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 80's sound with independent and mixed ramp, pulse, and sub outputs. But the twins don't 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.


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 or create new patches.

Version differences#

Illustration of Castor & Pollux I and Castor & Pollux II

Castor & Pollux has two different versions in the wild with significant changes between them. Fortunately, both versions are fundamentally the same brains with different user interfaces. Changes made by Castor & Pollux II include:

Both versions use the same firmware and behave the same way. This manual applies to both versions, however, the illustrations shown here depict Castor & Pollux II. We made sure that all interface elements use the same symbols and labels between the versions. If you run into trouble, don't hesitate to reach out.


Illustration of power connection

To install this into your Eurorack setup, connect a Eurorack power cable from your power supply to the back of the module. Note that even though there's a keyed power connector on the module, double check that the red stripe is on the side labeled red stripe! Once you've connected the power cable, secure your module to your rack rails using screws.


Castor & Pollux has two separate but intertwined oscillators. It's possible to control each oscillator independently, but they truly shine when used together. Castor & Pollux's design is focused around the connection between these two oscillators and modulating their parameters using the internal LFO.

Illustration of Castor & Pollux's interface with elements for the separate oscillators highlighted

Castor & Pollux's front panel is arranged so that Castor's inputs, outputs, and controls are on the left whereas the corresponding elements are mirrored on the right for Pollux. The center contains controls for the internal LFO and crossfade mixer:

Each oscillator, and β, is controlled by its associated knobs and CV input jacks:

Illustration of the knobs and input jacks

You can learn more about these inputs in the pitch behavior section.

Each oscillator has a single output jack, , and corresponding mixing knobs:

Illustration of the oscillator's outputs

Each oscillator generates three waveshapes: saw, pulse, and sub. The three mixing knobs control how much of each waveshape is present in the oscillator's output. You can read more about the sound of each waveshape in the waveshapes section.

Next, the crossfade mixer combines the output of the two oscillators together:

Illustration of the crossfade mixer and its output jack

The crossfader's knob, Σ, determines which oscillator is more prominent in the mix at the crossfade output, ♊️. The oscillator stacking section has more details and sound samples of combining the oscillators together.

Next up, in the very middle there's the LFO knob, φ:

Illustration of the LFO knob

This internal LFO can be used to modulate several parameters in interesting ways depending on the mode.

Last, but not least, there is a single button in the center that's used for changing modes and enabling the tweak overlay:

Illustration of the button

You can learn more about the different modes and tweaking in the modes & tweaking section.

First steps#

If you're not sure where to start with Castor & Pollux this section has a few patches to get you up and running. Grab your patch cables and follow along.

Patch one#

First, dip your toes in by putting together this patch:

Illustration of the first patch

You should hear a low, rumbly bass sound. Turn Castor's pitch knob (the big knob at the top left) and you'll hear Castor sweep through its range. This is called coarse behavior and it's explained in more detail in the pitch section.

You can keep playing around with this patch by moving the mix trimpots for each of the waveshapes to hear how they sound. You can also jump down to the waveshapes section to learn more about those.

Patch two#

Ready for more? Try out this patch:

Illustration of the second patch

Castor should now be playing notes based on the CV you're sending it. You can turn Castor's big pitch knob to tune/detune relative to the input. This is called fine behavior and it's also explained in more detail in the pitch section.

Patch three#

Okay, one last patch and you'll be ready for anything:

Illustration of the third patch

This is where things get fun- at this point, Castor & Pollux should both be playing (roughly) the same pitch and they're both coming through the crossfade output jack. Now, slowly wiggle Pollux's pitch knob. This is the most basic form of oscillator stacking and it's your gateway to the big, raw, and fun sounds of Castor & Pollux.

Now that you have a feel for Castor & Pollux, check out the rest of the manual to learn all about its various features, behaviors, and sounds.

Pitch behavior#

Due to the intertwined nature of Castor & Pollux's oscillators, the effect of the pitch CV jack and knobs take on different behaviors in different circumstances.


You can modify Castor & Pollux's pitch behavior using the settings editor.


Illustration of coarse pitch behavior

When nothing is patched into Castor's pitch CV jack, Coarse behavior is used. Castor's pitch is determined by its pitch knob which sweeps through six octaves and quantizes to the nearest semitone.


Illustration of fine pitch behavior

If there is a signal patched into Castor's pitch CV jack, Fine behavior is used. The input CV should be between 0 V and 6 V. The pitch knob offsets the input CV by ±1 octave. The pitch knob has a sort of "virtual notch" at the 12 o' clock position to help you dial in the frequency you're looking for.

Pollux also uses Fine behavior but follows Castor if nothing is patched into its pitch CV jack. Pollux doesn't ever use Coarse behavior.


Illustration of multiply pitch behavior

Finally, Pollux uses Multiply behavior when in Hard Sync mode. In this case, Pollux follows Castor and the knob adds up to three octaves.

Jack detection#

Castor & Pollux detects whether a signal is patched into the pitch jacks by checking if the signal is above 0 V. In many cases this works well, however, you may run into issues if you use a sequencer or controller that often sends 0 V signals. Castor & Pollux will think that a jack isn't detected and erroneously switch to a different pitch behavior. If this is something you're running into you can turn off jack detection using the settings editor.


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

To make tuning easier, Castor & Pollux provides two ways of using the pitch knob to dial in the perfect frequency.

440 Hz 440 Hz

First, the pitch knobs have a "virtual notch" because of their non-linear response: they're less sensitive in the middle of their range than the edges. It's usually easier to understand this visually, so try out the little illustration above and notice that with the non-linear response it's much easier to tune to frequencies around 440 Hz. This non-linear response only happens when the oscillator is using the Follow pitch behavior. You can configure how strong this effect is using the settings editor.

Illustration of tweak mode pitch tuning

Second, the tweak overlay allows extra-fine control over tuning. Holding down the button and turning the pitch knob allows you to apply an additional ±2.5 semitone offset. This offset is applied to the oscillator regardless of the pitch behavior or the current mode.


Just like the Juno, Castor & Pollux generates three waveshapes: 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 use internal low-frequency oscillator to modulate the pulse width in LFO PWM mode.

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 follows the pitch of 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:

Internal low-frequency oscillator#

Castor & Pollux's built-in low-frequency oscillator can be used to modulate various parameters depending on the current mode.

Illustration of LFO waveshape

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.

Modes & tweaking#

Castor & Pollux has four different modes that change the module's overall functionality:

Illustration of modes

Illustration of tapping the button

To cycle between modes, tap the button in the middle and the module will play a short animation to show that it has switched modes.

Illustration of holding the button

On the other hand, holding the button turns on the tweak overlay. This gives you access to additional parameters depending on the mode. When moving in and out of the tweak overlay, the knobs get "latched" so that they don't immediately cause changes - similar how many synthesizers work when loading patches. The parameter only starts changing once you've moved the knob. In all modes, the pitch knobs control the extra-fine tuning.


Castor & Pollux's default mode is the Chorus mode. This mode is inspired by the original Juno's analog chorus circuit, however, instead of applying the chorus affect after sound generation, Castor & Pollux's chorusing works by varying the frequency of the second oscillator using its internal low-frequency oscillator. This means you have to use both oscillators to hear this effect and it works best if Pollux is following Castor's pitch.

Illustration of chorus controls

The LFO knob, φ, determines the intensity of chorusing from none when fully counter-clockwise to its maximum at fully clockwise. The crossfade mixer, Σ, also has an impact on the intensity of the chorus.

Illustration of chorus tweak controls

When holding the tweak button, the LFO knob, φ, controls the LFO'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 the animation to start it otherwise you won't really be able to see the chorusing effect.

Here are some sound samples of chorusing:


LFO PWM mode uses the internal LFO to modulate the pulse width of each oscillator. This only affects the pulse waveshape.

Illustration of the LFO PWM controls

The LFO knob, φ, determines the frequency of the internal LFO. Meanwhile, each oscillator's pulse width knob controls the depth of modulation from none when fully counter-clockwise to its maximum at fully clockwise. Any signal patched into the pulse width jack is summed with the knob.

Illustration of the LFO PWM tweak controls

When holding the tweak button, each oscillator's pulse width knob controls the center of the pulse width modulation.


LFO FM mode uses the internal LFO to modulate the pitch for each oscillator. This is similar to the Chorus mode, except it applies to both oscillators instead of just Pollux.

Illustration of LFO FM controls

The LFO knob, φ, determines the frequency of the internal LFO. Meanwhile, each oscillator's pulse width knob controls the depth of pitch modulation from none when fully counter-clockwise to its maximum at fully clockwise.

Unlike the LFO PWM mode, the pulse width jack has no impact on modulation.

Illustration of LFO FM tweak controls

When holding the tweak button, each oscillator's pulse width knob controls the oscillator's pulse width.

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. Luckily, Castor & Pollux has two oscillators!

Illustration of hard sync where the first waveform resets the second waveform

Hard sync mode syncs Pollux's oscillator to Castor's frequency - Pollux's output retains the same base frequency as Castor but the waveshape changes dramatically with Pollux's pitch. In other words, it'll sound like the same note but have a much different, usually metallic, timbre.

Since hard sync only affects Pollux, you'll have to use either Pollux's output or the crossfade output to hear the change in timbre.

Here are some sound samples of hard sync:

Hard sync mode's controls are the same as chorus mode except that Pollux's pitch knob uses the Multiply behavior:

Hey, listen!

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.


Illustration of C&P & expander next to each other

Castor & Pollux II includes a small expander that provides individual output jacks for each oscillator's waveshapes.

Illustration of connecting the expander to C&P

To use the expander, connect the small ribbon cable to the back of Castor & Pollux in the header labeled Expander. Connect the other end to the matching header on the back of the expander. Secure the expander to your case using screws.

Lost cable?

Happens to the best of us. Contact us to get a replacement or grab a 10-pin 2x5 Socket-Socket 1.27mm IDC cable (also called a SWD cable) like these from Adafruit and SparkFun.

Connecting with USB#

You can connect Castor & Pollux to your computer using a standard micro USB cable, which lets you edit settings and update the firmware.

Illustration of connecting Castor & Pollux to USB

The micro USB port is located on the backside of the module. Once you've connected a cable, be careful not to put too much stress on the connector as it's possible to damage the connector with enough force.


Castor & Pollux II is happy to run on USB power when editing settings and updating firmware, but Castor & Pollux I needs power from your rack power supply while connected to USB.

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 lots of settings you can tweak over USB, such as the LED brightness, the range of the pitch knobs, quantization, and more.

Connect your module to your computer using a USB cable and then head over to the web-based settings editor to tweak to your heart's content. You don't need to install any software or drivers.

Updating the firmware#

  1. Download the latest firmware from GitHub. It's the .uf2 file.
  2. Connect your module to your computer using a USB cable.
  3. Place Castor & Pollux in bootloader mode by quickly pressing the Reset button twice. The reset button is the small button next to the USB port. Once in bootloader mode, you should see an external drive on your computer named GEMINIBOOT.
  4. Copy the .uf2 file from step one to the GEMINIBOOT drive. The module should restart by itself.

Open source hardware & software#

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

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 guarantee that the module will function under these conditions.


Ramp table swap#

Affected units
This applies to All Castor & Pollux II units shipped prior to February 7th, 2024. Castor & Pollux I units are not affected.
Due to an error during calibration, the internal ramp calibration tables for the two oscillators are swapped. This table is used to correct the amplitude of the ramp waveform, so when the tables are swapped the waveform appears distorted.
Corrective action
This errata can be resolved with just a computer and USB cable. Connect your unit to your computer via usb, open the settings editor using this specific link, press the "connect" button, then scroll all the way to the bottom and press the "swap ramp calibration" button. If this does not work for you, please reach out to us at for assistance.

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. We would also like to acknowledge the following individuals for their outstanding support: