When designing my Educational DIY Synth Thing I always had in mind that I might be able to hook it up to my Korg Volca Modular.

This post looks at the implications and possibilities.

Warning! I strongly recommend using old or second hand equipment for your experiments.  I am not responsible for any damage to expensive instruments!

CHECK YOUR LEVELS AND SIGNALS, DECIDE FOR YOURSELF, AND PLUG THINGS INTO YOUR KORG VOLCA AT YOUR OWN RISK!?

These are the key previous posts for the main concepts used in this project:

• Educational DIY Synth Thing.
• Korg Volca Modular Notes.
• Korg Volca Modular MIDI to CV PCB Design.

If you are new to electronics, see the Getting Started pages.

Korg Volca Modular Signals

According to the manual that comes with the synth, there are the following internal (i.e. patchable) signals:

• Audio: -3.3V to 3.3V (so 6.6V peak to peak).
• Unipolar CVs: 0 to 3.3V.
• Bipolar CVs: -3.3V to 3.3V.
• Gates and Triggers: 0 or 3.3V.

And of course, any of these signals as an output can be used as an input to another part of the synth.

One additional source of information comes from The Real Volca Modular Specs by Syntherjack where they have observed the following:

• Some fully modulated audio signals can get to 9V peak to peak.
• The carrier signal is a clean 4.5V peak to peak triangle wave.
• The function generator outputs are close to 0 to 3.3V, although the rise-and-fall function (shape/time) trigger appears to get up to 4.5V at times.
• The sequence gate outputs are 0 to 3.3V, but the pitch output gives a steady CV based on the currently playing note (from a sequence or the keyboard) across the range 0.5V up to around 2.2V. Apparently it follows a 0.5V/oct internal standard.
• The highest signal observed by Syntherjack was 9.4V peak to peak which implies that all of the Volca’s inputs should be able to take up to that voltage quite happily (otherwise there would be combinations of jumper wires that could fry the Volca!).

The 0.5V/oct comment on the pitch sequencer is interesting. That implies that the internal standard used by the Volca is half of what you’d expect in (say) a Eurorack 1V/oct common setup.

My own Synth Thing uses 1V/oct internally but only has a range of 0 to 3.3V.

So what I have learned? That the triggers and gates should be fine and compatible. That audio may be clipped as the Volca uses +/- 3.3V and I use 0-3.3V. But the Volca’s pitch CV is half that of the Synth Thing.

Volca to Synth Thing Experiments

Volca pitch CV to Synth Thing

The first experiment is to investigate that 0.5V/oct vs 1V/oct thing to see what that sounds like.

This demonstrates the basic connection from the Volca to the Synth Thing. A key point to note is that the GND must be connected together, but the only way to access that from the Volca is from the CV-In socket, which isn’t ideal.

Once the GND are linked, the Sequencer’s pitch output can be connected to the Synth Thing’s VCO1 pitch input and the pitch pot turned until the pitch matches the note played on the Volca. I tuned it to the lowest note on the keyboard and then could easily hear as it went up an octave that the Synth Thing hadn’t matched the pitch again.

As this is the pitch output for both the sequencer and the Volca’s keyboard, this allows the Volca’s sequencer to drive the Synth Thing too.

Some interesting micro-tonal effects are possible, but if I want pitch parity then I’d need to use some kind of amplifier with a gain of 2 on the output of the Volca to get it to match 1V/oct.

Volca Amplified Pitch CV to Synth Thing

There are lots of circuits around for a simple amplifier with a 2x gain, but perhaps one of the best for this type of application might be a classic non-inverting amplifier op-amp circuit.

At its simplest it is as follows:

The gain is 1 + R1/R2 so when R1 = R2 that gives a gain of 2. A MCP6002 is a “rail to rail” opamp so it can work essentially up to the power supply. In this case I’ll power it via the 5V supply from the Synth Thing.

Now there are a number of other considerations with a practical, accurate amplifier but for my purposes I’m putting on my “little knowledge is dangerous” hat and just going for it.

I’ve used two 220K resistors, but the accuracy of the resistors isn’t very good. The principle largely works, but I find that if I tune the two synths to the lower note on the Volca, I can’t quite get an accurate octave above. It may be that using variable resistors it might be possible to tune an octave.

There could easily be issues with non-linear tracking of input and output voltage and voltage drops due to impedance issues, but this is where my (limited) knowledge of electronics reaches its limits.

To be honest, I’m not entirely convinced the pitch tracking in the Synth Thing is very accurate anyway! I really ought to do some calibration tests with constant voltages vs frequency.

This is an interesting experiment and something worth exploring a little more in the future, but for now I’m leaving it here.

Volca and Synth Thing ADSR

The final audio output of the Volca can be obtained from the “Space Out” output. Pairing this with the gate from the sequencer allows me to use the Synth Thing’s ADSR envelope generator.

The key thing to watch out for is having the Volca’s release time too short. If it instantly releases then the release phase of the ADSR will seem not to be working.

The other thing to note is that the Volca sequencers gate output always has a break between notes so it isn’t a continuous gate. But playing quickly could easily overtake the release time from the ADSR.

I’m not sure if it is possible to completely bypass the built-in connection between the Volca’s modules, but it might be possible to connect the Source carrier out signal directly into the LPGs and onwards to the output. But that is pretty much bypassing most of what makes the Volca a Korg Volca Modular so I’m not sure why I’d want to do that

Synth Thing to Volca Experiments

It is probably prudent at this point to repeat the warnings:

• Check the voltage levels yourself before you attempt to plug anything into your Volca synth and then decide if you’re happy to do so.
• Do this entirely at your own risk.

I’m daft enough to take the consequences of toasting my Volca but I am not responsible for damage to anyone else’s!

Using the CV/Gate Input

This is by far the easiest, safest, and probably most reliable way to link into a Korg Volca Modular. I covered this before in my Korg Volca Modular Notes, but to recap:

• The TRS Left (Tip) is an audio or GATE signal input: “clipped to +/-5V and scaled down to +/- 3.3V”.
• The TRS Right (Ring) is for CV: “1V/octave signal (0 – +6V)”.

This is how to wire up the Synth Thing LFO to the CV input of the Volca.

It uses a stereo 3.5mm TRS to 3.5mm TRS lead – essentially a headphone lead. I’m using my Sparkfun 3.5mm TRS breakout and a solderless breadboard.

The CV signal is available on the lower of the two breakout jumper headers on the Volca. In the patch above it is connected to the control input (middle) of the first LPG.

Other good candidates for patching the CV to are:

• Source modulation control input.
• Source fold control input.
• Space out control input.

You can also get some curious effects by using the “a+bxc” utility module to combine the CV In with one of the internally generated control signals, e.g. the first LPG “+” output.

The CV In can be linked to the Source pitch input too, but there is no way that I’ve found so far of combining a pitch CV input with the keyboard, so the pitch becomes fixed by the CV input only at this point – i.e. it controls the carrier frequency, not the modulator (as far as I can see).

Synth Thing EG controlling the Volca

It is possible to trigger the Synth Thing envelope generator and use the resultant control signal back in the Volca via the CV in as shown below.

The key for this working is to use the Volca’s sequencer GATE output as the GATE and TRIGGER for the Synth Thing’s ADSR envelope generator and then feed the EG output into the Volca’s CV In. The Volca’s CV In is then connected to whatever is to be controlled by the EG – in the above case the first LPG.

Unlike the previous approach that fed the Volca’s output through the Synth Thing’s EG, this now replaces the built-in AHD attack and release EG of the Volca which leads to a much easier to understand set of controls.

Direct connections between Synth Thing and the Volca

Whilst it should be possible to connect the Synth Thing’s patch wires directly into various parts of the Volca, in the end, I decided there wasn’t anything at this point in time that couldn’t be achieved using the official CV In link.

So for the time being, I’ve not experimented further with any direct connections that bypass the official CV In.

Closing Thoughts

I’m intrigued by the 0.5V/oct thing and need to do some further testing of the Synth Thing’s response, but then I was never really happy with how the pitch and amplitude inputs for the VCOs were working. It is on my “todo” list to look into a more efficient way of sample the ESP32’s ADCs.

But I am quite impressed with the possibilities of the CV In on the Volca. Now I’ve explored and noted the basics I’ll have to see what the full “art of the possible” might be.

Kevin

https://diyelectromusic.com/2024/09/01/educational-diy-synth-thing-meets-korg-volca-modular/

Here are the build notes for my Korg Volca Modular MIDI to CV PCB.

Warning! I strongly recommend using old or second hand equipment for your experiments.  I am not responsible for any damage to expensive instruments!

If you are new to microcontrollers, see the Getting Started pages.

Bill of Materials

• Korg Volca Modular MIDI to CV PCB (GitHub link below).
• 1x ATtiny85 DIP-8.
• 1x 6N138 optoisolator DIP-8.
• 1x 1N914 or 1N4148 signal diode.
• 2x 220Ω resistors.
• 3x 1K resistors (one optional, for LED).
• 1x 4K7 resistor.
• 3x 100nF ceramic capacitors.
• 1x 3mm LED (optional).
• 1x 3.5mm stereo TRS pcb mount (see PCB and photos for footprint).
• Either 1x 3.5mm stereo TRS or 5-pin DIN socket (see PCB and photos for footprint).
• 2x 8-way DIP sockets (optional, recommended).
• Header pins (optional).
• 1x pin jumper (optional).

Power circuit (optional):

• 1x L7805 regular TO-220.
• 1x 10uF electrolytic capacitor.
• 1x 100uF electrolytic capacitor.
• 1x 2.1mm barrel jack socket (see PCB and photos for footprint).
• 1x on/off switch (optional).

Power Options

The PCB includes a regulator circuit to take a 7-12V input either via 2.1mm barrel jack or 2-pin jumper headers (red) to create the required 5V power supply.

The switch is optional. If not required, then the highlighted two pins (blue) can be connected with a wire bridge.

Alternatively, a regulated 5V supply can be provided directly via another jumper header (orange) in which case the barrel jack socket and all of the component footprints highlighted in the above photo can be omitted (other than the GND-5V header pins themselves).

When used with the built-in power supply, a link is required between “5V” (part of the orange highlight) and “REG”.

Note: when used with the barrel jack socket, I don’t see any reason why the VIN/GND (red) headers couldn’t be used for a “VOUT” connection too. If that had a couple of jumper wires to a second barrel jack, for example, it could potentially be used to power the Volca too.

See photos as part of the build for more details.

Build Steps

Taking a typical “low to high” soldering approach, this is the suggested order of assembly:

• All resistors and diode.
• DIP socket (if used) and TRS socket(s).
• Disc capacitors and LED.
• Jumper headers.
• Electrolytic capacitors.
• Other components.

Here are some build photos.

The order of installation for the larger components and connectors will depend on the exact configuration and components used. They can be installed last in whatever order makes sense.

Power Options

There are only three valid power configurations as follows.

• Left: Installation of the jumper between 5V and REG to use the on-board regulator (other header connections are redundant).
• Centre: External connection to regulated 5V power supply (regulator and power circuitry is redundant).
• Right: External connection to non-regulated 7-12V supply (barrel jack socket is redundant).

WARNING: Never connect a jumper between GND and 5V across the “GND 5V-REG” headers.

With hindsight, mixing the options for adding external jumper wires (so requiring GND/5V) and linking 5V to REG (so requiring a jumper) was probably unwise, but it seemed like a neat optimisation at the time.

Initial Testing

I recommend performing the general tests described here: PCBs prior to installing the ATtiny85.

It is also worth verifying the operation of whatever power option has been implemented by checking for GND and 5V at the ATtin85 socket:

Then a programmed ATtiny85 can be installed and MIDI reception checked, checking the CV and gate signals with an oscilloscope.

The CV/Gate TRS out is compatible with the Korg Volca Modular, which means it has the following connections:

• TRS Left channel = Gate: 0V (off) or 5V (on).
• TRS Right channel = CV: 0V to 5V range.

Programming the ATtiny85

The code for the MIDI to CV converter can be found here: https://emalliab.wordpress.com/2019/03/02/attiny85-midi-to-cv/

To program the ATtiny85 requires the following:

• A USB programmer. I use the Sparkfun “Tiny AVR Programmer“.
• The ATtiny85 Arduino core by Spence Konde: https://github.com/SpenceKonde/ATTinyCore – I have v1.5.2 installed at the time of writing.

The key things to remember are:

• Set the programmer to “USBTinyISP” – if you get a choice, I used the “slow” version. Note: you won’t see a “port” with this one, it using the programmer directly.
• The clock setting requires is “Internal” 8MHz.

There are options in the code for the MIDI channel to listen to, the default being channel 1.

There is also a setting for the MIDI notes that correspond to generating 0V and 5V. The default lowest note is 36 – C2, and the default highest note is 96 – C7, giving 5 octaves across the 0-5V CV range.

In Use

Here are some of the average CV voltage readings I’m getting for various MIDI notes:

• C2: avge -80 mV; 0V max
• C3: avge 918mV; 1.2V max
• C4: avge 1.88V; 2.24V max
• C5: avge 2.88V; 3.28V max
• C6: avge 3.84V; 4.16V max
• C7: avge 4.81V; 4.96V max

Plotting those readings out on a graph (max in orange; average in blue) gives:

So, actually that probably isn’t too bad. The gate seems to work pretty well too.

Controlling the Korg Volca Modular

To hook it up to a Korg Volca Modular requires 3.5mm stereo jack to jack cable between the CV output and the Korg’s CV-IN socket. The two signals (CV/Gate) are then available o the corresponding patch points.

Patch the CV signal to the oscillator (green wire below) and the Gate to the first (AHR) function (blue wire) as follows:

In use, to my ears, the tuning doesn’t sound too bad to me across the range C2 to C7. Not sure about that very bottom C2… but then I wasn’t convinced that 0V really was 0V anyway.

PCB Errata

There are no issues currently known with this PCB however please note:

Check and double check the outputs before you let them anywhere near your Korg Volca! This is “use at your own risk”. I am not responsible for any dead Volcas….

Enhancements:

• As already mentioned, with hindsight, I probably should have separated out the external, 5V regulated pin headers from the 5V-REG jumper.
• I wanted a power LED but MIDI activity might have been more useful.
• I need to get a better power switch footprint! That was a bit of an afterthought and isn’t particularly useful as it stands.
• Another thing that might be useful would be a second barrel jack socket to allow power to be taken from the same source into the Volca.

Find it on GitHub here.

Closing Thoughts

For a first practical application of both CV/Gate and the ATtiny85, I’m pretty pleased with how this works. It is certainly very responsive for the Korg. It is monophonic of course, as it only generates a single CV output but again that is fine for the Korg.

I know there are mods for a Volca Modular to give it MIDI directly – there are patch points readily available on the PCB inside, as I mentioned before – I just didn’t want to be opening mine up so soon and seeing as CV/Gate is readily available, I don’t really see the need.

I might see if I can knock-up a 3D printed case for this to make a nice, self-contained unit.

Kevin

https://diyelectromusic.com/2024/07/07/korg-volca-modular-midi-to-cv-pcb-build-guide/