Winterbloom's Neptune is a resonant diode ladder filter inspired by the designs of Moritz Klein and Arturia. It's intended to bring together sound and modulation into a complete, distinct synthesizer voice. Unlike typical multi-mode filters with separate modes or outputs, Neptune instead has separate low-pass and high-pass inputs into a single filter core. This filter core is coupled with a feedback modulated distortion circuit called salt that opens up a wide range of tonal possibilities from tame to dirty to outright chaotic.


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.


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.


Neptune illustration

Like other filters, Neptune subtracts frequencies from audio signals. Unlike most other filters, however, Neptune is a single filter with separate inputs for low-pass and high-pass. If you use just one of these inputs at a time, Neptune will act as a familiar low-pass or high-pass filter. However, using both inputs at the same time opens up new possibilities for combining and filtering multiple signals.

Neptune is also a resonant filter, which allows you to boost the frequencies around the cutoff frequency. Neptune rounds out its sonic character with Salt- a feedback modulation circuit that can add subtle grit when used sparingly and wild, chaotic distortion when applied with abandon.

Neptune belongs to diode ladder family of filters. These filters are known for their characteristic "dirty" sound compared to the modern, popular "smooth" OTA family. Diode filters behave quite differently from their counterparts, exhibiting increasingly non-linear behavior when overdriven or operated with high resonance. Neptune itself shares a lot of similarities with the Steiner filters found in Arturia's Brute series, but the addition of the salt feedback circuit allows for a broader range of unpredictable sounds.

Neptune is a collaboration between Carson Walls and Winterbloom. Carson crafted Neptune's voice, layout, and electronics design, while Thea & Maggie took on Neptune's visuals, interface, physical design, and manufacturing.


Audio inputs#

Audio inputs

Neptune has separate audio inputs for low-pass and high-pass labeled with lp and hp respectively. Each input has its own volume control knob, where fully counterclockwise mutes the input. The mark at 2 o' clock indicates unity gain, where the input and output volume will be the same.

Volume & resonance

Because of the architecture of this filter, the sound of the resonance is strongly affected by the input volume. The louder the input volume, the less obvious and "sweet" the resonance. If you want resonance to really catch the harmonics of the input, try reducing the input volume as you turn the resonance up.

Audio output and volume#

Audio output

Neptune's audio output, out, has a built-in VCA for volume control. The vol input responds to a 0 to 5 V signal with 0 V completely muting the output and 5 V giving maximum volume. When vol is left unpatched, the output volume defaults to maximum.

Cutoff frequency#

Cutoff frequency controls

Neptune dedicates several controls for the filter's cutoff frequency. Frequencies above the cutoff frequency are attenuated from the low-pass input signal, while frequencies below the cutoff are attenuated from the high-pass input signal.

The primary control for the cutoff frequency is the large cutoff knob labeled . Turning the knob fully counterclockwise sets the cutoff frequency to near 0 Hz, where the low-pass signal is almost entirely cut out while the high-pass signal is largely unaffected. Conversely, turning the knob fully clockwise sets the cutoff frequency to around 200 kHz, where the low-pass signal is largely unaffected and the high-pass signal is cut out.

Audio inputs

The cutoff frequency set by the knob can be modulated by two similar but distinct CV inputs labeled fm1 and fm2. Both inputs respond to ±5 V signals but fm1 has an attenuverter while fm2 has an attenuator.

The fm1 attenuverter controls both the level and polarity of fm1's CV input signal. At 12 o' clock, no signal is passed through. Turning clockwise gradually introduces the signal while turning counterclockwise gradually introduces an inverted version of the signal. The ring of LEDs around fm1 indicate the polarity and level of modulation: blue indicates a positive signal, while red indicates a negative signal.

Similarly, the smaller fm2 attenuator controls just the level of fm2's CV input signal. Fully counterclockwise completely attenuates the signal, while turning clockwise gradually introduces the signal. The white marking indicates where the cutoff frequency will roughly track at 1 Volt / octave, but note that due to the way the filter works this will not track perfectly.

Finally, the LEDs around the knob indicate the total combined effect of along with the fm1 & fm2 inputs and controls. LEDs to the left of the knob indicate that more of the low-pass signal is being passed, while the LEDs to the right indicate that more of the high-pass signal is being passed.

CV Range

Neptune's control voltages are nominally ±5 V, but it's generally fine with voltages beyond this range. Just note that it may not respond as expected when you get far out of that range.


Resonance controls

Neptune is a resonant filter, and like other filters, resonance increases the volume of frequencies near the cutoff frequency. Neptune's resonance is controlled with the reso knob and CV input. The reso knob introduces more and more resonance as you turn the knob clockwise. When the reso knob is turned beyond the white dot the filter will begin self-oscillating, indicated with a red LED. Likewise, the reso CV input responds to 0 to 5 V and is summed with the reso knob.

Unlike many filters, Neptune does not cut out bass frequencies as the resonance is turned up. While this preserves low-end frequencies, it can lead to the input volume overwhelming the resonance. If you want a "sweeter" resonance sound, try reducing the input volume when the resonance is turned up.


Salt controls

Neptune includes a feedback modulation circuit called Salt. When used sparingly, salt can add a bit of subtle grit and sonic character to your sound. When used with gleeful abandon, salt will create increasingly chaotic distortion with harsh overtones.

Salt is controlled by the salt knob and CV input. The salt knob introduces more and more feedback modulation as you turn the knob clockwise. Keep in mind that salt and resonance are inherently linked and interact strongly, meaning that more resonance will increase salt's effect and vice versa. There are many textures to be found in different combinations of salt, resonance, and input volume.

Example patches#

If you're not sure where to start with Neptune this section has a few patches to get you up and running. Grab your patch cables, an oscillator, and an output module to follow along.

Low-pass filter#

The first patch creates a familiar low-pass filter:

Low pass filter patch

Once patched, use the knob to sweep through the filter's frequency range. Try introducing some resonance using the reso knob and salt using the salt knob.

High-pass filter#

The second patch is similar to the first and creates a high-pass filter:

High pass filter patch

Once patched, use the knob to sweep through the filter's frequency range. Once again, try introducing some resonance using the reso knob and salt using the salt knob.

High/low shelf filter#

The next patch uses both inputs to create a high/low shelf filter:

High/low shelf filter patch

Once patched, you can use the lp and hp volume knobs as your low- and high-pass shelves while the Neptune ♆ knob controls the shelving frequency. You can use resonance as a frequency boost to emphasize the frequencies around the shelving frequency without cutting out the rest.

Spectral crossfader#

The next patch uses different signals on both inputs to create a spectral crossfader:

Spectral crossfader patch

Once patched, you can use the knob to crossfade between the frequency contents of the two signals, picking out the low frequencies of one signal and the high frequencies of the other.


The final patch explores self-oscillation:

Self-oscillation patch

Once patched, the filter should be self-oscillating and generating a sine waveform. You can use the knob to set the output signal's frequency. If you further increase resonance past the white dot, the waveform will saturate and become more harsh, like a square wave. Adding salt will cause the waveform to become more unstable and chaotic.

Further explorations#

These ideas go beyond the example patches and may help you explore Neptune's uses and sonic possibilities.

Basic synth voice#

Use any oscillator as the input for the lp in, preferably with a sharp waveform like a saw or square. Use an envelope going the fm1 and vol inputs. Trigger the envelope with a gate source from a keyboard or sequencer. Adjust the envelope timings to get long drones notes, or quick percussive ones. Adjust the cutoff frequency, fm1 amount, resonance, and salt to hear the range of possible sounds from the low-pass filter.

Spectral crossfading synth voice#

Patch one oscillator to the lp input and another oscillator to the hp input, or use different waveforms from the same oscillator. Make sure the oscillators are tuned close to each other for the most harmonious sound. Listen to how the filter can fade between the frequencies of the two oscillator signals. Plug one envelope to fm1 and a different envelope to vol. Trigger the envelopes with the same gate source, such as a keyboard or sequencer. Try different envelope shapes to get different responses, and listen to how different input signals and volumes effect the sound. Try different oscillator tunings, at octaves or ratios for nice harmonic sounds, or detuned for more dissonant ones.

Kick drum voice#

Leave the lp and hp inputs unpatched. Turn reso up until the filter starts self oscillating. Send a fast, exponential envelope to fm1 input and a second slower envelope to vol. The fast exponential envelope is the kick drum’s pitch sweep, whose intensity is adjusted with the fm1 attenuverter. Adjust the cutoff frequency to set the base pitch of the drum sound, for example set it to around 9 o’clock for a low bass drum sound. Try increasing the resonance or adding salt to increase the grittiness of the drum.

Chaotic oscillator#

Leave the lp and hp inputs unpatched and turn reso up until the filter starts self oscillating. Turn salt up until it just starts to get chaotic. The filter cutoff frequency will twist and wobble around, making a chaotic output which is a combination of feedback effects. Try adjusting the cutoff frequency, resonance, and salt to get a whole spectrum of sounds, from stable and harmonious, to wild and unstable.

Drum effects#

Send a drum mix, such as a recording or a mix of synthesized drums, to both the lp and hp inputs. This will make the filter act like a shelf filter. The lp and hp volume knobs act as the level for the low shelf and the high shelf, while the cutoff frequency controls the crossover point of the shelves. Try adding a bit of resonance, lowering the high pass volume a bit to make a low shelf filter, and adjusting the cutoff frequency to target specific frequencies in the drum mix. For example try to emphasizing the snare or getting rid of some of the high frequencies without cutting them out completely like on a traditional low pass filter.

Loop crossfader#

Use a device like an Octatrack, Digitakt, or any sampler that can output two different loops matched to the same tempo. If you only have a stereo output, you can hard-pan the loops to left and right channel before patching. Send one loop to the lp input and the other loop to the hp input. Adjusting the cutoff frequency will allow you to fade between the frequencies of the two loops. If the cutoff knob is fully clockwise you'll only hear the loop in the lp input, while if it's counterclockwise you'll only hear the loop in the hp input. You can adjust it to pick out the high frequencies of one loop, such as the cymbals, and the low frequencies of another loop, such as the bass drum.

CV filter#

Neptune is technically DC-coupled, so will pass and filter CV signals just as it does audio signals. If you send a signal to the lp input then Neptune will act like a slew limiter, limiting how fast the signal can move. If you use the hp input then Neptune will act like a gate to trigger converter, where sudden changes will be passed through but the output will settle back to the value at the lp input. Increasing resonance will cause Neptune to overshoot the input value and "ring".

Note that while Neptune can do some fun stuff with CV, it's very much designed with audio in mind which brings some limitations. First, the lowest cutoff frequency that the knob can reach is around 5 Hz, however, you can use the fm1 input along with its attenuverter to push the cutoff frequency even lower. Second, Neptune wasn't really designed for DC accuracy so it won't work well with sensitive CV signals like 1 V / Octave pitch signals. Finally, Neptune's output can have a DC offset of up to ±2 V, however, you can mitigate this a bit by mixing a constant voltage in with the lp input.

Open source hardware & software#

Neptune is completely open source and hacking is encouraged.

The hardware design is available under the permissive CERN-OHL-P v2 license, and is designed using KiCAD, which is also free and open source. You can open the hardware files using KiCAD, or you can download a PDF of the schematics.

Open Source Hardware Association mark Neptune 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 Neptune under the following conditions. These are provided for reference only; we do not guarantee that the module will function under these conditions.

Acknowledgments and thanks#

Neptune would not be possible without the excellent educational YouTube videos by Moritz Klein, and the help of him and others on his discord server. We would also like to acknowledge the following individuals for their outstanding support: