Building a Eurorack 3340 VCO – Part 5 Calibration

This is part five of the Eurorack 3340 VCO build guide. In part one, we covered the AS3340 schematic and bill of materials. In parts two and three, we built the three boards comprising the completed module. In part four, we connected the boards, made the panel, and tested our module.

With our module functioning, the final step is to calibrate it, specifically setting it up so that it tracks 1V/Octave across a useful range. Access to an oscilloscope, multimeter with frequency counting capability, or chromatic guitar tuner is required. A source of accurate control voltages is also necessary; this could be an analogue sequencer, MIDI to CV module, or something similar. 

Along with 1V/octave tracking, we’ll also centre our PWM and frequency controls and tweak the sine wave’s roundness and DC offset.

On this page

Photo of the CEM3340 / AS3340 VCO Eurorack module

Trimmers

Our module has six trimmers that are used for calibration and can be adjusted while the module is fully assembled.

Photo the 3340 Eurorack VCO illustrating the controls for calibration
Photo the 3340 Eurorack VCO illustrating the controls for calibration

Adjusting the 3340 VCO 1V/Octave Tracking

Eurorack follows the 1V per octave standard. One volt represents one octave, so the pitch produced by a control voltage of 2 V is one octave higher ( double the frequency ) than that produced by a control voltage of 1 V. Whether an oscillator exactly doubles its frequency for each additional volt is referred to as it’s tracking and we use the scale trimmer to adjust the tracking of our 3340 VCO. This can be a repetitive process, but you will get 1V/octave tracking over at least the 0-5V range with patience.

Basic principle

Our aim here is to measure the frequency with our oscilloscope, multimeter, or chromatic tuner at a known control voltage. Then, increase the control voltage by 1V and adjust the Scale trimmer so that we measure exactly double the frequency. The tricky part is that adjusting the scale also moves our starting frequency ( as we are stretching or expanding the frequency scaling range ), so we want to over-compensate slightly when we adjust the Scale Trimmer. We then return to our starting control voltage and adjust the controls until we are back at our starting frequency. We then repeat until no further scale adjustment is needed. 

Step by step
  1. Hook the triangle or saw output up to your oscilloscope, chromatic tuner or multimeter.
  2. Hook up an accurate CV source to the CV1 input and apply 0V
  3. Use the Coarse and Fine pitch controls to dial in a frequency that’s mathematically easy to double — e.g 50Hz. If you are using a chromatic tuner, picking a reference note is probably easier.
  4. Increase the CV by 1V and note whether the frequency is above or below what you’d expect – e.g greater or less than 100Hz if you started at 50Hz
  5. Adjust the Scale trimmer until you read double your reference frequency ( e.g. 100Hz). If you started above the expected frequency, adjust to be a little above double; if you started below, adjust to remain a little below. 
  6. Return the CV to 0V and adjust the fine pitch control back to your starting frequency or reference note.
  7. Repeat steps 4-6 until you no longer need to adjust the fine pitch control on returning to  0V CV
  8. Repeat steps 2-6 with increasing large CV increments ( 2V, 3V, 4V, 5V). At higher frequencies, the high-frequency tracking comes into play. We will adjust this next.
An alternative approach to calibrating the 3340 VCO

If you have access to a DAW with a tuner plugin and MIDI to CV interface, you could try this less laborious approach to calibrating your 3340 VCO. We’ve had good results with Ableton Live, but a hardware chromatic tuner and quantized analogue sequencer would probably work just as well.

1. Create a looped MIDI clip which plays notes chromatically across serval octaves.

2 Connect the MIDI output to your VCO via a MIDI to CV converter and the saw output of the VCO to an input on your audio interface via a module or mixer that reduces the VCO’s 10V peak-to-peak output to line level. 

3. Add a tuner plugin to the input channel in the DAW and monitor the incoming audio 

4. Play the sequence at a slow tempo ( so that the tuner has time to track notes).

5. As the sequence plays, observe the position of the indicator ( virtual needle ?  ) on the tuner. The indicator will maintain a static position when the VCO is tracking 1V/Octave. We don’t care about hitting the notes, just yet, just whether the indicator moves sharp or flat as the sequence plays.

6. Adjust the scale trimmer until the tuner’s indication stays static for at least the lower 4 or 5 octaves. Once it looks static, use the course and fine-pitch controls to move the indicator to “in tune”.

7. The tuner should display the correct notes as the sequence plays. If it drifts sharp or flat adjust the scale trimmer appropriately.

8. Adjust the HF trimmer and bring the upper octaves in tune as needed.

Extra Credit

ALFA, manufacturers of the AS3340, have an excellent guide covering how the AS3340 1V/octave tracking works and detail guide to calibrating it https://www.alfarzpp.lv/eng/sc/Tuning%20the%20AS3340.pdf

3340 High Frequency Tracking

As noted in the original CEM3340 data sheet, the 3340 provides internal circuitry to compensate for tracking errors that occur at frequencies greater than 5KHz. The output of this is arrives at pin 7. In our circuit, the HF trimmer feeds a portion of the current from pin 7 back to the CV input to provide this compensation following the schematic set out in the data sheet.

If your oscillator tracks 1V/octave well in the lower octave but is sharp or flat in the higher octave use, the HF trimmer can be used to correct the higher octave tracking.

Adjusting the base pitch

The Pitch Adjust trimmer can be used to set a musically useful base frequency for the oscillator when both the Coarse and Fine controls are centred. With the component values specified in the bill of materials, the combined range of the Coarse and Fine control is approx. ±3.5 octaves, so adjusting the pitch when both controls are centred to C5 ( 523.25Hz ) could be a practical starting point.

PWM Offset

The PWM offset trimmer is used to set the duty cycle of the pulse output to 50% when the pulse with control is centred, and no modulation signal is present at the PWM input. To adjust, centre the PW control and, using an oscilloscope, adjust the PWM offset trimmer until a 50% duty cycle is achieved. 

Sine wave

We use an overdriven differential pair of transistors (Q2, Q2)  to convert the triangle wave to an approximated sine wave. By adjusting the amplitude of the triangle signal across the base-emitter junction of Q2, we are able to change the amount of overdrive and, thus, the “roundness” of the sine wave approximation. The Sine Roundness trimmer,RT5, adjusts the roundness of the sine wave. Our sine wave may not be centred around 0V; the sine offset trimmer RT6 allows us to centre it. 

Calibrating the sine output

1.  Connect the VCO’s sine output to an oscilloscope. 

2. If the waveform isn’t centered around 0V adjust the sine offset trimmer to center it. If the waveform looks more square than sine, turn the roundness trimmer anit-clockwise to round it out.

3. Adjust the sine roundness trimmer until the waveform looks more or less sine-shaped. Turning clockwise increases roundness.

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