Castor & Pollux User Guide#

Castor & Pollux is a Roland Juno-inspired oscillator. It contains two digitally-controlled analog oscillators implemented using a faithfully modernized version of the original Juno 106 design. Like its inspiration, it can give you that wonderful 80s sound with independent and mixed ramp, pulse, and sub outputs. But the twins do not stop there- the second oscillator can be used to layer in thick detuned sounds, harmonically complex hard synced waveshapes, or as a fully independent second oscillator.


  • Available assembled or as a kit
  • 14 HP
  • +12v @ 100mA, -12v @ 35mA
  • 25mm deep
  • 2 separate oscillators
  • 7 octave range
  • 9 total outputs with waveform outputs and onboard mixers
  • Pitch CV ins (0 to 6v) and pitch knobs (-1 to +1 octave, configurable)
  • Pulse-width CV ins and knobs
  • Hard sync
  • Hardware chorusing
  • Internal low-frequency oscillator
  • ModularGrid page

Getting support and help#

We want you to have a wonderful experience with your module. If you need help or run into problems, please reach out to us. Email is the best way for product issues, whereas Discord and GitHub are great for getting advice and help on how to customize your module.

Installing the module#

To install this into your Eurorack setup:

  1. Connect a Eurorack power cable from your power supply to the back of the module. Note that even though the power connector on the module is keyed, double check that the red stripe is on the side labeled red stripe!
  2. Screw the module to your rack's rails.

Inputs and outputs#

Castor & Pollux contains two separate oscillators. Each has two inputs with associated knobs:

Inputs for one oscillator highlighted

  • A pitch CV input. The range is 0v to +6v.
  • A pitch knob. The range is -1 octave to +1 octave and is added to any incoming pitch CV. These have a special response curve that makes it easier to tune the oscillators.
  • A pulse width CV input. This changes the pulse width of the pulse wave. The range is 0v to +5v.
  • A pulse width knob. When combined with the pulse width CV it sets the minimum pulse width.

Each oscillator also has three dedicated waveshape outputs:

Outputs for one oscillator highlighted

  • A ramp wave output
  • A pulse wave output
  • A sub square wave output

Each waveshape is nominally 10v peak-to-peak. You can read more about the sound of each waveshape in the waveshapes section.

Each oscillator also has a waveshape mixer that contains level knobs for each waveshape and a mix output:

Mixer knobs and mixer output highlighted

The waveshapes section also shows how mixing the waveshapes affects the waveshape and sound.

There is also a combined output that mixes the output from both oscillators mixer together. It has a crossfader knob to determine which oscillator is more prominent in the mix:

Crossfader and combined output

The oscillator stacking section has more details and sound samples of combining the oscillators together.

Finally, there is a knob to enable chorusing, and a button to enable hard sync:

Chorusing knob and hard sync button

The chorusing section goes into more detail and has some sound samples. The hard sync section shows how hard sync affects the second oscillator.

Finally, here's a cheatsheet if you'd like to have a quick reference:

Module with labels for inputs and outputs


Castor & Pollux is an analog oscillator with a digital brain. It's calibrated during assembly to accurately track 1 volt/octave on the CV inputs and you can use the pitch knobs to adjust the tuning of each oscillator.

In order to make tuning easier, the pitch knobs have a non-linear response: they are less sensitive in the middle of their range than the edges. It's usually easier to understand this visually, so try out this little demonstration:

440 Hz 440 Hz

Notice that with the non-linear response it's much easier to tune to frequencies around 440 Hz. This is how Castor & Pollux's pitch knobs work. You can configure how strong this effect is using the settings editor.


Just like the Juno, Castor & Pollux has three waveshape outputs: ramp, pulse, and sub.

The ramp wave looks like this (click the image to start and stop the animation):

Simulation vs reality

These are simulations of perfect waveshapes. Castor & Pollux's waveshapes are generated by analog circuitry and they don't quite look exactly like these examples. That's a good thing - the analog weirdness is what adds character to the module.

The ramp sounds like this:

The pulse wave depends on the pulse width CV and knob. You can vary the pulse width in this animation to see how it affects the waveshape:

You can also configure the internal low-frequency oscillator to modulate the pulse width.

Here are some examples of the pulse wave's sounds:

Finally, there's the sub waveshape. It's a square wave that's one octave lower. Here's what it looks like:

And here are some sound samples of the sub waveshape:

These waveshapes can be mixed together to produce much more complex and interesting variants - try playing with the sliders under this animation to see how it affects the waveshape:

Here are some sound samples of various mixes:

Oscillator stacking#

Because Castor & Pollux has two oscillators, you can combine them together to build much more complex sounds. When there is no input into the second oscillator's pitch CV jack, it will follow the pitch of the the first oscillator. You can use the pitch knob of the second oscillator to detune the second oscillator and cause interesting interactions with the first oscillator. You'll take your output from the combined output (marked by the symbol ♊︎) and you can control the relative volume of the two oscillators using the crossfader (marked by the symbol Σ).

You can play around with this interactive animation to see how detuning and mixing changes the final waveshape:

This animation just uses the ramp waveshape, but the crossfader takes the mix from each oscillator's mixer, so you can combine many different waveshapes. Here are some sound samples of oscillator stacking:


Inspired by the original Juno's analog chorus circuit, Castor & Pollux contains an adaptation of chorusing that works well for a monophonic voice. Instead of applying the chorus affect after sound generation, Castor & Pollux's chorusing works by varying the frequency of the second oscillator using an internal low-frequency oscillator. This means you have to use both oscillators in order to hear this effect. The chorus knob (marked by the symbol φ) determines the intensity of chorusing from none when fully counter-clockwise, to its maximum at fully clockwise. The crossfader (marked by the symbol Σ) also has an impact on the intensity of the chorus. Finally, you can modify the speed of the chorus by changing the internal low-frequency oscillator's frequency.

You can play around with this interactive animation to see how the chorusing amount changes the final waveshape. You'll need to click on the animation to start it otherwise you won't really be able to see the chorusing effect.

Here are some sound samples of chorusing:

Hard sync#

Hard sync is a feature seen in several synthesizer voices that have two or more oscillators. However, it's not a feature seen on the original Juno because each voice only had one oscillator. Since Castor & Pollux has two oscillators to work with, it has an adaption of hard sync. Hard sync allows you to sync the second oscillator to the first - the output will retain the same base frequency as the first oscillator but the waveshape will be impacted by the frequency of the second oscillator. In other words, it'll sound like the same note but have a much different timbre.

To use hard sync tap the hard sync button - the LEDs should all change to the same color. You'll use the first oscillator's pitch CV to set the note and you'll use the second oscillator's pitch knob or pitch CV input to control the timbre. You'll take the output from the second oscillator (or the combined output) - since hard sync does not impact the waveshape of the first oscillator.

Here are some sound samples of hard sync:

Heads up

Due to the way hard sync works, you shouldn't set the second oscillator to a lower frequency than the first. It'll just result in the sound growing softer since the second oscillator doesn't have enough time to output a complete wave cycle.

Internal low-frequency oscillator#

Castor & Pollux has an built-in low-frequency oscillator (LFO) that's used for modulation. This LFO drives the chorus feature, but can also be assigned to some other parameters.

To configure the internal LFO, hold down the hard sync button. The module's lights should change. While holding the hard sync button you can modify the following settings:

LFO configuration knobs highlighted

  1. Moving the chorus knob (φ) will change the frequency of the LFO. The LEDs at the bottom of the module will pulse to show the LFO's frequency.
  2. Moving the pulse width knob of each of the oscillators will enable or disable LFO routing to the oscillator's pulse width. When the LFO is routed to an oscillator, its pulse width knob will control the intensity of the pulse width modulation instead of directly setting the pulse width. Turning the knob counter-clockwise will disable the LFO routing, turning it clockwise will enable it. The LEDs near the oscillator will illuminate if the LFO routing is enabled.

By default, the internal LFO is a straightforward triangle wave. However, the settings editor allows you to change the LFO's waveshape between triangle, sine, sawtooth, and square, as well as combine a second waveshape with the first to create interesting LFO effects.

Editing module settings#

Even though Castor & Pollux uses analog circuitry to make its beautiful sounds, there is a little microcontroller brain inside that's orchestrating everything. There are a few settings you can tweak over USB, such as the LED brightness, and the range of the pitch knobs. Head over to the web-based settings editor to tweak to your heart's content.

Updating the firmware#

  1. Download the latest firmware from GitHub. It will be a .uf2 file.
  2. Connect Castor & Pollux to your computer via USB. The USB port is located underneath the panel. Note that you still have to power the module from a Eurorack power supply.
  3. Place Castor & Pollux in bootloader mode by quickly pressing the reset button twice. The reset button is located just below the USB port. Once in bootloader mode, you should see a drive on your computer named GEMINIBOOT.
  4. Copy the .uf2 file from step 1 to the GEMINIBOOT drive. The module should restart by itself.

Open source hardware & software#

Castor & Pollux is completely open source and hacking is encouraged.

  • The firmware is available under the MIT License. Note that the firmware uses some third-party libraries that are under different, but compatible terms. Read the full text of the license for more details.
  • The hardware designs are available under the permissive CERN-OHL-P v2 license, and is designed using KiCAD, which is free and open-source. You can open the hardware files using KiCAD, or you can check out a PDF of the schematics for the mainboard and the jackboard.

Open Source Hardware Association mark Castor & Pollux is certified open source hardware.

Warranty, disclaimers, and limits#

Modules purchased from Winterbloom have a warranty of six months from the date of purchase. This warranty covers any manufacturing defects in the module. It does not cover damage due to incorrect handling, storage, power, overvoltage events, or modifications.

Please contact us if you are experiencing issues with your module. Modules returned under warranty will either be refunded, replaced, or repaired at our discretion. You will be responsible for the cost of returning the module to Winterbloom.

Because Eurorack has a vast number of possible combinations of power supplies, modules, cases, and accessories, it is impossible for us to guarantee compatibility with every possible configuration. We make a reasonable effort to test modules with several power supplies, cases, and other modules.

We encourage tinkering, however, we will not refund, repair, or replace any products that have been modified.

We have stress tested Castor & Pollux under the following conditions. These are provided for reference only; we do not guarentee that that the module will function under these conditions.

  • Continuous run time: at least 48 hours
  • Overvoltage from power supply: at least ±18 volts
  • Reverse voltage from power supply: at least ±18 volts
  • Undervoltage from power supply: as low as ±10 volts
  • Over and under voltage on inputs: at least ±8 volts
  • Output connected to another output: at least ±8 volts
  • Output shorted to ground: at least 60 minutes

Acknowledgments and thanks#

Castor & Pollux would not be possible without the help of the Adafruit, support from Microchip, and the volumes of literature documenting and analyzing the Juno's DCO design.