Thank you for buying KON by SYNTHFOX. KON is versatile voltage processor that does attenuversion and lag processing (slew limiting). In this manual, an overview of KON's features is given, as well as some additional information, such as a flowchart (logic diagram) of the module's insides and some patch ideas. Please, read the safety notice carefully!


  1. Installation / Safety Notice
  2. General Overview
  3. Controls
  4. Flowchart
  5. Calibration and settings
  6. Patch Ideas

Installation / Safety Notice

Thank you for buying KON! We at SYNTHFOX want people to have as much fun with and make as much good use out of our gear as possible. But firstly, we want users to be safe and their synthesizer systems to be fully functional. This device is not a consumer piece of electronics. This is a specific part (module) that is to be installed into Eurorack system and interfaced with other parts of it. The user handles the installation process - extra care should be taken!

KON has a keyed connector. The included ribbon cable is tested to comply with standard Eurorack boards, and it cannot be inserted the wrong way into the module without brute force. However, the power distribution boards may vary, and it is up to the user to well understand which way to connect the ribbon to the distribution boards. The red line on the ribbon cable marks the -12 volts line. The module should be connected like in the picture below: on the left side is the bus board connector, and on the right side is the connector on KON.

image by David Haillant

When installing the module:

The module should now be ready to play. If you have any troubles, feel free to reach out to SYNTHFOX through synthfoxmodular@gmail.com.


Enter KON, a versatile signal processor unit. The three main actions of KON is input signal attenuversion, lag processing and oscillating. This module has one signal input with a dedicated attenuverter knob, handy to scale and invert the input to taste. By default, about +12v is normalled to the input jack (except KON V1.0), and therefore the input attenuverter is an offset generator if nothing is patched into the module's singal input. This is handy for live performance - quickly turn the knob, and KON will slowly follow to the new value, while you play other parts of the system.

The input is then passed through a lag processor core. It consists of a core setup many Serge modules and their derivatives use for lag processing, stripped-down to basics and reconfigured for KON. The module's output is the output of this lag processor, indicated by a bicolor LED on top of the jack. Red colour indicates positive voltage, and blue colour indicates the negative.

The most common use of KON is lag processing, or slew limiting: put a signal to the input jack - and get a 'smoothed out' version of it at the output. When the input signal voltage changes, the output slides to the new voltage instead of reaching it immediately. Upwards time setting knob and a dedicated CV input jack determine the upwards glide time. Likewise, the downwards time is determined by the downwards time setting and its CV input. Both time settings can be controlled using the BOTH CV input. The more clockwise a time setting knob is, the smaller the lag time gets. While this sounds counterintuitive at first, higher time settings correlate to faster transitions and higher pitches in LOOP mode, so this way is, in fact, more intuitive then the other way around. Using KON as a lag processor on CVs will add glide, or portamento: squarewave LFOs can be turned into (dynamically controllable) trapezoids, sequences and other staircase-like voltages can get variously glided - from adding a little downwards lag for the acid feel, to smoothing them out into a sequential fluctuating curve.

The second main way KON can be used is - flicking the LOOP switch up and setting KON to oscillate. This may be used as an LFO in lower ranges and as an audio VCO in upper ranges of the upwards/downwards time knobs. The two controls are independent, so one can dial in any oscillation shape between sawtooth, triangle and reverse sawtooth. Of course, changing upwards or downwards time also changes the pitch of the oscillation. Controlling the overall frequency of the oscillation after the shape is dialed in is easy using the BOTH cv input. While it does track more or less exponentially, it does not have 1v/o properties, so it cannot be used to get tuned melodies using sequencers that stick to this standard, but plays well with hand-tunable sequencers and such.

Finally, looping and input lagging can be used both at the same time. The signal that causes KON to loop is merely added up to the input signal (see flowchart), so having some input signal along with it will cause interesting interplays of cycling and input processing. The looping oscillation usually occurs when the input voltage level is low enough, so scaling it with the attenuverter is necessary. In CVs, this will lead to occasional oscillations along with the lag-processed input signal at the output. At audio rates, this turns KON into an extremely potent dynamics and harmonics processor: looping adds a peak to the sound, similar to resonant peaks in VCFs. Controlling upwards, downwards or both times with an external CV will cause interesting timbral modulations, somewhere between a waveshaper, a filter and a VCA. Using this trick with LOOP disabled also works - but yeilds a tamer, non-'resonant' tone.

A lag processor and voltage scaler, KON is a very general and versatile module, yet has its own character. It can fulfil many roles beyound the ones mentioned above. For instance, one can also use it as an attack-release envelope by processing a gate signal from the keyboard with it, or as an envelope detector by passing audio through it, setting a short upwards and a slower downwards time. Creative possibilities are unlimited!


Ⓐ Upwards lag (glide) time setting
Ⓑ Downwards lag (glide) time setting
Ⓒ Upwards lag CV input (UCV)
Ⓓ Downwards lag CV input (DCV)

Ⓔ BOTH CV input
Ⓕ LOOP enable switch
Ⓖ Input signal attenuverter
Ⓗ Module input jack
Ⓘ Module output


Calibration and settings


There are two pin headers on KON V1.0. The RANGE header selects the range of lag times. The default position is "LOW" - it goes from moderately low CV times and up to mid audio times, and can be pushed above/below that using BOTH cv. "HI" position pushes these rates up - then, this module becomes more useful as an audio processor/filter and covers higher audio rates, but does not go as slow as with the "LOW" setting. The +12->IN pin header selects if a positive offset should be normalled to the input jack. If set to "YES", the offset will be normalled to the input jack. Setting it to "NO" removes the offset. The default setting has been "NO" starting V1.1 batch 4. This version of KON has no trimm potentiometers.


There is one pin header on KON V1.0 - it selects the range of lag times. The default position is "LOW" - it goes from moderately low CV times and up to mid audio times, and can be pushed above/below that using BOTH cv. "HI" position pushes these rates up - then, this module becomes more useful as an audio processor/filter and covers higher audio rates, but does not go as slow as with the "LOW" setting. This version of KON has no trimm potentiometers.

Patch Ideas

KON is a very general module, and shines the most in conjunction with other devices in a system. Its major uses have already been described in the overview. However, there is still a lot of KON-specific ideas to be explored - here are some of them!

Exponential Shape Hack

By itself, KON lags linearly. This means that it will have a steady volts per second increase or decrease. This also means that the LOOP-enabled oscillations will be linear - no matter the skew of it. However, there is an easy way to make it exponential. Use a passive splitter/multuple to get a copy of the KON output. Then run it through an attenuator or an attenuverter to have better shape control, and patch it back to one of the KON's CV inputs. Patching it to Upwards CV will affect the rising part of the oscillation. Likewise, patching it to Downwards CV will only affect the falling part, and both parts can be affected using the BOTH cv input. Adjust the attenuator to taste: the more of the KON input controls its parameter, the more exponential it becomes. If you are using an attenuverting, negative feedback will make the shape more logarithmic-like.

Attack-Release envelope

Just patch a gate signal (e.g. from a keyboard) to KON's input, set the input level knob full clockwise, and use the output to control whatever you'd like to control with an attack-release envelope. And there you have it! Upwards and downwards time controls now set attack and release, respecitvely. It's possible to use the previous patch example to change parts of the envelope to behave exponentially instead of linearly, too.

Dynamics and Harmonics Processor

Put your audio to the IN jack, and run the OUT to your system output. Set the up and down time cotnrols to about 2 o'clock, and use some sort of an LFO, envelope or sequence to control the times through the BOTH input. KON will now behave as a combination of a waveshaper, filter and VCA all at once, with a weird flavour. Voltage control over up and down times brings fascinating timbral movements, and controlling BOTH acts as a volume/timbre/cutoff control. Play with up and down time knobs and differences between them for interesting shaping possibilities. Engaging LOOP opens up another dimension of screechy, seemingly-resonant processing. NB! KON's output is not DC-decoupled and may have DC shifts, so using a DC-decoupler (a capacitor) or a dedicated audio mixer module is recommended for cleaner sound. Using the oscillator shape hack technique on top of this technique squeezes out even more peculiar fine settings, up to occasional suboctaving using negative feedback.

Envelope Follower

This trick is useful if you have a punchy sound source, like a drumline or a series of sound bursts, and a device processing them, like a VCF. KON can extract the 'volume movement' of the source audio as its output voltage by smoothing it out. It can be used to control the processor synchronously with the volume profile of the source audio. Simply mult your audio into the processor and KON's IN jack. Set KON's upwards time setting at about 3 o'clock, downwards at noon. Now the output is a voltage that jumps up quickly on every transient of the input sound, and falls down relatively slowly. Run KON's output to the processor module of your choice. Tune the time settings to taste after you complete the setup: the longer the time, the less sensitive the envelope following is, eating up smaller transients. Using a VCF with a bit of resonance, one can achieve an 'auto-squelch' on each transient of the audio by controlling its cutoff frequency with KON's output. The audio source has to be modular level (at least 5v peak to peak), so if you are using a line level signal, amplify it before running it through KON and the sound processing module.



v1.0: initial module release

v1.1: fixed 1.0 errors, added positive offset to input normal.


[20220216] 1.0: initial document

[20220430] 1.1: updated for module version 1.1

Happy patching!