oscillation actually stops until the pot is increased a little.  Then it breaks into a big pulse, later becoming a well-behaved triangle.  I consider this to be a feature, as you can gate the oscillator on and off by a square wave.

The second part is modulation by LFO.  For these, the initial pot is close to the middle.

The third part is modulation by VCO.  Here’s where the middle switch position shines!  It gets grungy.

Log FM Demo

The next recording demonstrates ‘log’ FM.  I don’t know why they used the term log, when it’s really just ordinary exponential FM.  In the demo the attenuator is turned up slowly and then back down.  First with an LFO and then with the 248 Hz sine wave.

Linear FM Demo

Same procedure as for the log FM demo, except using the linear FM input (which, as mentioned, was modified for more depth).

Pulse Width Modulation Demo

For this recording we listen to the pulse wave output.  The first part is a manual sweep of the initial width pot.  It goes from a thin pulse at one edge to a square in the middle and to similar pulse at the other side, without going so far as to stop the output.  Then we hear modulation by LFO and then VCO sine wave.  During the LFO and VCO demos the initial width pot starts out in the middle, but then I turned it left and right to show that with modulation the output can be made to stop, a useful feature.

I neglected to make a demo for the LFO feature.  With all the waveforms it will be really useful.