One of the most attractive features of modular synthesizers is the wide variety of modules and functions available – especially in the Eurorack format. However, along with that variety has come freedom for manufacturers to choose how their modules operate…which can occasionally cause issues for users combining modules from different manufacturers. One of those potential variations is how they respond to control voltages for pitch. Virtually everyone agrees that a change in 1.00 volts equates to change of one octave (halving or doubling in pitch) for an oscillator – but how many volts does a module require to produce a particular pitch?

If there’s a difference between the modules you own, in most cases it can be dialed out with a module’s own pitch knob, or perhaps a transpose function in your MIDI to CV (Control Voltage) interface. However, there are occasions when you need the assistance of an additional module – often referred to as a precision adder or octave switcher – to settle disagreements.

I’ve been reading manuals, talking to a number of manufacturers, and measuring my own modules, and have built a couple of tables for your reference: one for the default settings of MIDI to CV converters, and another for how specific oscillators respond to voltage. As I learn more, I’ll keep amending this list. The goal is to give you a head start in understanding how modules you own or are considering buying might play together, and to prepare you to compensate for some of the differences you many encounter.

Middle C, Where Art Thou?

I know there’s a bit of ongoing debate about the subject of “Middle C”: Is it MIDI note 48 or 60? Do we call it C3 or C4? 8′ or 16′? Is its pitch 130.8, 261.6, or 523.25 Hz (cycles per second), or some other number?

Without trying to settle any of those arguments, for the sake of this post I’m going to use MIDI note 60 (3C hex), C4, and 261.6 Hz for Middle C, which will be our reference throughout the rest of this post. For oscillators that have their octave switch marked in terms of feet, I’m going to use 8′ as the reference, as it is used for when concert A (440Hz) is above Middle C (261.6 Hz). If you or your modules prefer a different reference or labeling system, please offset the numbers below accordingly.

MIDI to CV Converters

I included in this category modules and standalone boxes that convert MIDI to control voltages, controller keyboards that output both MIDI and CV, and semi-modular synths that have their own MIDI input as well as a CV output.

All of the numbers below are based on the default settings at power up, as from the factory. Most of these devices have a way to set octave or finer transpositions between MIDI and CV using their front panel, through jumpers on the circuit board (Expert Sleepers FH-1), by using a web application (Intellijel µMIDI), etc.

For MIDI note 60, the resulting output voltage for the following devices is:

MIDI/CV Converter, Controller,
or semi-modular synth
Voltage for MIDI Note 60
at default settings
Moog Mother-320.0 v
Expert Sleepers FH-10.0 v
Kenton Pro 2 (older model)1.0 v
Kenton MIDI to CV converters (current)2.0v
Mutable Instruments Yarns2.0 v
MST MIDI to CV Converter2.0v
Doepfer A-190-1, A-190-2, & MCV42.0 v
Doepfer A-190-43.0 v
Akai MAX253.0 v
Pittsburgh MIDI 33.0 v
Roland SBX-13.0 v
Intellijel µMIDI3.0 v (was 5v in older versions of the firmware)
Arturia MiniBrute, MicroBrute, BeatStep, and KeyStep4.0 v

As you can see from the above, there can be up to five volts (which means five octaves) difference depending on which device you use to convert MIDI or a local keyboard to control voltages! If nothing else, I hope this gives you incentive to spend some quality time with your converter’s manual so you know how to change its response to better line up with your chosen oscillator(s).

Eurorack VCOs

testing pitch on a variety of VCOs

Transposing your keyboard or MIDI signal is one solution, but what if you are driving two different VCOs that have two different voltage requirements to play the same note?

I’ve listed below a cross-section of VCOs, as well as a few semi-modulars than include external CV input for their own oscillators. I’ve assumed that any pitch controls are set to their initial or center position (usually 12:00), with no external modulation. Most VCOs will – with some slight tweaking – play a C with these settings when fed an integer number of volts (i.e. 1.00, 2.00, etc.), so I’ve noted the voltage they’d like to see to play the same note: C4 (261.6 Hz). If there is an octave switch, it’s set at either +0 octaves or 8’. When given a choice between High, Mid, or Lo ranges, I went with High, as the others are often LFO modes. I’ve also included a conservative summary of the front panel tuning ranges of each module, to give you an idea of you how much you can reliably twist a knob to dial in an interval from C or match one VCO up with another:

VCOVoltage for C4 Coarse Tuning Range (octaves)Fine Tuning Range (semitones)
Synthesis Technology E3500.0 v07 (+4.5/-2.5 continuous)24 (+/-12)
Moog Mother-320.0 v ––24 (+/-12)
Intellijel Dixie II+0.0 v07 (+3/-4 in octave steps)12 (+/-6)
Intellijel Rubicon and Dixie II0.0 v11 (+/-5.5 continuous)12 (+/-6)
Verbos Harmonic Oscillator0.0 v12 (+/-6 continuous) ––
Verbos Complex Oscillator (master; modulation requires 4.0v)0.0v12 (+/-6 continuous)10 (+/-5)
Expert Sleepers Disting mk2 modes 4-c and 4-d1.0 v ––12 (+/-6)
AJH Minimod1.0 v04 (+/-2 in octave steps)16 (+/-8 or greater)
Roland 5121.0 v04 (+/-2 continuous)12 (+/-6)
Make Noise DPO & STO1.0 v09 (+/-4.5 or more, continous)10 (+/-5)
Expert Sleepers Disting mk3 modes 4-c and 4-d1.0 v20 (+8/-12 in octave steps)12 (+/-6)
Doepfer A-110-22.0 v02 (+/1 in octave steps)12 (+/-6)
Doepfer A-110-12.0 v04 (+/-2 in octave steps)12 (+/-6)
Roland System-1m2.0 v05 (+2/-3 in octave steps)02 (+/-1)
Doepfer A-1112.0 v07 (+4/-3 in octave steps)12 (+/-6)
Mutable Instruments Braids2.0 v08 (+/-4 continuous)02 (+/-1)
Waldorf nw12.4 v02 (+/-1 in octave steps)12 (+5/-7)
Pittsburgh Lifeforms SV-13.0 v17 (+/-8.5 continuous)16 (+/-8)
Arturia Minibrute & Microbrute4.0 v ––04 (+/-2)
Erica Black Varishape VCO24.0 v10 (+/-3 in octave steps, plus +/-2 continuous)04 (+/-2 for VCO2; VCO1 does not have fine tune)

At my arbitrarily-chosen default settings, when fed an integer number of volts the Synthesis Technology E350 plays roughly an E, while the Waldorf nw1 and the Make Noise DPO & STO play roughly a G. For the E350 and Make Noise VCOs which have a wide tuning range on their main pitch knob, I used the nearest C; the nw1 has a limited tuning range so I kept its in-between voltage.

As you can see above, the same voltage can result in an up to four octave difference in the resulting pitch. Most Eurorack oscillators have very wide front panel tuning ranges, and so you can merely dial out the difference, if there is any. But if you’re up against the limits of an oscillator’s tuning range just to get it in unison with another oscillator…and then you want to tune them to some other musical interval beyond that range…then you might have an issue. However, there is a solution.

The Takeaway: Consider a Precision Adder Module

Eurorack Transpose ModulesGiven the above, depending on your personal mix of gear, I think it would be wise for a modular synthesist to consider including a “precision adder” or other form of octave offset voltage processing module in their system. Split your pitch CV through a buffered mult, send one signal to one oscillator, and send a copy of this signal through your voltage processor to add or subtract the required number of volts before sending the signal on to your second oscillator. This is also a quick and easy way to add a precision octave switch to a module that otherwise just has a large continuous pitch dial (note: I’ve had oscillators that required retuning after changing their own octave switch)

Examples of octave offset voltage processors include the Doepfer A-185-2 Precision AdderALM/Busy Circuits ALM002 Beast’s ChalkboardFrequency Central Trans Europa, algorithm 1-a (Precision Adder) in the Expert Sleepers Disting, or even the rare Analogue Systems RS-420. All are useful, and each has a different feature set that you have to personally weigh. The Disting is my current favorite as it has a very wide +/-9v (octave) range, and is cost-effective as the module can do other things when you don’t need an offset control. An interesting alternative is the Ladik Q-010 Easy Quantizer, as it includes octave and semitone offset controls as well as quantization feature.

Is the Sky Really Falling?

There’s a reasonable chance your given collection of modules already play together well, or can have their differences resolved with a twist of a knob. But maybe not.

In my case, I installed a bright shiny new Waldorf nw1 into my rack, connected the “KB” output from a Moog Mother-32 to its “CV” input, latched a low arpeggio on my controller keyboard…and was convinced either the Waldorf or my brain was defective, as I could barely get it to play anything other than its absolute lowest note. I almost pulled the Waldorf out of the rack and returned it.

As it turns out, the nw1 doesn’t respond to anything below 0v, while the Mother-32 thinks that’s Middle C. Even if I transposed the controller keyboard up, tuned the Mother’s VCO all the way down, and tuned the nw1 all the way up, I could barely get them in unison – and I wanted to nw1 to play a high sound over the Mother’s strong bass. Fortunately, once I figured out what was going on, I could patch around it and get back playing again. I’m just trying to share what I’ve learned with you so you can keep the flow going as well.

If you have details on any other modules I can add to this list, please pass it along – feel free to contact me publicly in the comments below, privately through the Contact form at the bottom of the About page on this site, or on the Learning Modular Facebook page.

An issue that’s perplexed even more users is matching up modulation voltage ranges between modules: 0-5v? 0-8v? 0-10v? +/-5v? +/-10v? This is a little more free-form since modulation can do so many different things, but in the future I plan to tackle at least how to patch around the differences.

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